324 THE PRINCIPLES OF SCIENCE. 



for instance, that we are investigating the solution of some 

 salt in water. The first is a purely logical question : Is 

 there solution, or is there not ? Assuming the answer to 

 be in the affirmative, we next inquire, Does the solubility 

 vary with the temperature, or not ? In all probability 

 some variation wiU be found to exist, and we shall have 

 at the same time an answer to the further question, 

 Does the quantity dissolved increase, or does it diminish 

 with the temperature ? In by far the greatest number of 

 cases salts and substances of all kinds dissolve more freely 

 the higher the temperature of the water, but there are a 

 few salts, such as calcium sulphate, which follow the 

 opposite rule. A considerable number of salts resemble 

 sodium sulphate in becoming more soluble up to a certain 

 temperature, and then varying in the opposite direction. 

 We next require to assign the amount of variation as 

 compared with that of the temperature, assuming at first 

 that the increase of solubility is proportional to the in- 

 crease of temperature. Common salt is an instance of 

 very slight variation, and potassium nitrate of very con- 

 siderable increase with temperature. Very accurate ob- 

 servations will probably show, however, that the simple 

 law of proportionate variation is only approximately true, 

 and some more complicated law involving the second, 

 third, or higher powers of the temperature may ultimately 

 be established. All these investigations have to be 

 carried out for each salt separately, since no distinct prin- 

 ciples by which we may infer from one substance to 

 another have yet been detected. There is still an in- 

 definite field for further research open ; for the solubility 

 of salts would probably vary with the pressure under 

 which the medium is placed ; the presence of other salts 

 already dissolved may have effects yet unknown. The 

 researches already effected as regards the solvent power of 

 water must be repeated as regards alcohol, ether, carbon 



