FOOD 121 



some provender; but in the case of water the circumstances under which 

 it is collected are so varied, and the sources of contamination are so 

 numerous, that it is recognized by the experimental chemist as a fact 

 beyond question, that when he requires pure water for scientific purposes 

 he can only obtain it by the application of heat to convert the fluid 

 into a vapour, which he is bound to convey through perfectly sterilized 

 cold pipes, from which it will fall in the condition of water deprived of 

 all foreign matter. In this condition of purity, however, it has lost 

 all the flavour which makes it grateful to the palate, and at least certain 

 products which are beneficial to the system. It becomes, therefore, most 

 important to ascertain what impurities may be safely or advantageously 

 admitted. Water in its absolutely pure condition consists of oxygen 

 and hydrogen, and in this condition it may be obtained, theoretically, by 

 the process of vaporizing under conditions which render contamination 

 impossible. As it occurs, however, in seas and rivers, it holds in sus- 

 pension or in solution various substances which it obtains from the earth 

 and air through which it passes. Rain-water is sometimes referred to 

 as the purest form in which water can be obtained naturally, but this 

 implies that the rain shall fall and the water be collected in a place 

 quite remote from habitations of all kinds, otherwise the various gases 

 with which the atmosphere is charged, from the gaseous products of manu- 

 factures or living beings, are necessarily mixed with the falling rain, 

 contaminating it sometimes to an extent which renders it poisonous or 

 utterly unfit for use. Under ordinary conditions it is estimated that 

 rain-water, even in rural districts, contains about 2 grains of solid matter 

 to the gallon. In towns, particularly where large industries are carried 

 on, the quantity of solids is necessarily much larger. The gases which 

 contaminate rain-water are, in addition to carbonic acid, ammonia, sul- 

 phurous acid, and the emanations from drains and sewers. Other im- 

 purities are added as the rain passes over the roofs of buildings and along 

 the gutters which are arranged for carrying it to the drains. Decaying 

 vegetable and animal matters are frequently washed from the roofs of 

 buildings, and when the water is carried along lead gutters, or stored in 

 lead tanks, the sulphurous acid contained in the fluid, in addition to the 

 products of decomposing vegetable matter, leads to the solution of the 

 lead and renders the water poisonous, or at least highly injurious to the 

 animals which drink it. When the water reaches the ground it becomes 

 at once exposed to other sources of contamination. There is, first, the 

 presence of putrefying substances on the surface, animal and vegetable, 

 which leads to contamination with ammonia nitrates, nitrites, phosphates, 

 and other constituents of natural and artificial manures, and in passing 



