11 



IruublesoDie and hariiitul impurity when steam boilers are supplied 

 with it. Chalk is as nearly insoluble in pure water as most substances 

 with which we are ac(]uaiiited, one million |)arts of water dissolving 

 only eighteen parts of chalk. That is, were a gallon of water fully 

 saturated with chalk to be evaporated to dryness the residue would 

 weigh only about i)^ grains. We shall see, however, that under con- 

 ditions quite commonly found in nature the solubility of chalk may be 

 increased to 880 j)arts per million, /. c, a residue of 62 grains would be 

 obtained from a gallon of water saturated under these circumstances. 

 The condition referred to is the presence of free carbonic acid in the 

 water. Before illustrating this, let me indicate the laws which govern 

 the solution of gases in water. These are, briefly, (1st), the specific 

 nature of the gas; (2nd), the temperature; (3rd), the pressure. The 

 two gases, of which our atmosphere is essentially composed, are soluble 

 in water only to a very slight extent. At the ordinary temperature and 

 pressure of the air 100 gallons of water dissolve about 3 gallons of 

 oxygen, and nitrogen is only about half as soluble as oxygen. A fourth 

 law of gaseous solubility applies when a mixture of gases is exposed to 

 a solvent, as in the case of air and water. Each gas is, dissolved just in 

 such proportion as it would be were the other gas not present (the 

 pressure, of course, being correspondingly reduced). A consequence 

 of this is that while oxygen and nitrogen are present in air in the ratio 

 of I to 4 they are dissolved in water in the ratio of i to 2. Thus the 

 atmospheric gases present in water form a mixture very much richer in 

 oxygen than is the air, and the important consequences that follow from 

 this are not far to seek. It is from this dissolved oxygen that fish and 

 all water-breathers obtain the supply to arterialize their blood, and, 

 what bears more directly upon our subject to-night, it is by means of 

 this dissolved oxygen that the various processes by which the harmful 

 and even poisonous organic impurities of natural water are changed to 

 Innocent substances, are carried on. So emphatically is the presence 

 of oxygen m solution an essential condition of purity in a surface 

 water, that many chemists always estimate the dissolved oxygen m 

 water analysis. In illustration of this pomt I may quote the following 

 figures from a report upon the river Seine, above, at, and below 

 Paris : — 



