TAKING THE DENSITY OF VAPOUR OF VOLATILE LIQUIDS. 453 
supposing, of course, that the washing of the precipitate were continued as far 
as possible. 
The other error is absolute, not bearing any relation to the quantity of sub- 
stance operated upon. It may be illustrated by the error in weighing. Suppose 
the balance used in an analysis of a mineral were incapable of indicating differ- 
ences of less than a grain: when 100 grains were taken for analysis, the error 
of weighing would amount to 1 per cent. If, however, the quantity employed 
were diminished to two grains, the error would rise to 50 per cent. 
It is manifest that the test for absolute error in a process is extreme diminution 
of the quantities taken for experiment. 
Some of the absolute errors which might be expected to exist in the process 
which has been described, are the following :—Loss or gain in the weight of the 
glass on sealing the bulbs. Variation in the state of cleanliness or hygrometric 
condition of the surface of the bulbs. This must be reckoned absolute, inasmuch 
as the capacity of a vessel increases far more rapidly than its sides. 
Two determinations of the vapour-density of water have been made upon very 
small quantities. As will be evident on inspection, they have given results as 
nearly approximating to the truth as the balance will admit. If the method 
were affected by any considerable absolute errors, outrageous results ought to be 
obtained in these extreme cases. 
If. 
Barometer, 749-0 m. m. 
Temperature of balance-case, 7:5° C. 
Weight of tube in dry air = 93°8168 grm. 
Temperature at sealing, ‘73° C. 
Weight of tube + H + vapour = 93°6690 grm. 
Tube + water (at 8°5° C,) = 121-66 grm. 
Tube filled with water = 223-58 grm. 
Water column, 100 m. m. 
.. Vapour-density of water = 0°671. 
II. 
Barometer, 749° m. m. 
Temperature of balance-case, 7°5° C. 
Weight of tube in dry air = 89-2922 grm. 
Temperature at sealing = 91:9° C. 
Weight of tube + H+ vapour = 891805 grm. 
Tube + water (at 6-2° C.) = 117-72 grm. 
Tube filled with water = 188-35 grm. 
Water column = 103 m. m. 
Barometer, 750°8. 
.". Vapour-density of water = 0°654. 
Theoretical density of water, _ x 0691 = 0:6219. 
VOL. XXII. PART III. OA 
