CHEMICAL SCIENCE. 257 



Icctcd at four different times, at as many widely distant places. These give 

 in 100 parts 0'20-3, O'lOO, O'lG, and O'llj parts, of sulphate of lime. For the 

 sake of simplicity, take 0"2: then 1UOO gallons = 8000 Ibs. contain 1(5 Ibs. of 

 oypsum. The quantity of carbonate of baryta equivalent to this is 19 2 Ibs., 

 aiid of carbonate of lead 25 Ibs. Thus one ton (2000 Ibs.) of carbonate of 

 baryta, which some years since sold in England (in the form of ground with- 

 erite) for $20, is theoretically sufficient to separate the gypsum from 100,000 

 gallons of the water of the Atlantic. On referring now to the evaporative 

 power of different coals, as determined by Walter R. Johnson, for the 

 United States Navy Department (Taylor's Statistics of Coal, introduction, 

 p. Ivii; Knapp's Chem. Technology, Am. Ed. i. 43), we there find that 1 lb. 

 for instance, of Lackawauna anthracite (considered in New York superior 

 for steam purposes) produces 8] Ibs. of steam from cold water; so that to 

 evaporate 100,000 gallons would require *L> : 100 - OQO = 94,000 Ibs., or 47 tons. 



%2 



Forty per cent, of such a quantity is over 18 tons. 



I do not, however, by any means recommend the application of this prin- 

 ciple to ocean steamers, on account of practical difficulties, which I see no 

 way tp overcome. But on land the application is a matter of perfect sim- 

 plicity. It is only necessary to have the tanks from which the boiler is 

 supplied, larger, and to have more of them. While the process of mixture 

 and agitation with the carbonate, and precipitation by standing, is going on 

 in one tank, water may be used from another which has undergone the pro- 

 cess. The idea seems to me to be especially adapted to locomotives, for 

 which even sea-water may then be used. 



IMPURITIES IN DRINKING-WATERS. 



From a report by Edwin Lankester, F. R. S., on the drinking-waters of 

 London, published in the London Mechanics' Magazine, July 1828, we obtain 

 the following memoranda. 



Phosphates and silica exist in all London well-waters, in small quantities, 

 sulphate of lime, in the proportion of one to fifteen grains to the gallon. It 

 decomposes, in contact with organic matters, and produces sulphuretted 

 hydrogen. Very small quantities of organic matter serve to produce this 

 effect. 



Ammonia exists in Thames water in small quantities ; in much larger and 

 more appreciable quantities in the surface wells. This substance is the re- 

 sult of the decomposition of animal matter, and in the surface wells is 

 undoubtedly derived from human excretions. 



Nitrates were found in one well-water to the extent of fifty grains to the 

 gallon. 



The organic matters are not injurious when fresh or recent, but they assume 

 certain conditions of decomposition which occasionally render them deadly. 

 Their influence may be estimated by the case of the Lambeth and Yauxhall 

 Water Company's supply, during the years 1848 and 18-34, two years in 

 which cholera visited London. In 1818, both companies derived their supply 

 of water from the Thames at Battersea, and both supplied the same district 

 \vith water, and the houses supplied were equally visited with cholera. 



But in 18-34, the Lambeth Company obtained an improved supply high up 

 the Thames, at Ditton. The consequence was, according to Dr. Snow's cal- 

 culations, that the deaths amongst the population supplied by the Yauxhall 



22* 



