25 0 Messrs. Allen and Pep vs on the Changes produced in 
and yet it is of the utmost consequence in all calculations 
upon the effects produced, especially upon small portions of 
gas, that the state of the lungs should be accurately deter- 
mined ; this constitutes the great difficulty in the investiga- 
tions. We therefore commenced our labours by constructing 
an apparatus, in which we are able to respire from three to 
four thousand cubic inches of gas, conceiving, that in this 
quantity, the error arising from the residual gas in the lungs 
must be so much obviated as to permit the most satisfactory 
results. 
The apparatus consists of three gasometers, two of which 
are filled with mercury, and one with distilled water. 
The water gasometer which belongs to the Royal Institu- 
tion, is capable of holding four thousand two hundred cubic 
inches of gas, and each of the mercurial ones three hundred 
cubic inches: the apparatus was so arranged that the inspi- 
rations were all made from the water gasometer, and the 
expirations into the mercurial gasometers alternately. Each 
of the gasometers is furnished with a graduated scale, and they 
are all made to range with each other, so that the quantity of 
gas inspired and expired could be immediately and exactly 
ascertained : to each of the mercurial gasometers a glass tube 
is fixed, and made to enter a mercurial bath, from which 
portions of the expired air could at any time be taken for 
examination. 
By the kindness of our friend Silvanus Bevan, we are 
enabled to give an accurate drawing of the apparatus. 
