is a deep depression between the clinoid apophyses of the sphenoid bone. See Anatomy, p. 682.

SELTZER water, is a mineral water which springs up at Lower Seltzer, a village in the electorate of Trier, about 10 miles from Frankfort on the Main. It is a very useful medicinal water. It contains, according to some, a very small portion of calcareous earth, of a native mineral alkali, and an acid; but of these the quantity is too small to attribute any medicinal virtues to; but it contains also near 1/7th of its bulk of fixed air, which is more than is found in any other mineral water, and to this it owes its principal virtues. Others have said that it is of the very same nature with Pyrmont water, and contains a subtle aqueous fluid, a volatile iron, and a predominant alkali, all joined together into one brisk spirituous water. The consequence of these different opinions respecting its constituent parts is, that different methods have been recommended for imitating it.

According to the former analysis, artificial Seltzer water may be prepared by adding one scruple of magnesia alba, six scruples of fossil alkali, and four scruples of common salt, to each gallon of water, and saturating the water with fixed air or carbonic acid. According to the latter it may be imitated by adding to a quart of the purest and lightest water thirty drops of a strong solution of iron made in spirit of salt, a drachm of oil of tartar per deliquium, and thirty drops of spirit of vitriol, or a little more or less as is found necessary, not to let the alkali of the oil of tartar prevail too strongly, tho' it must prevail a little. If the proportions be carefully observed, and the whole of these ingredients shaken briskly together, the artificial Seltzer or Pyrmont water thus made will strongly resemble the natural, and have the same good effect in medicine.

But as fixed air is the only efficacious medicinal part of the composition of Seltzer water, the best method of imitating it is by impregnating common water with that acid by a process for which we are indebted to Dr Priestley. The first idea of this kind occurred to him in 1767, when, having placed shallow vessels of water within the region of fixed air, on the surface of the fermenting vessels of a brewery, and left them all night in that situation, he found that the water had acquired a very sensible and pleasant impregnation. He proceeded to accelerate the impregnation by pouring the water from one vessel into another, while they were both held within the sphere of the fixed air. The method of effecting this by air dislodged from chalk and other calcareous substances did not occur to him till the year 1772, when he published his directions for this purpose, together with a drawing of the necessary apparatus, which he had before communicated to the Board of Admiralty. That apparatus has now given way to another invented by Dr Nooth, which is made of glass, and stands on a wooden vessel (fig. 1.) resembling a tea-board: the middle vessel B has a neck which is inserted into the mouth of the vessel A, to which it is ground air-tight. The lower neck of the vessel B has a glass stopper S, composed of two parts, both having holes sufficient to let a good quantity of air pass through them. Between these two parts is left a small space, containing a plano-convex lens, which acts like a valve, in letting the air pass from below upwards, and hindering its return into the vessel A. The upper vessel C terminates below in a tube r t, which being crooked, hinders the immediate ascent of the bubbles of fixed air into that vessel, before they reach the surface of the water in the vessel B. The vessel C is also ground air-tight to the upper neck or the middle vessel B, and has a stopper p fitted to its upper mouth, which has a hole through its middle. The upper vessel C holds just half as much as the middle one B; and the end t of the crooked tube goes no lower than the middle of the vessel B.

For the use of this apparatus: Fill the middle vessel B with spring or any other wholesome water, and join to it the vessel C. Pour water into the vessel A (by the opening m, or otherwise) so as to cover the rising part of its bottom: for this about three-fourths of a pint will be sufficient. Fill an ounce phial with oil of vitriol, and add it to the water, shaking the vessel so as to mix them well together. As heat is generated, it will be best to add the oil by a little at a time, otherwise the vessel may be broken. Put to this, through a wide glass or paper funnel, about an ounce of powdered raw chalk or marble. White marble being first granulated, or pounded like coarse sand, is better for the purpose than pounded chalk, because it is harder; and therefore the action of the diluted acid upon it is slower, and lasts to a considerable time. On this account the supply of fixed air from it is more regular than with the chalk: and besides, when no more air is produced, the water may be decanted from the vessel A, and the white sediment washed off, and the remaining granulated marble may be employed again, by adding to it fresh water and a new quantity of oil of vitriol. The funnel in this process is made use of, in order to prevent the powder from touching the inside of the vessel's mouth; for if that happens, it will stick so strongly to the neck of the vessel B as not to admit of their being separated without breaking. Place immediately the two vessels B and C (fastened to each other) into the mouth of the vessel A, as in the figure, and all the fixed air which is disengaged from the chalk or marble by the oil of vitriol will pass up through the valve in S into the vessel B. When this fixed air comes to the top of the vessel B, it will dilute from thence as much water as is equal to its bulk; which water will be forced up through the crooked tube into the upper vessel C.

Care must be taken not to shake the vessel A when the powdered chalk is put in; otherwise a great and sudden effervescence will ensue, which will perhaps expel part of the contents. In this case it may be necessary to open a little the stopper p, in order to give vent, otherwise the vessel A may burst. It will be proper also to throw away the contents and wash the vessel; for the matter will stick between the necks of the vessels, and cement them together. The operation must then be begun afresh. But if the chalk be put into the vessel looily wrapped up in paper, this accident will be still better guarded against. When the effervescence goes on well, the vessel C will soon be filled with water, and the vessel B half filled with air; which will easily be known to be the case by the air going up in large bubbles through the crooked tube r t.

When this is observed, take off the two vessels B and C together as they are, and shake them so that the water and air within them may be much agitated. A great part part of the fixed air will be absorbed into the water, as will appear by the end of the crooked tube being considerably under the surface of the water in the vessel.

The shaking them for two or three minutes will be sufficient for this purpose. These vessels must not be shaken while joined to the under one A, otherwise too great an effervescence will be occasioned in the latter, together with the ill consequence above mentioned. After the water and air have been sufficiently agitated, loosen the upper vessel C, so that the remaining water may fall down into B, and the unabsoled air pass out. Put these vessels together, and replace them into the mouth of A, in order that B may be again half filled with fixed air. Shake the vessels B and C, and let out the unabsoled air as before. By repeating the operation three or four times, the water will be sufficiently impregnated.

Whenever the effervescence nearly ceases in the vessel A, it may be renewed by giving it a gentle shake, so that the powdered chalk or marble at the bottom may be mixed with the oil of vitriol and water above it; for then a greater quantity of fixed air will be disengaged. When the effervescence can be no longer renewed by shaking the vessel A, either more chalk must be put in, or more oil of vitriol; or more water, if neither of these produce the desired effect.

Mr Magellan has still further improved this contrivance. He has two sets of the vessels B and C. While he is shaking the air and water contained in one of these sets, the other may be receiving fixed air from the vessel A. By this means twice the quantity of water may be impregnated in the same time. He has a wooden stand on which to fix the vessels B, C, when taken off from A, which is very convenient. He has a small tin trough for measuring the quantity of chalk or marble requisite for one operation, and a wide glass funnel for putting it through into the vessel A, to prevent its sticking to the sides, as mentioned before.

He has also contrived a stopper without a hole, to be used occasionally instead of the perforated one p. It must be of a conical figure, and very loose; but so exactly and smoothly ground as to be air-tight merely by its pressure. Its use is to compress the fixed air on the water, and thereby increase the impregnation. For by keeping the air on the water in this compressed state, the latter may be made to sparkle like champagne. And if the vessels are strong, there will be no danger of their bursting in the operation.

The water thus impregnated may be drawn out at the opening k. But if it is not wanted immediately, it will be better to let it remain in the machine, where it has no communication with the external air; otherwise the fixed air flies off by degrees, and the water becomes vapid and flat. But it may be kept a long time in bottles well stopped, especially if they are placed with their mouths downwards.

Dr Withering of Birmingham has lately contrived a new apparatus for impregnating water with fixed air, which, he says, is preferable to that in common use, because it can be made at less expense, and is more easily prepared; because the whole quantity of fixable air produced is converted to use, without any waste of the vitriolic acid; because it impregnates three times the quantity of water at one time more completely and with less trouble; and the impregnated water will always retain its virtue, if the joints and cocks of the machine are made perfectly air-tight; for which purpose they should once a year be supplied with a small quantity of unsalted lard. This apparatus is exhibited by fig. 2, and consists of a glass vessel A, about ten inches high in the cylindrical part, and six inches and a half in diameter; another glass vessel B, about twelve inches high in the conical part, one inch and a half in the neck, and five inches in diameter at the bottom; a copper pipe C passing through the stopper of the vessel B, and tied fast in the flexible tube D, made of strong leather, air-tight, and kept hollow by means of a spiral wire passing through its whole length; a conical brass pipe E, with a stop-cock fastened to the tube D; another conical pipe F, with a stop-cock G, into which the end of the tube E is accurately ground so as to be air-tight, and cutting off all communication with the atmosphere when the pipe E is removed; two large hog's bladders H, H, each of which ought to hold two quarts; a stop-cock I to prevent the water rising into the bladders when the vessel A is agitated; a bladder K tied to the crooked tube with the stop-cock L, which occasionally opens or shuts the communication with the vessel B; a glass funnel M, accurately fitted with the glass stopper N; an aperture O, fitted with a glass stopper or a silver cock, from which the impregnated water is to be drawn for use; and, lastly, the tube P opening into the vessel A. When this apparatus is used, let the vessel A be filled with pure water, and any other ingredients that are required, in a proper proportion; into the vessel B put as much marble or whiting, in small lumps, as will cover its bottom to the height of about two inches, and pour in water to the height represented by the dotted line; let the mouth of the vessel A be well fitted with a cork, and through a hole in the cork pass the tube P, putting upon the cork melted sealing-wax of the softest kind, or modeling-wax, so as to make the whole air-tight. Let the mouth of the vessel B be stopped with a piece of mahogany, turned into a conical figure in a lathe, and of a size somewhat larger than the mouth of the glass will admit; put this piece of wood into melted bees-wax, and heat the wax till the wood begins to grow black; when cool, turn it again till it fits the mouth of the vessel; the tubes C, L, and M are fitted into holes and bored through the wooden stopper previous to its being immersed in the wax; push these tubes through the holes, and press the stopper into the orifice of the vessel B, and cement the whole with sealing or modeling-wax; shut the stop-cocks I and L, having previously pressed the air out of the bladder K; open the stop-cocks G and E; then squeeze the air out of the bladders H, H, and afterwards press the conical pipe E into the pipe F; pour about a large spoonful of oil of vitriol through the funnel M, and stop it with its stopper N. The fixable air let loose by the effervescence in the vessel B, rising through the tube C, passes into the bladders H, H, and distends them. In this case open the stop-cock I, and from the aperture O draw out about a quart of water; and the space before occupied by the water will be filled with fixable air, which soon begins to be absorbed by the remaining water, and is still supplied from the bladders H, H, and from the effervescing mixture in the vessel B. When the bladders are considerably collapsed, more vitriolic acid must be added through the funnel. funnel M, so that they may be always kept pretty fully dilated. When an impregnation is speedily required, turn the stop-cocks at G and E, and open that at L; then separate the pipe E from the tube F, and agitate the vessel A; the fixable air will pass into the bladder K, and may be passed into the two other bladders, when the parts of the apparatus are united. During the agitation, the stop-cock at I should be closed, and opened only occasionally to supply out of the bladders H, II, the fixable air absorbed by the water. If a strong impregnation be required, this process should be carried on in a room, the heat of which does not exceed forty-eight degrees of Fahrenheit's thermometer.

Dr Withering observes, that the impregnated water receives no taste from the bladders; and that if the vessel A with its impregnated water be separated from the vessel B at the conical parting E, F, it may be inclosed in a pyramidal mahogany case, out of the lower part of which the silver cock at O projects; and thus serve for an ornamental as well as luxurious and salubrious addition to the side-board, particularly in the summer and autumnal seasons.

The artificial mineral waters thus made, are more pleasant to the palate than the natural Pyrmont or Seltzer waters; which, besides their fixed air, contain saline particles of a disagreeable taste, which are known to contribute little or nothing to their medicinal virtues, and may, in some cases, be hurtful. They are likewise considerably stronger. According to Sir John Pringle, these waters may be made more nearly to resemble genuine Pyrmont water, by adding to each pint of them from eight to ten drops of *unctura maris cum spiritu salis*. Or this may be done, by adding to the water in the middle vessel B (fig. 1.), in the proportion of about thirty grains of Epsom salt, ten grains of common salt, a scruple of magnesia alba, and a dram of iron filings or iron wire, clean and free from rust, to one gallon of spring water, and impregnating the whole with fixed air in the manner already described. Let them remain, till the other ingredients and as much of the iron as necessary are dissolved; which will be in two or three days: or the magnesia may be omitted, and then the operation will be finished in less than half that time. These waters may be rendered ferruginous or chalybeate very easily, by putting in the middle vessel two or more slender phials, filled with cuttings of fine iron-binding wire, or with small iron nails; because the impregnated water will dissolve the iron so fast, as to become well saturated with it in a few hours, according to the experiments of Mr Lane. But the method of rendering these artificial waters chalybeate, used by Dr Hulme, is to add one grain of salt of steel to each pint (sixteen ounces) of water already impregnated with fixed air.

But the ingenious Mr Bewley has invented a still better method of exhibiting fixed air as a medicine. He directs a scruple of alkaline salt to be dissolved in a sufficient quantity (a quarter of a pint, or less) of water, which is to be impregnated with as much fixed air as it can imbibe: this is to be taken at one dose. Mr Bewley directs it to be prepared in larger quantities at a time, and calls it his mephitic julep. If immediately after it a spoonful of lemon juice, mixed with two or three spoonfuls of water, and sweetened with sugar, be drunk, the fixed air will be extricated in the stomach; and thus a much greater quantity of it may be given than the same quantity of water alone can be made to imbibe. Fixed air acts as a corroborant; and therefore may be given with succeds in weaknesses of the stomach, and in vomitings arising from that cause. It has also been given with succeds in the stone and nephritic complaints. When the lungs are purulent, fixed air mixed with the air drawn into the lungs has repeatedly been found to perform a cure. The bark also may be given with advantage in water impregnated with fixed air, as they both coincide in their effect. Fixed air may be applied by means of a syringe, funnel, or otherwise, to inflamed breasts, putrid ulcers, mortified parts, ulcerated sore throats, and has been found in such and similar cases to have very remarkable efficacy. It may also be given internally at the same time. In phthisic dysenteries, and in putrid stools, fixed air may be given by way of clyster. Fermenting cataplasms are of service, chiefly as they supply fixed air to the part. In cases of putridity fixed air has been successfully applied to the surface of the body exposed to streams of it. It is also found an excellent cooling as well as strengthening beverage in hot relaxing weather, and has the advantage of being pleasant to the taste.