PIONEER INVESTIGATIONS 11 
physiologists is found in a paper by Helmholtz, who, independently of Mayer, 
announced the same fundamental principle of the conservation of energy. 
In this paper Helmholtz saj's: 
The majority of the physiologists in the last century and in the beginning of this cen- 
tury were of the opinion that the processes in the living bodies were determined by one 
principal agent which they chose to call the "vital principle." The physical forces in the 
living body they supposed could be suspended or again set free at any moment by the in- 
fluence of the vital principle, and that by this means this agent could produce changes in 
the interior of the body so that the health of the body could be thereby preserved or restored. 
The present generation, on the contrary, is hard at work to find out the real causes 
of the processes which go on in the living body. They do not suppose that there is any 
other difference between the chemical and the mechanical actions in the living body and 
out of it than can be explained by the more complicated circumstances and conditions under 
which these actions take place, and we have seen that the law of conservation of force 
legitimizes this supposition. This law, moreover, shows the way in which this fundamental 
question, which has excited so many theoretical speculations, can be really and completely 
solved by experiment. 
Although the calorimetric experiments of Despretz and Dulong, espe- 
cially after their interpretation by Liebig, aided greatly in verifying experi- 
mentally the law of the conservation of energy in the animal body, it was not 
until the classical experiments of Rubner b in 1893 on dogs, in which the income 
and outgo were accurately measured, that the law of the conservation of 
energy in the animal body was demonstrated. Subsequently, Atwater and 
his associates, working with the respiration calorimeter at Wesleyan Univer- 
sity, Middletown, Connecticut, amplified the results obtained by Rubner, 
and by means of complete balance experiments, demonstrated this uniformity 
in the income and outgo of energy in the human body, thus proving the law 
of the conservation of energy. Simultaneously Laulanie in Toulouse made a 
series of observations with animals in an animal calorimeter which also further 
substantiated this important deduction. 
Shortly after the announcement of the law of the conservation of energy 
by Mayer, an English engineer, Joule, computed the mechanical efficiency of 
a horse by comparing the amount of work performed with the energy in the 
food eaten. The following paragraph is from a paper by Scoresby and Joule : c 
A horse, when its power is advantageously applied, is able to raise a weight of 24,000,000 
pounds to the height of one foot per day. In the same time (24 hours) he will consume 12 
pounds of hay and 12 pounds of corn. He is therefore able to raise 143 pounds by the 
consumption of one grain of the mixed food. From our own experiments on the combus- 
tion of a mixture of hay and corn in oxygen gas, we find that each grain of food, consist- 
ing of equal parts of undried hay and corn, is able to give 0°.682 to a pound of water, a 
quantity of heat equivalent to the raising of a weight of 557 pounds to the height of a foot. 
Whence it appears that one-quarter of the whole amount of vis viva generated by the com- 
bustion of food in the animal frame is capable of being applied in producing a useful me- 
chanical effect, — the remaining three-quarters being required in order to keep up the 
animal heat, etc. If these theoretic views be correct, they would lead to the interesting 
conclusion (which is the same as that announced by Matteucci from other considerations) 
that the animal frame, though destined to fulfill so many other ends, is, as an engine, more 
perfect in the economy of vis viva than the best of human contrivances. 
As the problem of the mechanical efficiency of the body came more and 
more to the front, many estimates with regard to the mechanical output of a 
a Helmholtz, Proc. of the Roval Institution, 1861, 3, p. 35G. 
6 Rubner, Zeitschr. f. Biol., 1894, 30, p. 73. 
* Scoresby and Joule, Philosophical Magazine, 1846, 28, p. 454. 
