xiii.] MEASUREMENT OF PHENOMENA. 279 



and more careful measurement. We cannot lay down 

 any invariable series of questions which must be asked 

 from nature. The exact character of the questions will 

 vary according to the nature of the case, but they will 

 usually be of an evident kind, and we may readily illus- 

 trate them by examples. Suppose that we are investigat- 

 ing 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 will exist, and we 

 must have 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. Accurate observa- 

 tions 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 will 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 elfected as regards the solvent power of 

 water must be repeated with alcohol, ether, carbon 

 bisulphide, and other media, so that unless general laws 

 can be detected, this one phenomenon of solution can 



