é CHEMICAL AND PHYSICAL NOTES. 109 
tents—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 sensibly as the amount of chlorine which it contains. 
The writer always uses the hydrometer, with which he can make sure of 
the density to one or two units in the fifth decimal place, relatively to 
that of distilled water at the same temperature determined at the same 
time and with the same instrument. The chlorine method is quite 
unsuitable for use at sea, because the quantity of chlorine is so large 
that the amount of water convenient for analysis is too small for 
exact measurement, 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 distinguish- 
ing oceanic waters, the writer considers the chemical method untrust- 
worthy, 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 hydrometric method. 
The most important factor is the weight of the hydrometer. The 
following particulars refer to the determination of the weight of the 
instrument, 1893, No. 16. 
The weight iz vacuo of the hydrometer was determined indepen- 
dently on three different days as follows :— 
November 25, 1893.—Barometer 736°98 mm.; tempcrature, dry bulb 10°-3 C., 
wet bulb 9°°0 C. 
Weight of hygdrometer in air : 182°5875 grms. 
Add weight of displaced air . . ‘ 071960, 
Weight in vacuo of hydrometer 182°7835 ,, 
December 9, 1893.—Barometer 730°35 mm.; temperature, dry bulb 6°°0C.,, 
wet bulb 4°°6 C. 
Weight of hydrometer in air . .  . 182°5850 grms. 
Add weight of displaced air . . . 0:1975 i, 
Weight in vacuo of liydrometer » 182°7825 
December 16, 1893.—Barometer 754°64 mm.; temperature, dry bulb 11°:0 C., 
wet bulb 9°°9 ©. 
Weight of hydrometer in air de 8 . . 182+5828 grms. 
Add weight of displaced air ie Go 0°2002 ~—C«,, 
Weight in vacuo of hydrometer_ . 182°7830  ,, 
Taking the mean of these three independent and concordant 
weighings we obtain for the 
Weight in vacuo of hydrometer . . . 182-7830 grms. 
