ON THE SOLUBILITY OF SALTS. 30.") 



are many others which I may hereafter try to make use of, and some of 

 which have been employed for analogous purposes by others ; such, for 

 example, as the influence which solutions exert upon the spectrum, as shown 

 by the ingenious experiments of Dr. Gladstone, who I hope may be induced 

 to take up the whole question of the optical relations of saline solutions. 



Relative Compressibility of Saline Solutions. — Among the most important 

 physical phenomena the more complete study of which promises to throw 

 light upon the nature of solution and the action of salts in solution upon each 

 other, may be mentioned their compressibility. As all the experiments upon 

 the solubility of salts at high temperatures must be conducted under high press- 

 ure, I must necessarily begin by investigating the action of pressure alone 

 upon saline solutions in the cold. With the exception of the experiments of 

 Grassi, made with Regnault's piezometer, we have no records of any in which 

 saline solutions were examined. Grassi's, too, are mere isolated examples, 

 not sufficient to enable a law to be established ; the pressures, too, were very 

 limited. The experiments of W. Thomson and Bunsen show that the point 

 of solidification of bodies is lowered by pressure; hence we may expect that 

 solutions of salts, if subject to pressure, would crystallize. There is a re- 

 markable experiment of Perkins which bears out this supposition ; he ex- 

 posed glacial acetic acid with ^th water to a pressure of 1100 atmospheres, 

 and found that |-ths of it had crystallized in a few minutes. In some trials 

 which I have commenced, and in which I have used Epsom salts, nitrate of 

 potash and common salt, high pressure alone appeared to produce crystal- 

 lization. I obtained the pressure by the method employed by Degen* to 

 ascertain whether oxygen and hydrogen combined with one another under 

 great pressure. This method consists in decomposing water in a closed tube, 

 by means of a voltaic current. I employed a U-tube, in one leg of which I 

 placed the solution of the salt, and in the other the water to be decomposed. 

 The decomposition was effected by means of two platinum wires soldered 

 into the glass. Degen introduced a manometer-tube into his apparatus, by 

 which he was able to register the pressure. In my first experiments 1 did 

 not think this necessary, as they were only tentative. As it will be necessary 

 to make a series of pressure experiments upon every salt the solubility of 

 which I may seek to determine at high temperatures, in order that I may be 

 able to determine what is due to pressure and what to temperature in the 

 phenomena of solution at high temperatures, I must use a manometer in all 

 future experiments. I am in hopes that a series of experiments made with 

 Regnault's piezometer upon the comparative compressibility of concentrated 

 solutions of single salts, and of mixtures of salts according to the schemes of 

 classification, according to form and chemical composition above suggested, 

 may give results which can serve to indicate molecular changes not other- 

 wise recognizable. It is also probable that many new hydrates may be pro- 

 duced under the influence of great pressure; and that the character of the 

 precipitates, the form, specific gravity, and other physical properties of salts 

 crystallized under the joint influence of pressure, and the presence of other 

 substances in solution, may present interesting modifications. 



The experiments of Grassi having shown that the compressibility of 

 distilled water free of air decreases as the temperature rises, while the com- 

 pressibility of all the other fluids experimented upon increases with the tempe- 

 rature, the action of pressure upon the solutions of salts in water must be 

 different from that which it exerts upon similar solutions in alcohol, ether, 

 &c. I will not discuss this subject further here, as I hope very soon to be 



* Poggendorff's Annalen, vol. xxxviii. p. 454. 

 1859. ' x 



