324 
DR. MARSHALL HALL ON THE INVERSE RATIO 
young of certain birds which are hatched without feathers, and of certain 
animals which are born blind ; and in hybernation it is almost extinct. 
To ascertain the quantity of respiration in any given animal, with extreme 
minuteness, was a task of great difficulty. It was still more difficult to deter- 
mine this problem, so as to represent the quantities of respiration in the dif- 
ferent kinds, ages, and states of animals, in an accurate series of numbers. 
The changes induced in a given volume of air made the subject of experi- 
ment, by changes in the temperature and pressure of the atmosphere, and by 
variations in the height of the fluid of a pneumatic trough, which it is so diffi- 
cult to appreciate minutely ; the similar changes induced by the humidity of 
expired air, and by the heat of the animal itself, were so many and complicated, 
that it appeared almost impossible to arrive at a precise result. These difficul- 
ties, in fine, were such as to lead one of the first chemists of the present day to 
give up some similar inquiries in despair. 
Fortunately I have been enabled to devise an apparatus which reduces this 
complex problem to the utmost degree of simplicity. I now beg the indulgence 
of the Society whilst I give a detailed description of its construction and mode 
of operation. 
This apparatus, which I shall designate the Pneumatometer, consists of a glass 
jar a b (Plate XI.) inverted in a mercurial trough c d, so grooved and excavated, 
as accurately to receive the lower rim of the jar and the lowest part of the tube 
efg, and also to admit of the animal which is made the subject of experiment, 
being withdrawn through the mercury. This jar communicates, by means of 
the bent tube efg h, with the gauge ij, which is inserted into a larger tube, 
k /, containing water. A free communication between the jar and the external 
air is effected and cut off, at any time, by introducing and withdrawing the 
little bent tube m n, placing the finger upon the extremity m, whilst the extre- 
mity n is passed through the mercury. 
If the jar be of the capacity of one hundred cubic inches, the gauge is to 
contain ten, and to be graduated into cubic inches and tenths of a cubic 
inch ; so that each smallest division shall be the thousandth part of the 
whole contents of the jar. 
Attached to the same mercurial trough is placed a little apparatus, op, 
termed an Aerometer, and consisting of a glass ball o, of the capacity of ten 
