SG 



A. Pedler— 



On the ijast and present 



[No. 2, 



The primary form of natural water is rain, and although at first sight 

 it might appear that rain water should be very pure, yet it has been clearly 

 shown* that it is very seldom that such is the case, and that rain 

 water almost always contains, as impurities, small quantities o£ organic 

 matter, ammonia, and ammonium salts, derived from the atmosphere. 

 In large towns especially, the rain water is so impure, that it cannot be con- 

 sidered a safe water supply for drinking and other domestic purposes. On 

 reaching the ground the water becomes charged to a greater or less extent 

 with the various soluble constituents of the soil, and with any other 

 matters which may have accumulated in it. If it falls on land either 

 cultivated or imcultivated, it rapidly drains off, and finds its way into 

 streams and rivers, which in the earlier parts of their course certainly, will 

 be tolerably fi'ee fi'om organic impurity, except that derived from any 

 manure, etc. which may have been on the land. Unless the river water 

 is subsequently rendered impure by the admission of sewage from towns, 

 villages, etc., or by the admission of manufacturing refuse, it will form, 

 generally speaking, a comparatively pure and wholesome supply of water. 

 In some cases, however, such water is used by the inhabitants of towns on its 

 banks, and is after use returned to the river in the form of sewage, which 

 will be charged with impurity derived from animal excreta, household and 

 manufacturing refuse, soap, and other filth. Water contaminated in such 

 a way is clearly unfit for domestic use. After returning to the stream it will 

 perhaps in its course towards the sea become partially purified by slow oxida- 

 tion of the organic matter and by the absorbent action of vegetation, but 

 as will be subsequently shown this process of purification is an extremely 

 slow one. 



In the case of rain water falling in towns such as Calcutta, it will, as 

 pointed out previously, be impure from the presence of organic matter, 

 ammonia, etc. ; of this impure water a considerable proportion of it as before 

 shewn will find its way into the river or into smaller streams communicating 

 with it, but another portion will be collected in the tanks, which are dug for 

 this purpose, and a third portion after percolating through the soil will find 

 its way into numerous shallow wells. These tanks and shallow wells may 

 therefore be considered as being merely pits for the accumulation of drainage 

 from the immediately surrounding soil. In the case of Calcutta the town 

 is densely populated, and as the manners and customs of the native inhabi- 

 tants are in many respects very primitive, the soil must be inevitably 

 charged with excretal and other refuse, so that the water when it reaches 

 the tank or well, will be largely contaminated with the impurities deri- 

 ved from these sources. In the absence of any system of drainage, as was 

 the case in Calcutta some years ago, such tank or well water could only 

 * Angus Smith on Aii- and Eain. 



