Xlvi REPORT^— 1853. 



stored exactly to its primitive temperature and condition, — Carnot's second 

 fundamental proposition asserts that the quantity of heat which passes out of 

 the body into surrounding space, or into other bodies, in the form of heat, 

 during the second operation, is precisely the same as that which passed into 

 the body during the first operation. This view does not recognise the possi- 

 bility of heat being lost by conversion into something else, and in this parti- 

 cular is at variance with the new theory, which asserts that heat may be lost 

 by conversion into mechanical effect. To elucidate this distinction, suppose 

 a quantity of water to be poured into an empty vessel. It might then be 

 asserted, that, in emptying the vessel again, we must pour out just as much 

 water as we had previously poured in. This would be equivalent to Carnot's 

 proposition with respect to heat. But suppose a part of the water while in 

 the vessel to be converted into vapour ; then it would not be true that in 

 emptying the vessel, the same quantity of water, in the form of water, must 

 pass out of the vessel as had before passed into it, since a portion would have 

 passed out in the form of vapour. This is analogous to the assertion of the 

 new theory with regard to heat, which may be lost, according to that theory, 

 by conversion into mechanical effect, in a manner analogous to that in which 

 water may be said to be lost by conversion into vapour. But the new theory 

 not only asserts generally the convertibility of heat into mechanical effect, 

 and the converse, but also more definitely, that, whatever be the mode of 

 converting the one into the other — and whether heat be employed to pro- 

 duce mechanical effect, or mechanical force be employed to produce heat— 

 the same quantity of the one is always the equivalent of the same quantity 

 of the other. This proposition can only be established by experiment. Rum- 

 ford, who was one of the first to adopt the fundamental notion of this theory 

 as regards the nature of heat, made a rough attempt to determine the rela- 

 tion between the force producing friction and the heat generated by it ; but 

 it was reserved for Mr. Joule to lay the true foundation of this theory by a 

 series of experiments, which, in the philosophical discernment with which they 

 were conceived, and the ingenuity with which they were executed, have not 

 often, perhaps, been surpassed. In whatever way he employed mechanical 

 force to produce heat, he found, approximately, the same quantity of heat 

 produced by the same amount of force, the force btir.g estimated \n foot- 

 pounds according to the usual mode in practical mechanics, i. e. by the motive 

 power employed in raising a weight of 1 lb. through the space of 1 foot. 

 The conclusion adopted by Mr. Joule is that 1° Fahr. is equivalent to 772 

 foot-pounds. 



These results are unquestionably among the most curious and interesting 

 of those which experimental research has recently brought before us. When 

 first announced some ten or twelve years ago, they did not attract the atten- 

 tion which they deserved ; but more recently their importance has been fully 

 recognized by all those who cultivate the department of science to which they 

 belong. Of this Mr. Joule received last year one of the most gratifying 

 proofs, in the award made to him by the Council of the Royal Society, of one 

 of the medals placed annually at their disposal. It may not be known to 

 many of you that we have in Mr. Joule a pupil, a friend, and fellow-towns- 

 man of Dalton. 



This theory is in perfect harmony with the opinions now very generally 

 entertained respecting radiant heat. Formerly light and heat were regarded 

 as consisting of material particles, continually radiating from luminous and 

 heated bodies respectively; but it may now be considered as established 

 beyond controversy, that light is piopagated through space by the vibrations 



