This is prepared by saturating fresh lemon-juice with carbonate or bicarbonate of potassa. Four fluidounces of the juice are a convenient quantity. The salt is to be slowly added until effervescence ceases. A solution of citric acid in water, flavoured with oil of lemons, may be substituted for the lemon-juice. (See Citric Acid, II. 92.) in the present Pharmacopoeia, the preparation made with lemon-juice, and that with solution of citric acid, are distinguished by different names; the former being called Mistura Potassae Citratis, the latter Liquor Potassae Citratis. The bicarbonate of potassa is directed in the Pharmacopoeia, preferably to the carbonate, because the latter is apt to contain impurities, especially silicate of potassa, which is decomposed by the citric acid, with the deposition of hydrated silica in a flocculent state. This, however, has no other disadvantage than its unsighthliness; as it has no effect on the system. By the reaction between the carbonate or bicarbonate and the citric acid, in the preparation of the solution, citrate of potassa is formed, which remains dissolved, and carbonic acid is liberated, partly escaping with effervescence, and partly dissolved by the water; so that the preparation is a solution of citrate of potassa in water, impregnated with carbonic acid. When fresh lemon-juice is used, the solution has a slightly greenish colour, in consequence of changes in the organic constituents of the juice. it is, moreover, of variable strength, as the quantity of acid in lemon-juice varies. Nevertheless, if the juice be from good sound lemons, and have a very sour taste, the preparation will be sufficiently uniform for use; and it has seemed to me more agreeable to the palate and the stomach, than when made with the solution of citric acid.

Another mode of preparing the neutral mixture is simply to dissolve three drachms of citrate of potassa in four fluidounces of water. To give it a greater resemblance to that prepared in the former methods, a drop or two of oil of lemons may be first rubbed up with the citrate, and this may be dissolved in carbonic acid water, instead of pure water. Upon the whole, I prefer the solution made with fresh lemon-juice.

The dose of the neutral mixture is a tablespoonful, or half a fluidounce every two hours. The interval may be shortened to an hour, or extended to three or four hours, according to the severity of the fever. it should be diluted with at least an equal quantity of water, when administered. Some prefer the preparation made originally with a mixture of equal measures of lemon-juice and water, instead of the undiluted juice. The advantage of this is that the dose contains more absorbed carbonic acid, than when the preparation is diluted after saturation. Of the preparation made in this way the dose would of course be double, or a fluidounce. "When a strong sedative impression on the circulation is desired, small quantities of tartar emetic may be added to the mixture, say one-twelfth or one-eighth of a grain for each dose. Should there be nervous symptoms, as restlessness, morbid vigilance, muscular twitchings, startings, etc., attendant on the hot dry skin which calls for the neutral mixture, a little sweet spirit of nitre, or Hoffman's anodyne, may be added to it with advantage. Not unfrequently the mixture slightly disturbs the bowels, or causes griping pain. in such cases, it may be very properly combined with a little laudanum, or solution of sulphate of morphia, which is often useful also by aiding the diaphoresis, and composing irritation. Sugar may be added, if desired, in order to render it more palatable.