phylis: a discontinuation, or analysis, of the structure of a mixed body; whereby, what was one, and contiguous, is divided into little parts, either homogeneous or heterogeneous.

Diffusion, then, is a general name for all reductions of concrete bodies into their smallest parts, without any regard either to solidity or fluidity: though in the usual acceptation of the word among authors, it is restrained to the reduction of solid bodies into a state of fluidity; which is more properly expressed by solution, as a branch of diffusion.

According to the opinion of Fr. Tertius de Lanis, Boerhaave, and some other learned men, the power or faculty of diffusing is lodged in fire alone. See Fire and Heat.

According to this hypothesis, other fluids commonly supposed diffusives, only produce their effect by means of the fiery spicula they abound with; and even air, which is judged a powerful menstruum, owes all its force to the rays of light diffused therein.

Sir Isaac Newton accounts for all dissolutions, and the several phenomena thereof, from the great principle of attraction; and, in effect, the phenomena of diffusion furnish a great part of the arguments and considerations whereby he proves the reality of that principle. The following is a specimen of that great author's way of philosophizing on the subject of diffusion.

"When salt of tartar dissolves by lying in a moist place, is not this done by an attraction between the particles of the salt of tartar and those of the water which float in the air in form of vapours? and why does not common salt, or salt-petre, or vitriol, do the like, but for want of such an attraction? And when aqua-fortis, or spirit of vitriol, poured on steel-filings, dissolves the filings with a great heat and ebullition; is not this heat and ebullition effected by a violent motion of the parts? and does not that motion argue, that the acid parts of the liquor rush towards the parts of the metal with violence, and run forcibly into its pores; till, getting between the utmost particles and the main mass of metal, they loosen them therefrom, and set them at liberty to float off into the water? When a solution of iron in aqua-fortis dissolves lapis calaminaris, and lets go the iron; or a solution of copper dissolves iron immersed in it, and lets go the copper; or a solution of mercury in aqua-fortis poured on iron, copper, tin, or lead, dissolves the metal, and lets go the mercury; does not this argue, that the acid particles of the aqua-fortis are attracted more strongly by the lapis calaminaris than by iron; by iron than by copper; by copper than by silver; and by iron, copper, tin, and lead, than by mercury? And is it not for the same reason, that iron requires more aqua-fortis to dissolve it than copper, and copper more than the other metals; and that of all metals iron is dissolved most easily, and is most apt to rust; and next after iron, copper? When aqua-fortis dissolves silver, and not gold; and aqua-regia dissolves gold, and not silver; may it not be said, that aqua-fortis is subtle enough to penetrate the pores of gold as well as of silver, but wants the attractive force to give it entrance; and the same of aqua-regia and silver? And when metals are dissolved in acid menstruums, and the acids in conjunction with the metal act after a different manner, so that the taste of the dissolution compound is milder than that of the simples and sometimes a sweet one; is it not because the acids adhere to the metallic particles, and thereby lose much of their activity? And if the acid be in too small a proportion to make the compound dissoluble in water; will it not, by adhering strongly to the metal, become inactive, and lose its taste, and the compound become a tasteless earth? for such things as are not dissoluble by the moisture of the tongue are infipid."

Dr Freind gives us a mechanical account of diffusion, in the instance of salt dissolved in water, which is the most simple operation that falls under this head. This motion he attributes to that attractive force, which is so extensive in natural philosophy, that there is no kind of matter but what is under its influence. It may be observed, says he, that the corpuscles of salts, which are the most simple of any, are withal very minute, and for their bulk very solid; and therefore exert a very strong attractive force, which, ceteris paribus, is proportional to the quantity of matter. Hence it comes to pass, that the particles of water are more strongly attracted by the saline particles than they are by one another: the particles of water, therefore, cohering but loosely, and being easily moveable, approach the corpuscles of salts, and run, as it were, into their embraces: and the motion of them is quicker or slower, according to their less or greater distances; the attractive force in all bodies being strongest, at the point of contact. Therefore, if salt be thrown into the middle of a dish full of water, we shall find the aqueous particles which are in the middle of the dish sharp and pungent to the taste, but the water upon the sides of the vessel almost insipid; so that, when such a motion once arises, the aqueous particles are carried with an equal force towards the salts, and the moment of them is to be estimated from the ratio of their weight and celerity conjunctly. By the force of this impulse, they open to themselves a passage into the pores of the salts, which are very numerous; and at length to break and divide their texture, that all cohesion of their parts is destroyed: hereupon, being separated, and removed to a convenient distance from one another, they are dispersed, and float here and there about the water.

The simple diffusion of saline substances of every kind in water, may indeed be plausibly enough explained on the hypothesis of attraction; but where the diffusion is attended with heat, the emission of vapour, &c. it seems necessary to seek for some other principle than mere attraction to solve these phenomena. When diluted oil of vitriol, for instance, is poured upon iron-filings, a great quantity of vapour arises, which, if it was attempted to be confined, would certainly break the containing vessel.—It is impossible to imagine any connection between attraction and the emission of a vapour; and what is still more unaccountable, this vapour is inflammable, though neither the oil of vitriol nor the iron are so by themselves. Another very strong objection against the hypothesis of attraction may be derived from the phenomena of metallic dissolutions in general; for they do not dissolve completely in acids, as salts do in water. By dissolution they are always decomposed, and cannot be recovered in their proper form without a good deal of trouble. One metal, indeed, will very often precipitate another from an acid in its metallic form; but this is attended with the decomposition of the second metal; so that this can by no means be reckoned a fair experiment. But, whatever other method is used, the dissolved metal is always recovered in form of an earthy powder, that we could scarcely imagine capable of ever becoming malleable, and assuming the splendid appearance of a metal. Now, if there was a strong attraction between this and the acid, we might very justly conjecture, that the dissolution happened by means of that attraction; but so far from this, after a metal has been dissolved by any acid, and the calx has been separated from it, it is always difficult, and very often impossible, to procure a dissolution of the calx in the same acid. The action of the acid in this case seems not unlike that of fire upon wood or any other inflammable substance. Dry wood, thrown into the fire, burns and flames with great violence; but the same wood reduced to ashes, instead of burning, extinguishes fire already kindled. In like manner, a piece of clear metal thrown into an acid, dissolves with great violence; but the same metal, deprived of its phlogistic principle, and reduced to a calx, cannot be acted upon by acids, in whatever manner they are applied; at least, not without the greatest difficulty; and the more perfect the calx is, i.e., the more completely it is deprived of its inflammable principle, the greater the difficulty is of combining it afterwards with an acid.

Another thing in which the dissolution of metals by an acid resembles the burning of combustibles by fire is, that in both cases there is a separation of the principle of inflammability. In the case of oil of vitriol and iron-filings, this is exceedingly obvious; for there the vapour which arises from the mixture takes fire, and explodes with great vehemence. In all other cases it is very easily proved; for the calx is always capable of being revived into metal by the addition of any substance containing phlogiston. The calces prepared by fire, and by precipitation from acids, also resemble one another too much, that in many cases they are scarce to be distinguished.

These considerations seem to favour the hypothesis of Dr Boerhaave; and much more does the following, namely, that almost all metallic solutions produce some degree of sensible heat. In some metals this is very considerable; but the greatest heat producible by an aqueous solution of any substance is by dissolving quicklime in the nitrous acid. The heat here greatly exceeds that of boiling water. In some dissolutions of inflammable matters by a mixture of the vitriolic and nitrous acids, the heat is so great, that the whole mixture takes fire almost instantaneously. Hence the Boerhaavians think they have sufficient grounds to conclude, that fire alone is the agent by which all dissolutions are performed.

These appearances have also been explained on the principles of attraction; and it has been said, that the heat, &c., were owing to nothing but the violent action of the particles of the acid and metal upon each other. But the late discoveries made by Dr Black, with regard to heat, show, that it is capable of remaining concealed in substances for any length of time, and afterwards breaking out in its proper form. It is probable, therefore, that the heat produced in these dissolutions is no other than what existed before, either in the acid or in the metal. But for a full discussion of this subject see the articles Cold, Congelation, Evaporation, Fire, Heat, &c.