
DYNAMICAL THEORY OF HEAT. 267 
in form, either is a consequence of the other. The reasoning in each demonstra- 
tion is strictly analogous to that which Carnor originally gave. 
15. A complete theory of the motive power of heat would consist of the ap- 
plication of the two propositions demonstrated above, to every possible method of © 
producing mechanical effect from thermal agency.* As yet this has not been 
done for the electrical method, as far as regards the criterion of a perfect engine, 
implied in the second proposition, and probably cannot be done without certain 
limitations ; but the application of the first proposition has been very thoroughly 
investigated, and verified experimentally, by Mr Jouxe, in his researches “ On 
the Calorific Effects of Magneto-Electricity ;” and on it is founded one of his ways 
of determining experimentally the mechanical equivalent of heat. Thus, from 
his discovery of the laws of generation of heat in the galvanic circuit,t it follows 
that, when mechanical work by means of a magneto-electric machine is the source 
of the galvanism, the heat generated in any given portion of the fixed part of the 
circuit is proportional to the whole work spent; and from his experimental 
demonstration that heat is developed in any moving part of the circuit at exactly 
the same rate as if it were at rest, and traversed by a current of the same strength, 
he is enabled to conclude— 
(1.) That heat may be created by working a magneto-electric machine. 
(2.) That if the current excited be not allowed to produce any other than 
thermal effects, the total quantity of heat produced is, in all circumstances, 
exactly proportional to the quantity of work spent. 
16. Again, the admirable discovery of PeLtiEr, that cold is produced by an 
electrical current passing from bismuth to antimony, is referred to by JouLE, as 
shewing how it may be proved that, when an electrical current is continuously 
produced from a purely thermal source, the quantities of heat evolved electrically 
in the different homogeneous parts of the circuit are only compensations for a loss 
from the junctions of the different metals, or that, when the effect of the current 
is entirely thermal, there must be just as much heat emitted from the parts not 
affected by the source as is taken in from the source. 
17. Lastly,{ when a current produced by thermal agency is made to work an 
* «here are [at present known] two, and only two, distinct ways in which mechanical effect 
can be obtained from heat. One of these is by the alterations of volume which bodies experience 
through the action of heat, the other is through the medium of electric agency.” —Account of Car- 
nor’s Theory, § 4. (Transactions, Vol. XVI, Part V.) . . - . A paper by Mr Joutz, 
containing demonstrations of these laws, and of others on the relations of the chemical and thermal 
agencies concerned, was communicated to the Royal Society on the 17th December 1840, but was not 
published in the Transactions. (See abstract containing a statement of the laws quoted ahage: in the 
Philosophical Magazine, vol. xviil., p. 308). It was published in the Philosophical Magazine in 
October 1841 (vol. xix., p. 260). 
{ That, in a given fixed part of the circuit, the heat evolved in a given time is proportional to 
the square of the strength of the current, and for different fixed parts, with the same strength of 
current, the quantities of heat evolved in equal times are as the resistances. 
t This reasoning was suggested to me by the following passage contained in a letter which I 
