13 



marked increase in solubility in the case of solid bodies. The follow- 

 i diagram (see Roscoe and Schorlemmer's Treatise on Chemistry, 

 vol. ii., p. 45) will serve to illustrate graphically this point. You will 

 observe that while the rate of increase in solubility for increased tem- 

 perature varies with the specific nature of the salt, it is pretty generally 

 true that the solubility increases as the temperature rises. In the case 

 of sulphate of soda we have a peculiarity in that the maximum of solu- 

 bility is found at about 90" Fah. In common salt we find another 

 interesting peculiarity in that for temperatures between the freezing point 

 and boiling point of water the solubility is practically constant at about 

 four pounds of salt per gallon of water. In the case of sulphate of lime 

 we find the very slight solubility of this salt in cold water is even lowered 

 as the temperature reaches the boiling point, although the decrease in 

 solubility is too small to be well marked. However, did this diagram 

 indicate temperatures as high as those found in steam boilers, where 

 water boils under artificial pressure, we should find that at 270° Fah., a 

 temperature which corresponds to the boiling point of water under a 

 pressure of two and a half atmospheres, or about 40 pounds per square 

 inch — a very ordinary boiler pressure — the solubility of gypsum is 

 reduced to one-twentieth part of its solubility at 212° Fah.: and as a 

 consequence of this nineteen-twentieths of the sulphate of lime in solu- 

 tion in a feed-water is deposited as a coherent and very hard crust on 

 the inner surface of the boiler. 



The remaining condition which affects the solubility of solids in 

 water is the presence of other substances in solution. There is pro- 

 bably no exception to the statement that the solubility of a solid is 

 influenced more or less by the presence of other dissolved bodies. All 

 the phenomena of precipitation depend upon this principle. I shall 

 have occasion to illustrate this in the course of the evening, but I may 

 now ask you to observe how promptly chlorides are thrown out of 

 solution by salts of silver, salts of iron by ammonia or other alkali, lead 

 salts by carbonates or sulphates, all of which reactions are of great value 

 to water consumers, whether the water be used for household or manu- 

 facturing purposes. I can only make detailed reference to two cases of 

 great importance in this connection. The first is the solubility of lead 

 in water, and is of great importance from the extensive use of lead 



