17 



which is boiling in the flask upon the left contains sand and clay in 

 suspension, sulphate and carbonate of lime in solution, as well as salts 

 of ammonia and common salt. We shall look in vain in this distillate 

 (the condensed steam) for any traces of these, and although we may 

 find traces of carbonic acid and ammonia, since these readily volatile 

 substances may come over with the first portions of the water vapour, 

 yet if we reject the first portion of distilled water, we shall find the 

 remainder to be absolutely pure, since the salts mentioned above are 

 not converted into vapour at the temperature at which water boils, and 

 they therefore remain behind in the flask. Even the ammonia might 

 have been prevented from coming over had we taken proper precau- 

 tions in treating the water before applying heat. It will, however, be 

 evident that distillation is too expensive a method to be practically 

 available on the large scale for water purification ; and it is only in such 

 cases as on shipboard that water for drinking purposes is obtained in 

 this way. A process quite analogous to this is, nevertheless, carried on 

 by natural agencies on the large scale. The formation of clouds, and 

 the precipitation of their watery burden, as rain, snow, etc., is but a 

 vast distilling of the surface waters of the earth ; and were it not for 

 the impurities washed out of the air by it, rain water would be quite as 

 pure as the distilled water flowing from this condenser. Indeed, were 

 proper pains taken to reject from cisterns the first portions of each 

 shower, as containing the bulk of the impurities of the air, and the dust 

 and dirt from the roofs on which it falls, rain water might be collected 

 and stored so as to form a perfectly wholesome and even palatable 

 drinking water, since it is well aerated, and the insipidity due to absence 

 of dissolved ?olids is less and less noticed as people become habituated 

 to its use. I have figured in this diagram two original devices, by 

 means of either of which a definite portion of each rain-fall may be 

 automatically prevented from entering the cistern, and only the later 

 portions of the shower allowed to flow into it ; and I think that every 

 cistern should be provided with a contrivance fitted to effect this 

 separation of the earlier from the later portion of each rain-fall. 



For purposes of brevity I shall omit any mention of sea water, or 

 lake water; and devote the remainder of the evening to some remarks 

 upon river and well waters; and in order to make it possible to define 



