86 PRINCIPLES OF CHEMISTRY 



It is_ evident that the conception of the partial pressures of gases 

 should not only be applied to the formation of solutions, but also to all 

 cases of chemical action of gases. Especially numerous are its appli- 

 cations to the physiology of respiration, for in these cases it is only the 

 oxygen of the atmosphere that acts. 38 



The solution of solids, whilst depending only in a small mea- 

 sure on the pressure under which solution takes place (because solids 

 and liquids are almost incompressible), is very clearly dependent on 

 the temperature. In the great majority of cases the solubility of 

 solids in water increases with the temperature ; and further, the 

 rapidity of solution increases also. The latter is determined by the 

 rapidity of diffusion of the solution formed into the remainder of the 

 water. The solution of a solid in water, although it is as with gases, 

 a physical passage into a liquid state, is determined, however, by its 

 chemical affinity for water ; which is particularly clear from the fact 

 that in solution there occurs a diminution in volume, a change in the 

 boiling point of water, a change in the tension of its vapour, in. the 

 freezing point, and in many similar properties. Were solution a physical, 

 and not a chemical, phenomenon, it would naturally be accompanied 

 by an increase and not by a diminution of volume, because generally in 

 melting a solid increases in volume (its density diminishes). Con- 

 traction is the usual phenomenon accompanying solution, and takes 

 place even in the addition of solutions to water, 39 and in the solution 



58 Among the numerous researches concerning this subject, certain results obtained 

 by Paul Bert are cited in Chapter III., and here we will point out that Prof. Sechenoff, 

 in his researches on the absorption of gases by liquids, very fully investigated the 

 phenomena of the solution of carbonic anhydride in solutions of various salts, and 

 arrived at many important results, which showed that, on the one hand, in the solution 

 of carbonic anhydride in solutions of salts on which it is capable of acting chemically (for 

 example, sodium carbonate, borax, ordinary sodium phosphate), there is not only an 

 increase of solubility, but also a distinct deviation from the law of Henry and Dalton ; 

 and, on the other hand, that solutions of salts which are not acted on by carbonic anhy- 

 dride (for example, the chlorides, nitrates, and sulphates) absorb less of it, by reason of 

 the competition of the already dissolved salt, and follow the law of Henry and Dalton,. 

 but all the same show undoubted signs of a chemical action between the salt, water, and 

 carbonic anhydride. Sulphuric acid (whose co-efficient of absorption is 92 vols. per 100), 

 when diluted with water, absorbs less and less carbonic anhydride, until the hydrate 

 H 2 SO 4 ,H 2 O (co-eff. of absorption then equals 66 vols.) is formed ; then on further 

 addition of water the solubility again rises until a solution of 100 p.c. of water ia 

 obtained. 



39 Kremers made this observation in the following simple form : He took a narrow- 

 necked flask, with a mark on the narrow part (like that on a litre flask which is used for 

 accurately measuring liquids), poured water into it, and then inserted a funnel, having a 

 fine tube which reached to the bottom of the flask. Through this funnel he carefully 

 poured a solution of any salt, and (having removed the funnel) allowed the liquid to 

 attain a definite temperature (in a water bath) ; he then filled the flask up to the mark 

 with water. In this manner two layers of liquid were obtained, the heavy saline solution 



