32 



IRRITABILITY. 



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 determine this problem, so as 

 to represent the quantities of respiration in the 

 different 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 experiment, by changes in the tem- 

 perature and pressure of the atmosphere, and 

 by variations in the height of the fluid of a 

 pneumatic trough, which it is so difficult to 

 appreciate minutely; the similar changes in- 

 duced by the humidity of expired air, and by 

 the heat of the animal itself, were so many 

 and complicated, that it appeared almost im- 

 possible to arrive at a precise result. These 

 difficulties, 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 pro- 

 blem to the utmost degree of simplicity. I 

 now beg the indulgence of the reader whilst 

 I give a detailed description of its construction 

 and mode of operation. 



This apparatus, which I shall designate the 

 pnewnatumeter, consists of a glass jar (fig. 1, 

 , b,) inverted in a mercurial trough Cc, d,) so 

 grooved and excavated, as accurately to receive 

 the lower rim of the jar and the lowest part of 

 the tube (e,f, g,) and also to admit of the ani- 

 mal which is made the subject of experiment, 

 being withdrawn through the mercury. This 

 jar communicates, by means of the bent tube 

 (e,J\ g, h,) with the gauge (i,j,) which is in- 

 serted into a larger tube (/c, /,) 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 (in, ,) placing the finger upon the ex- 

 tremity (in,) whilst the extremity () is passed 

 through the mercury. 



If the jar be of the capacity of one hun- 

 dred 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 (o, p,) termed an 

 aerometer, and consisting of a glass ball (o,) 

 of the capacity of ten cubic inches, commu- 

 nicating with a tube (p, <?,) bent at its upper 

 part, of the capacity of one cubic inch, di- 



Fig. 1. 



