GAS APPARATUS. 
315 
with the water in the cistern ; and for this reason, the che 
mist must refrain as much as possible from keeping the 
hands upon either ; or, if it is suspected that they have been 
heated, he must cool them again by means of the water in 
the cistern. The height of the barometer and thermometer 
during the experiment is of no consequence. When the 
marks have been thus ascertained upon the jar, for every 
ten cubical inches, a scale is made upon one of its sides, by 
means of a diamond pencil. 
Glass canes are graduated in the same manner for using 
in the quicksilver trough, only they must be divided into 
cubic inches. The bottle for gauging these should hold 
one cubic inch, and, consequently, the gauge-bottle must 
hold precisely 252 grains 58, or ten pennyweights, a half 
pennyweight, and half a grain of distilled water ; or the 
specific gravity of the quicksilver at hand is to be taken, the 
weight of a cubic inch calculated, and used to fill the gauge. 
Another mode of determining the volume of elastic aeri 
form fluids, consists in transferring the unknown volume of 
air into a narrow cylindrical vessel, standing on the shelf of 
the pneumatic trough, and then marking, by means of a slip 
of paper pasted on the jar, the exact height of the elastic 
fluid. This being done, the jar is turned up and filled with 
water, exactly up to the mark ; and by weighing the water, 
the volume of the gas may be found, either in ounce and 
grain measures, or by reduction, into cubic inches. This 
method, like that of graduating jars by pouring in succes 
sive portions of water, or quicksilver, requires a correction 
for the difference of level between the liquid in the jar and 
that in the trough. 
In measuring gases with these graduated jars, two cor 
rections are necessary, the one for variation in their bulk, by 
the increase or diminution of the pressure of the atmosphere, 
as marked by the barometer, and the difference of level be 
tween the surfaces of the liquid that is used to confine them; 
the other for the temperature at which they are measured. 
It might certainly be beneficial to science, that one uni 
form pressure and temperature should be adopted, at least, 
in each country, but this cannot be hoped for. 
The barometer is to be observed, and if the jar was not 
graduated in the first of the methods here mentioned, the 
difference of the levels in the quicksilver trough is to be 
deducted from the bárometric height, or the difference of 
the levels in the water-trough being taken, 3/ioths of an inch 
of baromctric pressure is to be deducted for every four inches