in chemistry, the name by which one of the general classes of salts are distinguished. The characteristic marks of them are, 1. The peculiar taste which we call sour; though this does not hold universally; for the acid of arsenic, which in other respects manifests a strong acid power, has not this sour taste; nor are the volatile sulphureous acid, or those of tungsten and molybdena, lately discovered by Mr Scheele, very distinguishable in this way. On the other hand, the strong acids of vitriol, nitre, and even sea-salt, are altogether caustic, and cannot be tasted until they have been largely diluted with water. 2. With water they combine into a fluid, the specific gravity of which is not a medium between the water and acid separately taken. This holds good with the strong acids, which grow hot with water, and shrink into less bulk by reason of their emitting a quantity of the fire they contain; but whether it also takes place in the weaker acids, has not yet been ascertained; though the probability is, that it will take place in them also. 3. With spirit of wine, they unite into a very volatile and inflammable substance called ether. This also must be understood only of the strong mineral acids, or of the acetous when very much concentrated; for the acids of tartar, borax, arsenic, lapis ponderosus (tungsten), and molybdena, do not produce any. 4. They change the blue colour of vegetables to red, and heighten the colour of those which are already red.—This property is more universal than those we have yet mentioned; but the volatile sulphureous acid, those of tungsten and molybdena, are exceptions. 5. They unite with all kinds of earths excepting the siliceous (though the fluor acid dissolves this also), with fixed and volatile alkalies, and with metals, in such a manner as to form compounds considerably permanent, and whose ingredients cannot be separated without some difficulty. This is the most universal and distinguishing mark; and there is not any acid but what shows its attraction for one or other of these substances, especially the alkaline salts. Oils and fats, indeed, will unite with alkalies; but they may be separated by the weakest known acids, so that there is no danger of confounding the two together. 6. When mixed with any fermentable liquor, they prevent that process from taking place; or, if it has already begun, they will put a stop to it. This also must be understood only of the stronger acids, or at least will require a considerable quantity of the weaker to effect it. 7. They cannot be frozen but in a degree of cold below the freezing point of water. This property is likewise not universal, but is remarkable only in the stronger acids.

The nature of acids has long been a matter of speculation, and of late has engaged the attention of philosophers very considerably. Some have supposed them to be simple chemical elements, while others imagined them to be composed of water and earth. Both these opinions, however, are inadmissible; the former, because we are certain that most acids may be entirely decomposed, and resolved into aerial vapours of different kinds, which could not happen if they were simple and unchangeable elements; the latter, because there is not the smallest probability that two ingredients, seemingly so infipid and inactive as water and earth, could by their union produce a compound endowed with such powerful and even destructive properties as many of the acids possess.—The late discoveries concerning air of different kinds have suggested a new theory, first published by M. Lavoisier, and thereafter's hypothesis maintained by the French chemists, viz. That the acid principle is contained in the air; and, accordingly, as the acid combines itself with different substances, forms acids of different denominations.

This theory he considers as established by numerous indubitable experiments. These cannot here be detailed; but his conclusions from the whole are, That "deplogisticated air enters as a constituent part into Ba's of the composition of several acids, particularly the phosphoric, vitriolic, and nitrous; that this pure and highly respirable air is the constitutive principle of acidity common to all acids; and that the difference by which principle they are distinguished from each other is produced by the union of one or more principles besides this air, so as to constitute the particular form under which each acid appears." To deplogisticated air in its state of fixity, therefore, he gives the title of the acidifying or oxygenous principle; and concludes farther from his experiments, 1. "That, when combined with the matter of fire, heat, and light, this principle produces deplogisticated air; though he considers this position as not capable of absolute demonstration. It must not, therefore, be confounded with the following; which, he says, are supported by experiment and positive proofs. 2. That the same acidifying principle, combined with phlogistic substances or charcoal, forms fixed air. 3. That with sulphur it forms vitriolic acid. 4. That with nitrous air it forms nitrous acid. 5. That with Kunckel's phosphorus, it forms the phosphoric acid. 6. With sugar it forms the acid of sugar," &c.

The opinion of Mr Lavoisier concerning the composition of acids has in part been adopted by Mr Kirwan; who, in his treatise on Phlogiston, published in 1787, informs us, that he is now of opinion "that deplogisticated air becomes an essential constituent part of acids. All acids (he adds) consist of two principles: one peculiar to each, which, in the opinion of the antiphlogistians, has not as yet been decomposed, and consequently must be looked upon, relatively to the present state of our knowledge, as a simple substance: the other, pure air, in a concrete state; that is, deprived of the greater part of its specific heat, and condensed into a smaller volume. The first they call the acid basis; the last, the oxygenous principle; thus the vitriolic acid, according to them, consists of sulphur as its basis, and pure air in a concrete state as its acidifying or oxygenous principle. This doctrine of the composition of acids has been admitted by some of the ablest defenders of phlogiston, and particularly by that distinguished philosphic chemist M. de Morveau, with this single modification, that the bases of acids contain phlogiston, which they lose on uniting to pure air: yet it seems very difficult to conceive how pure air can unite to phlogiston, a substance to which it has the greatest affinity, without forming a new compound endowed with very different properties from those which it possessed before such union. It seems therefore more reasonable to conclude, either that it forms water, as Mr Cavendish thinks; or fixed air, as I shall afterwards endeavour to prove."

In his explanation of the formation of acids, Mr Kirwan Kirwan first states the opinion of the antiphlogistians, viz. That the vitriolic acid, when considered abstractedly from the water it contains, always consists of sulphur (which they consider as a simple substance) united to a large portion of the oxygenous principle. "In my opinion (says he), it consists of a basis or radical principle, which, when saturated with phlogiston, constitutes sulphur; when saturated with fixed air, becomes common fixed vitriolic acid; and, when combined partly with the one and partly with the other, becomes volatile vitriolic acid. That sulphur, during its conversion into vitriolic acid, unites to air of some sort or other, is evident from the quantity of air which it absorbs, in whatever way that conversion is brought about. Thus, firstly, during combustion in respirable air, 100 grains of sulphur absorb 420 cubic inches of pure air, or about 143 grains; but the proportion of this pure air united with a given quantity of sulphur is not easily determined, because it is vitriolic air that is constantly formed; and this air essentially contains some portion of sulphur in solution, which portion is variable. Secondly, Pyrites, during their decomposition, absorb a considerable proportion of pure air, as Mr Lavoisier has observed; so also does liver of sulphur exposed to the atmosphere, for after some time it is converted into tartar vitriolate."

Mr Kirwan next proceeds to inquire, whether the air absorbed during the combustion of sulphur continues to be pure air; or whether it be converted into water or fixed air? He inclines to the latter opinions, for various reasons* which he specifies.

With regard to the nitrous acid, the experiments of Mr Cavendish, as well as of the French chemists, leave no room to doubt that it is produced during the deflagration of dephlogisticated and inflammable air. Mr Cavendish has shown that the nitrous acid may be formed by taking the electric spark in a mixture of three measures of phlogisticated air and seven of dephlogisticated air, or, in weight, one part of the former and about 2.6 of the latter. Mr Lavoisier, as has been already mentioned, supposes the nitrous acid to be composed of nitrous air united to the oxygenous principle, or basis of pure air; and 100 grains of dry nitrous acid consist of 64 grains of nitrous air united to 36 of pure air deprived of its specific fire; or, according to Mr Kirwan's calculation, 173 cubic inches of nitrous air and 105 of pure air. But nitrous air, as Mr Lavoisier himself has observed, is a compound; 100 grains of it, according to him, containing 32 of phlogisticated and 68 of pure air; consequently 64 grains of it contain 20.5 of phlogisticated air, and 43.5 of pure air. Hence, according to him, 100 grains of dry nitrous acid contain 79½ of pure air and 20½ of phlogisticated air. Mr Kirwan is of opinion that 100 grains of pure, dry, and colourless nitrous acid contain 38.17 grams of fixed air as its acidifying principle, 57.06 of nitrous basis, and 4.77 of phlogiston united to the nitrous basis. With regard to the nitrous basis itself, he says that one third of its weight is phlogisticated and two thirds dephlogisticated air, both in a concrete state.

"Nitrous basis (says Mr Kirwan), saturated with phlogiston, constitutes nitrous air: 100 grains of this basis take up nearly 22 of phlogiston. Hence the constituent principles of nitrous acid are fixed air, dephlo-

gisticated air, phlogisticated air, and inflammable air, all in their concrete state.

"Red, yellow, green, and blue nitrous acids, whose colours are intense, owe their origin to the absorption of nitrous air; and consequently the proportion of their principles is variable, though all have the dephlogisticated acid for their ground. Thus Dr Priestley, having exposed (strong pale-yellow nitrous acid, whose specific gravity could not be less than 1.400 to nitrous air, found that 100 grains of this acid absorbed, in two days, 247 cubic inches of nitrous air: now, 100 grains of this spirit must have contained, by my calculation, about 21 grains of dry acid, and these 21 grains took up 91.39 grams of nitrous air. When about 20 cubic inches of nitrous air were absorbed (that is, about seven grains), the acid became of an orange colour; when 50 cubic inches were absorbed (about 18 grams), it became green; and when nearly the whole was absorbed, it evaporated in the form of nitrous vapour, carrying off part of the water with it. Hence we see, that nitrous vapour consists of nitrous acid united to three or four times its weight of nitrous air and a little water."

Mr Kirwan next proceeds to contest Mr Lavoisier's opinion, that nitrous air is a constituent principle of the nitrous acid. "The following experiments (says he) show that nitrous air is not a constituent principle of the nitrous acid, but that fixed air is. 1. There is not a doubt but that pure nitrous acid enters entire, and without decomposition, into fixed alkalis, and forms nitre. Now if nitre be distilled in a good earthen retort, it will be wholly decomposed; and so also will the acid itself, except a few drops which pass in the beginning of the distillation, and nothing but dephlogisticated air, more or less pure, and consequently intermixed with phlogisticated air and a slight proportion of fixed air, will be found: these, therefore, are its true constituent parts when disengaged from substances that cannot communicate phlogiston to it in any remarkable quantity, such as alkalis and earths; but if it be separated from substances that contain phlogiston, such as metals, it will then indeed be resolved into nitrous air, and dephlogisticated air more or less pure, the phlogiston of the fixed air being detained by the metal. Mr Berthollet, who seems to have made the experiment with the greatest exactness, produced 71.4 cubic inches of dephlogisticated air from a Troy ounce of nitre. This, however, was far from being of the purest kind; and Dr Priestley, Mr Berthollet, and Mr Succow, observed, that the air which first passes contains fixed air, and renders lime-water turbid. Here then we have three of the constituent parts of the nitrous acid, with scarce any nitrous air; which the antiphlogistians suppose to be one of the constituent parts of the acid, and to make two thirds of its bulk when exhibited in an aerial form."

To obviate an objection that the quantity of fixed air thus obtained is too small to deserve to be ranked among the constituent parts of the nitrous acid, Mr Kirwan first inquires in what proportion it ought to exist there; and though this is variable, according to the different states of the nitrous acid with respect to phlogistication, he reckons it at one-third of the acid as existing in the nitre; and, from the decomposition of this this fixed air, and the phlogiston emitted by it of consequence, he attributes the phlogistication and redness of the nitrous acid when exposed to more heat. As a proof that fixed air may be decomposed in this manner, he adduces two experiments of Dr Priestley. In one of these, dephlogisticated air was obtained by means of acetous acid in that concentrated state in which it is called radical vinegar. Having mixed half an ounce of the acid with two ounces of calcined whiting, he obtained from it 350 ounce-measures of air; of which about one-third was fixed more in the first portions, and less in the last. The standard of the residuum in the first portions was 1.66, in the second 1.42, and in the third, 1.38; which is very near the goodness of common air. The whiting then weighed 760 grains.

On adding a quarter of an ounce more of radical vinegar, and repeating the operation, 120 ounce-measures of air were obtained, and the whiting was reduced to 730 grains. A third operation, in which another quarter of an ounce of vinegar was added, reduced the matter to 489 grains; but the last portion of air extracted had no fixed air, and was considerably better than that of the atmosphere. ——The other experiment was made with lime-stone alone; from four ounces of the white crystals, of which 830 ounce-measures of air were obtained, the first portion of which had only one-fourth of fixed air, and the standard of the residuum was never better than 1.56, nor worse than 1.66; so that it was nearly of the goodness of common air.

Our author then proceeds to relate several other experiments in which the nitrous acid was decomposed; but a particular relation of them would swell this article beyond its due bounds. At last, however, he concludes in the following manner. "If spirit of nitre be made to boil, and its vapour received through a red-hot earthen tube, it will be converted into dephlogisticated air, in which a portion both of phlogisticated and fixed air is found, as Dr Priestley has discovered: the water through which this air passes will also contain fixed air. Here then are several ways of decomposing the nitrous acid; and in one only it is resolved into nitrous and dephlogisticated air; and in this way it may, at least, be strongly suspected to receive an addition of another principle. Why then should these be regarded as its constituent principles? And as in the two simplest methods of decomposition, in which the reaction of no foreign substance can be suspected, it appears in the form of dephlogisticated, phlogisticated, and fixed air (the former always containing a portion of the two last), why then should not these be accounted its true constituent parts?"

This theory is further confirmed by reflecting on the manner in which nitrous acid is generated by nature. Mr Thouvenel found that this acid is constantly produced when chalk is exposed to a mixture of putrid air and common air, or putrid and dephlogisticated air; but if the putrid air be passed through lime-water, it is never generated; and that it is rarely produced by the exposure of quick-lime or fixed alkalis to these airs. The reason that alkalis, though aerated, are not so proper, is, that they do not combine with phlogisticated air as calcareous earths do. Mr Cavendish, indeed, produced nitrous acid without any apparent mixture of fixed air; but the atom of it necessary for the formation of the small quantity of nitrous acid he produced (about one-third of a grain), might well be contained in the phlogisticated air he employed, or perhaps formed in the operation."

Having thus far stated the different opinions of the most celebrated French and English philosophers concerning the composition of acids, it is necessary to take notice of some experiments made by Mr Watt, in order to determine whether the dephlogisticated air produced from nitre really proceeds from which a decomposition of the acid, or what quantity of the latter is required to constitute a determinate quantity of the former. To ascertain this*, 240 grains of mercury were put into a glass retort with 480 grains* Philof. of diluted dephlogisticated nitrous acid, which was the quantity necessary to dissolve the whole of the mercury; and as soon as the common air was expelled, a proper vessel was applied to receive the air produced in the operation. Sixteen ounce-measures of nitrous air came over during the solution, and on changing the receiver, a quantity of dilute, but highly phlogisticated nitrous acid, was obtained. The air receiver being again applied, four ounce measures of strong and pure nitrous air were obtained, which, by the dephlogisticated air that arose immediately after, were reduced to half an ounce measure. The production of dephlogisticated air continued very rapid, the mercury being all the while received, until the operation was ended by the distillation or sublimation of the whole of the mercury. Two hundred and eighteen grains of the metal were obtained in its running form, and 22 remained in the form of an orange-coloured sublimate in the upper part of the retort.—The 16 ounce-measures of nitrous air, first obtained, were then converted into nitrous acid by the gradual admission of common air, and then added to the water in the basin in which the receiver had been inverted; the whole quantity being about two quarts, and very acid to the taste, sparkling at the same time with nitrous air. To determine the quantity of acid thus recovered, as well as that which remained in the sublimate, a solution of alkali of tartar was made; and by experiment it was found, that 120 grains of the acid, originally employed in dissolving the mercury, saturated 352 grains of this solution; the orange coloured sublimate and all the acid liquor recovered being saturated by 1395 grains of the same. Hence it appears, by the rule of proportion, that out of 480 grains of nitrous acid originally employed, only five were lost; "a smaller quantity (as Mr Watt justly observes) than what might reasonably be supposed to be lost in the process by the extreme volatility of the nitrous acid." His conclusion therefore is, that "the nitrous acid does not enter into the composition of dephlogisticated air: it seems only to serve to absorb phlogiston from the watery part of the mercurial nitre."

This experiment was repeated with cubic nitre, and only 30 ounce-measures of air distilled from an ounce of the mineral alkali exactly saturated with nitrous acid. The water through which the air passed was acid, and the residuum in the retort alkaline; but on mixing the two together, the solution was found to be exactly neutral by every possible test.

Not satisfied with these experiments, Mr Watt distilled an ounce (480 grains) of common nitre, stopping the process when 50 ounce-measures of air had been produced. This air had a strong smell of the nitrous nitrous acid, from which it could not be freed by washing with the water in the basin. The residuum in the retort was alkaline as before, and the water slightly acid; nor was the saturation completed by mixing the two together. Ten grains of weak nitrous acid, 105 grains of which contained the acid of 60 of nitre, completed the saturation. These ten grains contained the acid of 57 grains of nitre; which, by Mr Kirwan's experiments, is equal to two grains of real nitrous acid.

"We have therefore (says Mr Watt) 34 grains weight of deplogificated air produced, and only two grains of real acid missing; and it is not certain that even this quantity was destroyed, because some portion of the glaas of the retort was dissolved by the nitre, and some part of the materials employed in making the glaas being alkali, we may conclude, that the alkali of the nitre would be augmented by the alkali of that part of the glaas it had dissolved; but as the glaas cracked into small pieces on cooling, and some part of the coating adhered firmly to it, the quantity of the glaas that was dissolved could not be ascertained."

To avoid the force of objections drawn from these experiments, and which seem ready to overthrow his hypothesis, as well as that of Mr Lavoisier entirely, Mr Kirwan makes the following reply.—"My ingenious friend Mr Watt, as well as Mr Cavendish, are of opinion, that the whole quantity of deplogificated air, produced from the distillation of nitre, arises from the deplogification of the water it contains, it being decomposed by the nitrous acid, which then becomes phlogisticated. This opinion is exposed to insurmountable difficulties. For, in the first place, nitre affords deplogificated air at the rate of 146.125 cubic inches for every 100 grains of nitre, which, by the proper allowances for phlogisticated air, should weigh 46.77 grains: but then deplogificated air is only one of the constituent parts of water, for it contains 13 per cent. of inflammable air, that is to say, 87 grains of deplogificated air; to form 100 grains of water requires an addition of 13 grains of inflammable air; consequently 46.77 grains of deplogificated air require nearly 7 of inflammable air, and would then form 53.77 grains of water, which exceeds half the weight of the nitre; a quantity of water, as Mr Watt owns, certainly inadmissible.—Mr Watt found, that the water over which the air proceeding from the decomposition of 960 grains of nitre had been received, contained only the acid belonging to 120 grains of nitre; and even this small quantity he inferred only from my experiments. But my experiments are totally inapplicable in this case; for I used only the deplogificated nitrous acid; and alkalis are saturable by a much smaller quantity of phlogisticated than of deplogificated acids, as is evident in the case of the deplogificated marine acid, as Stahl long ago observed; for he says, that the volatile acid of sulphur saturates 10 times as much alkali as the fixed. Mr Bergman and Mr Scheele observed, that melted nitre is still neutral, though it be phlogisticated; therefore it is air, and not water, which it wants. Accordingly Dr Priestley found it to injure common air by attracting its deplogificated part; but if it be kept in fusion for some time, it loses its acid, and becomes alkaline; and the air it receives must surely be deemed rather to recompose the acid than to form water; of whose formation, in the temperature of the atmosphere, we have no sort of proof. On the contrary, the imposibility of accounting for the loss of acid in this case is an evident proof of the fallacy of that hypothesis.

—By Mr Lavoisier's analysis, 100 grains of nitre contain 57 of caustic alkali; by Mr Bergman's, 49; by Mr Wenzel's, 52; by Mr Wiegels, 46½; by...ind in mine, 63: the mean of all which is 53½; which leaves nitre 46.5 for acid and water, which is very nearly the weight of the air expelled. The different quantity of acid assigned by different persons to nitre, is in part owing to its degree of phlogistication in nitre. I believe at present, that 100 grains of nitre contain 34 of acid and about 12 of water, including the water in the acid and that of crystallization."

Mr Kirwan next proceeds to consider, in a manner similar to that above related, the composition of the other acids.—The marine acid, according to him, consists of a peculiar basis united to phlogiston, and a certain quantity of fixed air; to both of which the basis seems to have a strong affinity. On depriving it of this phlogiston, the affinity of the acid to fixed air becomes much stronger, and it saturates itself so largely with it, that its attractions for other substances containing little or no phlogiston, become nearly as weak as those of fixed air itself when equally condensed; but with respect to bodies that contain a considerable quantity of phlogiston, its affinities are much stronger, as its basis attracts the phlogiston, while those bodies attract its excess of fixed air. In this state it does not expel fixed air from aerated fixed alkalis or earths until it is heated; and then deplogificated air separates from it, and it becomes, in all respects, common marine acid. For as it contains an excess of fixed air, it acts nearly as an acid of the same nature; but when heat is applied, its basis deplogificates its own fixed air, which then becomes deplogificated air, at the same time that the acid becomes common marine acid, and acts as such.

Mr Lavoisier, and other philosophers, who deny the existence of phlogiston, are of opinion, that the common marine acid consists of a peculiar basis united to a small proportion of pure air, or oxygenous principle, and the deplogificated marine acid differs from it only by containing an excess of this principle.—This opinion they are chiefly induced to maintain, because the acid in its deplogificated state is procured by distilling common marine acid from manganese; and the manganese, if distilled by itself, before the acid is distilled from it, affords deplogificated air; but after the acid is distilled from it, it yields none.—"This experiment, however, (says Mr Kirwan), proves no more but that the manganese contains some air which is de-phlogisticated during the calcination. And that this air is fixed air, appears from the following considerations: The black calx of manganese almost always gives out fixed air at first, before any deplogificated air appears; whence it is natural to think, that the deplogificated air proceeds from the deplogification of the fixed. And hence, if it be distilled with filings of iron, or in a gun-barrel, it scarce gives out any other than fixed air; if at any time it gives out deplogificated air, with little or no mixture of fixed air, this is owing to a very perfect deplogification of the calx, and to its containing very little moisture. Thus Dr Priestley, having having passed the steam of boiling water through manganese heated in an earthen tube, obtained a very large quantity of fixed air, and scarce any other; though on repeating this experiment with manganese well freed from calcareous earth, I obtained a large portion of dephlogisticated air; but I believe much depends on the degree of heat to which the tube is subjected. But having distilled manganese, which yielded of itself some fixed air with common spirit of salt, I obtained dephlogisticated marine acid, and not a particle of fixed air; which shows that this last combined with the dephlogisticated basis, and formed the dephlogisticated acid. Mr Hermitadt having dissolved the black calx in common marine acid, and precipitated it with an acerated fixed alkali, obtained, as usual, a white precipitate; which, when heated, afforded a great part of the fixed air it had absorbed from the alkali; but when heated to such a degree as to be of a brown red colour, and consequently deplogisticated, it converted common spirit of salt into a dephlogisticated acid, which could proceed only from some fixed air yet unexpelled: Yet if sal-ammoniac be distilled with the black calx of manganese, it will be expelled in a caustic state; for the fixed air unites to the dephlogisticated marine basis in preference to the volatile alkali.*

Several other experiments are related by Mr Kirwan, which the limits of this article will not allow us to experiment in fert; but the following, he is of opinion, fully confirms his hypothesis, and subverts that of the antiphlogistians.

"Six cubic inches of inflammable air were mixed with as much dephlogisticated marine air over lime-water. In about ten minutes after the greater part of the diminution had taken place, a white cloud appeared on the surface (a) of the lime-water, and by agitation it became still more turbid. As it was possible that the manganese might be mixed with calcareous earth, some dephlogisticated marine air was extracted from another portion of it, and received on lime-water; but it was wholly absorbed, without forming the least cloud, tho' there was lime enough; for, on adding acerated water, a cloud appeared."

The other acids particularly treated of by Mr Kirwan are the phosphoric and saccharine. In his treatise on the former, he adopts the analysis of Mr Lavoisier, changing only his acid principle of dephlogisticated for fixed air. From this it appears, that the phosphoric acid consists of a peculiar basis united to 2.265 of its weight of the acid principle; or, in other words, 100 grains of dry phosphoric acid contains about 69 of fixed air and 31 of its peculiar basis: 100 grams of the phosphoric basis take up 226.5 of fixed air, or 32.9 of phlogiston when it becomes phosphorus; and 100 grams of phosphorus contain 75.24 of basis and 24.76 of phlogiston.—The basis of this acid is the only one that can be procured free, both from the phlogiston and the acidifying principle; it is called, though improperly, as it is not soluble in water, the glacial phosphoric acid. Mr Lavoisier and others are of opinion, that phosphorus is a simple substance containing no phlogiston, and that the acid consists of the oxygenous principle united to it.

With regard to the acid of sugar, Mr Kirwan observes, that sugar itself is a compound of fixed air with a much larger proportion of inflammable air, and some saccharine water, all condensed to a degree of which we are ignorant, but retaining, upon the whole, much more specific heat than either oil or charcoal; though he seems inclined to the hypothesis of Mr Morveau, that this substance has for its basis a fine ethereal oil, to which a large proportion of condensed inflammable air is superadded. The acid of sugar, then, according to him, consists of this peculiar basis deprived of its superfluous phlogiston, and united to a great quantity of fixed air in a concrete state. He also of opinion, that it does not exist ready formed in the sugar, but is produced in the operations that substance undergoes: that it derives most of its acid principle from the nitrous acid employed; the nitrous basis taking up the phlogiston, and the fixed air of the nitrous acid combining with the saccharine basis. He contests strongly an opinion of Mr Lavoisier, that sugar is a sort of charcoal, which, uniting with the oxygenous principle of the nitrous acid, decomposes it, sets loose the nitrous air, and forms the saccharine acid; and that, towards the end of the operation, the saccharine acid itself is decomposed; the consequence of which is the production of fixed air, which, according to him, is only the oxygenous principle combined with charcoal. On this Mr Kirwan remarks, 1. "That, according to this theory, the acid of sugar should be the same with fixed air, since both are composed of the oxygenous principle united with charcoal: or, if Mr Lavoisier should reply, that sugar is different from common charcoal, he reminds him, that, according to his own table of affinities, the oxygenous principle has a much stronger attraction for charcoal than for sugar, and consequently that the latter ought to be decomposed by the former; nay, that it should be regenerated by various metallic substances, which, according to him, have a greater attraction for this principle. 2. According to this hypothesis, the saccharine acid ought to weigh more than the sugar employed in the operation; which is so far from being the case, that it is universally agreed to be much less; Bergman making it only 4/10, Mr Chaptal from 4/10 to 5/10ths, and Mr Sage 5/10ths.

3. If the saccharine acid consisted of sugar, or consisted of that substance undecomposed, and barely united to the oxygenous principle, it ought to be formed by treating sugar with the black calx of manganese, or with dephlogisticated marine acid; both of which, according to him, have less attraction for the oxygenous principle than sugar. Lastly, (says Mr Kirwan), If the acid of sugar be distilled, it is wholly converted into water, fixed inflammable air, and not a particle of coal or dephlogisticated air is found in it. It is not therefore reasonable to look on either of them as its constituent principles; but as fixed air alone can be extracted from all vegetable acids, it seems to be the true acidifiable principle."

Having given a view of the present opinions relative to the original formation of acids, it remains to treat a little more particularly of each of the different kinds.

(a) On mixing these, a dense white cloud appears; one half the bulk of both disappears, and the residuum explodes like a mixture of inflammable and dephlogisticated air. Acids. They are divided into three different classes, expressive of their origin, viz. the Mineral, Vegetable, and Animal. The mineral acids are those of vitriol, nitre, sea-salt, borax, amber, fluor, arsenic, tungsten, molybdena, &c. The vegetable are, those of vinegar, tartar, sugar, benzoïn, apples, citrons, lemons, tamarinds, sorrel, cork, &c. The animal acids are, the microfic or acid of urine, and that of bones, both of which are also called the phosphoric, though this might be accounted a vegetable acid, as it is procured by distilling mustard and some other vegetables by a violent fire. Besides these, there are the acids of ants, wasps, bees, silk-worms, milk, &c. It has also been discovered, that the human calculus is formed for the most part of a peculiar acid, which has received the name of lithiafic acid. Lastly, As an acid distinct from all these, we may now add fixed air, by force called the aerial, and by others the cretaceous acid; the latter appellation it derives from creta, chalk, because it is found in that substance in great quantity. See AEREOLOGY.

The general properties of acids have already been enumerated; the most remarkable of which is their attraction for alkaline salts, earths, and metals. Though this is common to all, yet very considerable differences are observed among them in this respect, and on those differences depend almost all the phenomena of that part of Chemistry which treats of salts. As these phenomena are particularly considered under that article, we shall here only in general take notice, that the three acids named the vitriolic, nitrous, and marine, are the strongest of them all; that is, if any other acid be united to an alkali, earth, or metal, the union will be broken by adding to that compound any of the three acids just mentioned. Neither are these equal in power among themselves; for the vitriolic is stronger than the nitrous, and the nitrous stronger than the marine. The rule, however, is liable to certain exceptions and variations, depending chiefly on the circumstances of heat or cold, moisture or dryness, and particularly on the state of the marine acid with regard to its being in the form of an aqueous fluid or reduced to a dry vapour. In this last case it seems stronger than either the vitriolic or nitrous; and even when in an aqueous state, both the nitrous and marine acids, when added in great quantity, seem to oppose and overwhelm the stronger vitriolic acid, so that they will partly expel it from an alkaline salt. This does not depend on the mere quantity of acidity they possess: for the acetous acid may be concentrated to such a degree as to become stronger in this respect than spirit of salt; yet it will always be inferior in point of real strength, when tried with an alkali in competition with the latter. The aerial acid is the weakest of all; and may be expelled not only by vinegar, but by the acid juices of fruits, tartar, and the acids of tungsten and molybdena.

Some acids have the property of resisting the fire, and melting into a kind of glaas, such as that of borax and phosphorus. This circumstance gives them an advantage over the stronger acids which are volatile; and thus the two just mentioned, as well as those of arsenic and tungsten, will, in a very strong heat, expel the acid of vitriol itself, though the latter will, in the cold, expel any one of them with great ease.

Both the vitriolic and nitrous acids have a very strong attraction for phlogiston; and unite with certain oily and inflammable matter so vehemently as to occasion great heat, and sometimes even violent and unextinguishable flame. This is particularly the case with the nitrous acid, or with a mixture of the two; and indeed the nitrous acid, though weaker than the vitriolic, flows itself in every instance to be far more active, and to perform all its operations with vastly greater rapidity, than the other. All these particulars, however, as they properly fall under the article Chemistry, are there explained at length: together with the origin and peculiar methods of preparing each of the acids, and the various uses to which they may be applied in arts and manufactures. See also their different titles as they occur in the order of the alphabet; as, Nitre, Vinegar, Vitriol, &c.

ACIDULOUS denotes a thing that is slightly acid; it is synonymous with the word sub-acid.

ACIDULÆ. Mineral waters that contain a brisk spirit, when unaccompanied with heat, are thus named; but if they are hot also they are called THERMAE. See MINERAL WATERS.