GAS APPARATUS. 
319 
trough is made to coincide, or the difference is measured 
and noted. The cock is then shut, and the globe being de 
tached is again weighed, the weight of the gas it contains 
divided by that of the air that was extracted from it, gives 
the specific gravity of the gas. 
The chemists are contented, for the most part, with 
making a gross allowance of one-third part of a grain for the 
weight of the moisture contained in every hundred cubic 
inches of the gas used in this experiment. When the specific 
gravity of the gas does not greatly differ from that of atmos 
pheric air, and they both stand over water, so as to be in the 
same state as to moisture, this correction for the weight of 
the moisture is neglected. 
The air-pump is sometimes useful in getting liquids to 
pass through filters. For this purpose, in a small way, it is 
sufficient to put a funnel in the opening at the top of a re 
ceiver, and lute the joining with soft wax ; the filter, sup 
ported by a strong cloth, is then placed in the funnel, and a 
few strokes of the pump being taken, the atmosphere will 
force the liquid through the filter. This method is used on 
the large scale to force liquids through numerous folds of 
flannel, or other filtering stuffs, the air-pump being worked 
by a steam engine. 
The air-pump has been lately used to extract part of the 
air out of distilling vessels, in order to prevent the liquor in 
them from attaining such a high temperature as it would if 
subjected to the whole pressure of the atmosphere. 
Condenser. 
The condenser is less employed than the air-pump ; it is 
not, however, without its uses, for the manufacturers have 
employed large apparatus of the same kind to force size 
into paper, and dye stuffs into cloths, with evident advantage 
The same machine is sometimes made to serve at pleasure, 
either as an exhausting or condensing apparatus. 
It is necessary that a laboratory condenser should have 
pipes and cocks to screw on to it, that it may not only con 
dense the air in the proper receiver that forms a part of it, 
but in any other vessel, as an Indian rubber bottle. 
Transferring Syringe. 
This is meant to serve as a substitute for the air-pump and 
condenser ; but is far inferior in power. It is, however, 
sometimes useful to transfer either gases’ or dense liquids 
from one vessel to another, or from a vessel to force them