338 THE POPULAR SCIENCE MONTHLY. 



specified substance, in order that its temperature may be raised by 

 one degree ; no discontinuous change of physical state occurring. A 

 part of this heat, it is thought, is used in raising the temperature of 

 the substance, and thus increasing the real heat or thermal contrast 

 of the body ; while the remainder is expended in producing, as it 

 were, some change in the potentiality of intermolecular distance, or 

 molecular motions, not indicated by the thermometer, but in general 

 attended by the expansion or contraction of the body heated. The 

 energy existing in this latter form, and measured in heat-units, has 

 been called by Clausius the ergonal content of the body. 



If we were, therefore, to suppose the following effects produced, 

 in a specified manner, during the reception of a quantity of heat by 

 any portion of an elastic fluid, namely, an increase of temperature, a 

 change in the mean distance or motions of the molecules not causing 

 any variation of temperature and a performance of external work by 

 the consequent increase of volume against exterior resistance, it is 

 evident that we could not consider any one of these effects to be the 

 dynamical equivalent of the whole acquisition of heat. Much criticism 

 upon the original reasoning of Mayer has therefore been called forth 

 by this fact, that, without proving the absence of the second effect 

 above mentioned, or in any way referring to the possibility of its dis- 

 turbing influence upon the calculation, he arbitrarily assumed that the 

 mechanical energy expended in compressing atmospheric air should 

 be regarded as the mechanical equivalent of the heat thus rendered 

 sensible. 1 



But though erroneous in principle, this method of determining the 

 mechanical equivalent of heat was afterward shown by Joule to in- 

 volve no sensible inaccuracy of result in the case of air and other per- 

 manent gases. 2 



The experiment by which this conclusion was attained consisted in 

 the repetition, with a slight but very important modification, of one 

 originally designed by Gay-Lussac to investigate the effect upon the 

 temperature of a gas of its free expansion into a vacuum. 



The apparatus consisted of two reservoirs, H and JEI, which 

 might be joined by connecting-tubes and a coupling-nut, and each 

 closed independently by a very perfect stopcock. Into one of these 



1 Besides, the analogy which he drew between the heat produced upon the sudden 

 stoppage of a falling body, constituting a diminution of the earth's bulk, and the forcible 

 compression of an elastic body, is by no means an admissible one, and in seeking to jus- 

 tify this view by the following statement : " Yet just as little as it may be inferred from 

 the relations of falling force to motion, that falling force is motion, so little is the conclu- 

 sion admissible in the case of heat " (that heat is motion). " We much prefer to adopt 

 the opposite conclusion, that in order to become heat, the motion either simple or vibra- 

 tory, as light, radiant heat, etc. must cease to exist as motion " he succeeded only in 

 rendering the subject more indefinite and confused. 



2 " On the Changes of Temperature produced by the Rarefaction and Condensation 

 of Air." (Philosophical Magazine, 1845, (3) xxxi., p. 376.) 



