238 Mr. J. J. Woodward on the Modern 
organic compounds that supply the animal world with food. 
Nevertheless, while I yield my hearty assent to this generali- 
zation, and freely admit that it is more than a mere deduction 
from the general doctrine of the conservation of energy—that, 
in fact, it affords the most satisfactory explanation yet suggested 
for a large number of observed phenomena—it is my duty to 
caution you against the erroneous supposition that any one 
has ever yet succeeded in affording a rigorous demonstration 
of the truth of the generalization by an adequate series of 
actual experiments. 
Various attempts have indeed been made of late years to 
determine experimentally both for animals and for man the 
potential energy contained in the food of a given period, and 
the actual energy liberated during the same time in the form 
of heat and work. I think, however, that all practical physi- 
ologists who have looked into the question will agree with me 
that the numerical results hitherto obtained must be received 
with the utmost caution *. Difficulties exist on both sides of 
the problem. It is comparatively easy no doubt to obtain a 
close approximation to the quantity and composition of the 
food; but to represent numerically what becomes of it in the 
body, to deduct correctly what passes through unchanged, 
and ascertain with reasonable accuracy the amount of carbon 
dioxide, water, and urea into which the rest is transformed, 
these are questions which have taxed the utmost resources of 
investigators, and as to which our knowledge is yet in its 
infancy. 
On the other hand, the direct measurement of the resulting 
heat and work has hitherto proved still less satisfactory. It 
would seem to be a very simple thing to place an animal in 
a calorimeter and measure the heat-units evolved in a given 
time, as Lavoisier and Laplace attempted to do in the latter part 
of the last century ; and we have been told that “ Lavoisier’s 
guinea-pig placed in the calorimeter gave as accurate a return 
for the energy it had absorbed in its food as any thermic 
engine would have done” 7. But this assertion is not sup- 
ported by the results of actual experiment. We know now 
that many precautions, unknown to Lavoisier, must be taken 
to secure any approach to accuracy in calorimetric experiments 
with animals ; and just as the method is being brought to 
something like perfection, by arranging for the respiratory 
process and its influence on the results, and by other neces- 
* See, for example, M. Foster, ‘Text-book of Physiology’ (2nd edit. 
London, 1878, p. 355). 
+ Barker, op. cit. supra 
