in the Ticenty Years before 1895 65 



dissolved contents can, for almost all purposes, be held to 

 be constant, and that, therefore, a water is generally charac- 

 terised by the amount of its dissolved contents, by its salinity. 

 This salinity, within the limits met with in the ocean, varies 

 directly with the density. The density can be determined with 

 great accuracy even at sea by means of a suitable hydrometer. 

 It has been found, also, that the preponderance of chlorides 

 over other salts in sea-water is such that the salinity of a sea- 

 water varies approximately as the amount of chlorine which 

 it contains. I myself always use the hydrometer, with which 

 I determine directly the density to one or two units in the fifth 



:nal place, as against distilled water of the same temperature 

 determined at the same time and with the same instrument. 

 The chlorine method is quite unsuitable for use at sea; first, 

 because the quantity of chlorine is so large that the amount 

 of water convenient for analysis is very small, and it cannot 

 be weighed at sea. Then at sea nothing is free from chlorine 

 the air and everything is impregnated with chlorides; so 

 that, as a means of specifying and di>tinguishing oceanic 

 I consider the chemical method absolutely untrust- 



iiy, except when made with all refinements in a laboratory 

 on land. There is, of course, no comparison in the amount 

 of time required compared with the hydrometer method. 

 Many writers, in passing judgment on the hydrometer 



i inst; ument for the determination of the density of liquids, 

 have only in their minds the hydrometer whose indications 

 are determined by comparison with another or standard 



ument; or by immersion in solutions, the densities of 



h have been otherwise ascertained. These instruments 

 no greater value than that of more or less carefully 



tructed copies of a standard, the method and the principle 

 "t the construction of whi< h U not always given, kijitly, 



tore, they prefer the density as d-tn mined by weighing 



--el tilled with the liquid .nid romp. Hint,' it with tl 



\vater of the same teinpri,ituie ullm- the -ame 



hydrometer which I ron>tnietrd ti the "Chalta 



id u- d during the \\lmlr ,f it. i- ii"t an hvdro- 



B. III. 5 



