dryness in a drying oven, when the weight of the residue will indicate the weight of the dissolved substance, and subtracting this from the weight of the solution gives the weight of alcohol. From these data the ratio of solubility is calculated in the manner already explained in the example, given for determining the solubility at normal temperature.

Rapid simple solution of solid bodies is always accompanied by a fall in temperature, while a solution of gases causes a rise in temperature; these phenomena are in accordance with the laws governing the state of aggregation of bodies. Solids, for the assumption of the fluid state, require a certain amount of energy or heat, which is withdrawn from the surrounding liquid and becomes latent, while gases when condensing to liquids give out an amount of heat corresponding to that required for maintenance of the gaseous state. Four ounces of ammonium nitrate or potassium iodide rapidly shaken in a bottle with two ounces of pure water will produce sufficient cold to condense the moisture of the air on the outside of the bottle and freeze it into thin sheets of ice.

Since rapid simple solution causes a decided fall in temperature, advantage is taken of the fact that some substances hasten the liquefaction of others in the production of so-called freezing mixtures; thus, 5 parts each of ammonium chloride and potassium nitrate dissolved in 19 parts of water will cause a drop of temperature of 20° C. (36° F.); a mixture of 2 parts of snow and 3 parts of crystallized calcium chloride will cause the temperature to fall from 0° C. (32° F.) to -45.5° C. (―50° F.) and freeze mercury; the usual mixture for ice-cream freezers consists of salt with twice its weight of snow or crushed ice, which produces a temperature equal to about -20° C. (—4° F.), the cream in the cylinder freezing by reason of the great abstraction of heat necessary for the rapid liquefaction of the ice and snow surrounding it-not, as some persons believe, because intense cold is imparted to it from the outside.

Salts which have been deprived of their water of crystallization, and thus been converted into anhydrous amorphous powders, will cause a more or less marked rise in temperature when brought into solution; the heat thus generated must be looked upon as due to chemical action involving the restoration of water necessary for the assumption of the crystallized state by the anhydrous salt. If crystallized sodium carbonate be shaken with twice its weight of water, a marked fall in temperature will be noticed, whereas anhydrous sodium carbonate shaken with twice its weight of water causes a rise in temperature, thus proving the correctness of the preceding supposition. When liquids are dissolved in other liquids no change of temperature will occur in the mixture unless contraction of volume takes place, as in the case of alcohol and water or sulphuric acid and water.

Saturated solutions, in a pharmaceutical sense, are such as cannot take up any more of the dissolved body at ordinary tem

perature; in other words, the solvent has become charged with as much soluble matter as it is capable of retaining in intimate union at the ordinary temperature. The statements of ratio of solubility in the Pharmacopoeia and elsewhere always refer to the formation of saturated solutions at the temperature named; thus the official statement that cane-sugar is soluble at 25° C. (77° F.) in 0.46 part of water and 137.2 parts of alcohol, in part of boiling water and 28 parts of boiling alcohol, means that with the proportions of water and alcohol named sugar forms saturated solutions at the temperatures indicated. Supersaturated solutions are those in which the solvent, by artificial means, has been made to take up more of the soluble matter than it is capable of retaining under ordinary circumstances; they are very unstable and present a peculiar condition of solubility. If 3 parts of sodium sulphate be dissolved in 1 part of water at 30° C. (86° F.), the solution carefully filtered into a perfectly clean dry bottle free from dust, and allowed to cool gradually, it will remain clear as long as it is not disturbed, although supersaturated, since water at 15° C. (59° F.) can dissolve only about one-third of its weight of the salt; but if the bottle containing the supersaturated solution be shaken, or a little broken glass be introduced, the whole contents will suddenly congeal to a crystalline mass. Saturated solutions of salts are frequently capable of dissolving other salts, and thus may be used for purposes of purification; if potassium nitrate be treated with a saturated aqueous solution of the same salt, no more potassium nitrate can be taken up, but impurities present will enter into solution and are thus removed.

The effect which the presence of one substance may have upon the solubility of another is interesting as well as of practical value in pharmacy. Corrosive sublimate is far more soluble in water in the presence of alkali chlorides, and red mercuric iodide is readily dissolved in a solution of potassium iodide; in both cases union takes place between the mercuric and alkali salts. The increased solubility of potassium chlorate in the presence of sodium bicarbonate is well known; mutual decomposition, no doubt, results, the newly formed salts, sodium chlorate and potassium bicarbonate, requiring only 1.1 part and 3.2 parts of water at 15° C. (59° F.) respectively for solution, as against 16.7 and 12 parts for the original salts. Ordinarily iodine requires about 5000 parts of water for solution, but if mixed with twice its weight of potassium iodide it will readily dissolve in 20 times its weight of water. In this case no chemical union takes place, as the iodine has every appearance of being dissolved but not combined; it retains its characteristic color and odor, and if the solution be heated in a test-tube the iodine can be completely volatilized, a portion subliming in the cooler part of the tube in its original condition.

A marked example of the effect of the presence of one substance on the solubility of another is found in the well-known concentrated solution of sodium phosphate, largely used by physicians.

Sodium phosphate contains ordinarily about 60 per cent. of water of crystallization, and is soluble at 15° C. (59° F.) in 6 parts of water; if 100 Gm. of the salt be triturated with 13 Gm. of citric acid and 2 Gm. of sodium nitrate until liquefied, and enough water then added to bring the volume up to 100 Cc., the solution will keep. This solution, which represents about 60 grains of sodium phosphate in each fluidrachm, is the result of chemical action, and is called by some solution of sodium citrophosphate and by others compound solution of sodium phosphate.

In striking contrast to the above examples may be mentioned the insolubility of potassium sulphate in a solution of ammonium. sulphate and of potassium nitrate in a solution of ammonium nitrate.

Solutions of solids always measure more than the liquid used to prepare them, but never as much as the combined volumes of the solvent and dissolved body. The increase in volume will naturally vary considerably, and be greatest when the substance to be dissolved is very soluble, as sugar, sodium salicylate, or potassium iodide in water. Another factor determining the volume of the solution is the presence of large proportions of water of crystallization. The following table of saturated solutions, prepared at the temperature of 15° C. (59° F.), is of interest :

(Borax, ferrous sulphate, magnesium sulphate, sodium phosphate, and sodium sulphate contain water of crystallization varying from 45.31 per cent. to 60.3 per cent. of the weight of the substance.)

Percentage Solutions. This term is applied to solutions of definite strength, containing a specified amount of soluble matter in 100 parts of the solution; thus a 1 per cent. solution is composed of 1 part of the soluble substance and 99 parts of the solvent; or a 5 per cent. solution is composed of 5 parts of the soluble substance and 95 parts of the solvent, etc. For solids and gases percentage solutions should always be prepared by weight, while for liquid substances either weight or volume may be employed. The quantity of soluble substance and solvent necessary to make a specified quantity of any particular percentage solution may be readily ascertained by the following rule: Multiply the quantity of solution desired, in

grammes or grains, by the number expressing the percentage, divide the product by 100, and the quotient will indicate the quantity of soluble substance necessary; subtract this from the total quantity of solution desired, and the remainder will indicate the necessary quantity of solvent.

Examples: Wanted 500 Gm. of 10 per cent. carbolized oil: 500 X 10= 5000, and 5000 100= 50; 500- 50 450. Answer: Dissolve 50 Gm. of crystallized carbolic acid in 450 Gm. of olive oil.

Wanted 750 grains of 4 per cent. cocaine hydrochloride solution: 750 X 4 = 3000, and 3000 ÷ 100=30; 750—30720. Answer: Dissolve 30 grains of cocaine hydrochloride in 720 grains of distilled

water.

Wanted 640 Gm. of 2 per cent. mercuric chloride solution: 640 X 2 = 1280, and 1280 ÷ 100 = 12.8; 640 12.8 = 627.2. Answer: 12.8 Gm. of mercuric chloride must be dissolved in 627.2 Gm. of distilled water.

Wanted 480 grains of 20 per cent. quinine oleate : 480 × 20 = 9600, and 9600 100 96; 480- - 96384. Answer: Dissolve 96 grains of quinine alkaloid in 384 grains of oleic acid.

Sometimes a percentage solution of two or three substances is wanted; in such a case the absolute quantity of each active ingredient is first ascertained by the rule given above; the sum of their weights is then subtracted from the total quantity of solution desired to find the necessary weight of the solvent; for instance: Wanted 250 grains of 8 per cent. cocaine hydrochloride solution, containing also 2 per cent. of boric acid: 250 × 8 = 2000, and 2000 ÷ 100 20; 250 X 2 = 500, and 500 100 5; 20+ 525; 250 — 25 =225. Answer: Dissolve 20 grains of cocaine hydrochloride and 5 grains of boric acid in 225 grains of distilled water.

When a definite volume of a weight percentage solution is wanted, the quantity nearest in volume to that required must be made; although this sometimes involves a slight loss, there is no other method known if accuracy is to be preserved. Thus, if 2 fluidrachms of a 4 per cent. solution of any soluble chemical are wanted, 5 grains of the substance must be dissolved in 120 grains of water; the 125 grains of solution will measure a trifle more than 2 fluidrachms. If 8 fluidounces of a 10 per cent. solution are wanted, 4000 grains of solution must be made by using 400 grains of the medicinal agent and 3600 grains of water; 8 fluid ounces of water weigh 3646 grains hence the excess of solution will not be large. If a quart of 1 per cent. mercuric chloride solution is desired, 15,000 grains of solution must be made, as the weight of a quart of water is 14,583 grains, which is only 267 grains less than the quantity of water necessary; 150 grains of mercuric chloride dissolved in 14,850 grains of water yield only a little over fluidounce more of the solution than is wanted. If 500 Cc. of a 5 per cent. solution are desired, 530 Gm. of the solution must be made, the excess of solution being 3.5 Cc.,

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for 5 per cent. of 530 is 26.5, and as each Cc. of water equals 1 Gm., 530 26.5 503.5. When solvents other than water are used, having a higher or lower specific gravity, due allowance must be made for this fact, as the volume of a liquid compared with that of an equal weight of water varies with the specific gravity of the liquid; thus, if 4 fluidounces of a 5 per cent. solution of iodoform in alcohol are desired, it will suffice to make 1600 grains, of which 80 graius must be iodoform and 1520 grains alcohol; this will insure the full volume desired, as the specific gravity of official alcohol is 0.820, and 4 fluidounces will therefore weigh only 1494.7 grains (for 455.7 X 4 X 0.820-1494.69), whereas 4 fluidounces of water weigh 1822.8 grains. If a definite volume of a percentage solution in glycerin is required, it becomes necessary to make a larger quantity by weight than for the same volume of an aqueous solution, because the specific gravity of glycerin is 1.25, or one-fourth higher than that of water, while its specific volume is only 0.8, or one-fifth lower than that of water. To make 250 Cc. of io a per cent. solution of borax in glycerin would require 35 Gm. of borax and 315 Gm. of glycerin, yielding 350 Gm. of solution; this quantity will not be much in excess of 250 Cc., since the volume of 315 Gm. of glycerin is 252 Cc. (3151.25), and the presence of the borax will not materially influence the volume. When strong percentage solutions of saline substances are made the latter often increase the volume of fluid markedly, and particularly so if they contain much water of crystallization, as shown in the table on page 125.

Solutions of arbitrary strengths are frequently employed, and although not as accurately made as percentage solutions, nevertheless seem to answer the purposes well for which they are intended. They are usually prepared as follows:

(It is evident that if metric weights and measures are used, much greater accuracy will be insured.)

The so-called normal salt solution used by physicians for transfusion and other purposes must not be confounded with the normal sodium chloride solution used by chemists in volumetric analysis. The former, preferably called physiological salt solution, contains