452 DR LYON PLAYFAIR AND MR J. A. WANKLYN ON A MODE OF 
included on sealing the bulbs. We will answer some objections which might 
naturally be raised. 
It might be feared that the vapour would not be perfectly absorbed by water ; 
but when it is considered how much water there is, and how little vapour has to 
be acted upon by it, it will be evident, that only in those instances where the 
vapour is nearly insoluble in water, will there be any chance of appreciably 
incomplete absorption. Only in those instances where the substance is at once 
insoluble in water and possesses a high tension at ordinary temperatures, could 
error arising from this source become serious. 
With regard to the absorption of hydrogen by water, it may be remarked, 
that since between 8° and 11°C. the absorption-coefficents of hydrogen and com- 
mon air, as determined by BuNSEN, are nearly identical, the measurement of 
hydrogen over water is so far unobjectionable. 
As an example of the method, we subjoin the particulars of a vapour-density 
determination of absolute alcohol, made at 30° C. below its boiling point :— 
Barometer (corrected to 0° ©.) = 763-094 m. m. 
Temperature of the balance-case, 7°5° C. 
Weight of apparatus in dry air, = 69-959 grm. 
Temperature at the time of sealing, 48° C. 
Weight of the apparatus + H + vapour = 69-5275. 
Weight of apparatus + water (at 5:2° C.) = 191-76 grm. 
water column, 122 m.m. 
Weight of apparatus filled with water = 545°36 grm. 
Volumes corrected 
at 0° C.and 760 m.m. pressure, 


Cubic centimetres, grm. 
Hydrogen - vapour, 406-43 weighing 0°1695 
Hydrogen, 341:27 4 0:0306 
65°16 0°1389 
Therefore, 65°16 cub. c. of aleohol vapour weigh -1389 grm, 
But 65°16 cub. ¢. of air weigh -0843 grm. 
F 1389 
Vapour density of alcohol = ———~ = 1-648. 
apour density of alcoho 0843 1-648 
In all quantitative operations there are two kinds or orders of inaccuracy. 
The one is relative, bearing a tolerably constant proportion to the quantities 
operated upon: thus, for instance, in a precipitation, the precipitate is liable 
to retain a certain very small proportion of the soluble matter. The error 
in percentage on an analysis in such a case would be neither increased nor dimi- 
nished by varying the quantities of substance taken for the analysis—always 

