ON ANIMAL HEAT. 
613 
" During the past week I was induced to resume the subject 
afresh ; and, upon making an experiment with the aorta of a 
bullock, I felt much gratification in being able to verify my pre- 
vious conjecture. The experiment was performed in the following 
manner : — Having cut off a circular portion of the descending 
arch of the aorta, about an inch in length, I laid it open and 
carefully dissected out the elastic coat, and, taking hold of it by 
each extremity, I pulled it to and fro with a continuous jerking 
motion (in imitation of the systole and diastole of the artery) for 
the space of about a minute, when, placing it upon the bulb of a 
thermometer, I had the satisfaction to find that, after it had re- 
mained two minutes, the mercury had risen as many degrees. 
On removing the thermometer, its heat immediately began to 
diminish . To be certain that the heat did not arise from any other 
source than the one in question, I took the precaution of covering 
my fingers with a double layer of flannel, to prevent the commu- 
nication of heat from the body. I also covered my mouth with 
a handkerchief, to guard against the warm breath affecting the 
thermometer whilst watching the progress of the experiment. I 
may likewise state that the experiment was performed in a room 
without fire, the temperature of the air at the time being 55°. 
“ There were several difficulties to contend with during the in- 
vestigation, and it was not until after repeated trials that the ex- 
periment succeeded to my satisfaction. The chief impediment, 
I think, must have been owing to the moisture of the artery, 
which, by its evaporation, must have had a constant tendency to 
carry off the heat. Having, however, performed the experiment 
twice consecutively in the same satisfactory manner, I think 
there can be but little doubt entertained as to its conclusiveness. 
“ My attention was often arrested, whilst conducting the expe- 
riments, by the striking mechanical analogies between caoutchouc 
and the elastic coat of arteries. Like the former, the latter could 
be elongated to twice its ordinary length, and, on withdrawing the 
tension, would return to its usual dimension with considerable 
force and a snapping noise. I was also surprised to find, on 
slightly drying it, that it would erase black-lead pencil marks 
from paper without leaving a stain. This latter circumstance is, 
perhaps, of trifling importance : it serves, however, to shew that 
strong mechanical resemblance may exist between bodies widely 
differing in their chemical properties. 
“ From the foregoing observations, I think I am entitled to 
conclude that the whole of the heat developed in the animal eco- 
nomy can now be satisfactorily explained. Physiologists have 
often proved that the greater part of animal heat is occasioned 
by the chemical changes which take place during respiration; 
