40 
2. That their distances may he changed by dilatation or 
contraction, work being used up in accomplishing this 
change, just as when weights are raised against gravity, i.e., 
when their distance from the centre of the earth is altered. 
3. That any outside forces acting on the body — e.g., pres- 
sures in the case of a gas — might have their points of 
application altered. 
The two first are internal effects — the last external. 
Mayer and Joule both of them applied the principle of 
the transference of heat into work to air compressed by 
work. With this substance Joule showed experimentally 
that the first two effects might be disregarded altogether in 
comparison with the third. Mayer assumed this without 
any proof, or apparently without an idea that he was 
making any assumption. The argument of Mayer was there- 
fore only an ingenious speculation — Joule’s was a complete 
proof. 
Joule, however, went much farther than Mayer. He 
showed by frictional and electromagnetic experiments of the 
most varied kind that the supposed equivalence was an 
absolute law of nature, and he found means in all these 
cases to disengage the result from the complications to 
which we have referred, and to determine the definite 
amounts of work and heat which are equivalent to each 
other. He perceived and stated the principal applications 
of his discovery to the laws of chemical combination and 
vital action, and Professor Tait says — “In all the scientifi- 
cally legitimate steps which the early history of the principle 
records, Joule had the priority. The reader should clearly 
recognise the fact that the experimental foundation of the 
principle in its generality, and the earliest suggestions of 
many of its most important applications, belong unquestion- 
ably to Joule. Trained to accurate experiment and pro- 
found reflection in the school of Dalton, the pupil has not 
only immortalised himself, but has added to the fame of the 
master.” 
