494 PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 



other losses were taken into consideration, Joule was able to calculate 

 bow many work-units would have to be employed in the friction in order 

 to produce one heat-unit. 



Joule then modified this experiment in various ways by having the 

 friction produced in other liquids and under different circumstances. 

 He changed the size of the descending weight, and then the height to 

 be traversed ; the result was that in friction the same amount of work 

 is always employed in the production of a heat-unit, the mean being 

 425 kilogram-meters. This important figure is called the work equiva- 

 lent of the heat-unit, or the mechanical equivalent of heat.* 



If 425 kilogram-meters are equal to one heat-unit, one kilogram-meter 

 (that is, the work-unit) will amount to -^- s of the heat-unit. This numeri- 

 cal expression of T i- of the heat-unit, which is the equivalent of the 

 work-unit, is called the heat equivalent of the work-unit. 



Joule has furthermore determined the mechanical equivalent of heat 

 which shows itself in the compression of air. Him did similar work in 

 relation to the heat produced by the collision of two bodies ; other in- 

 vestigators at a later period ascertained the equivalent of other sources 

 of heat. The results of all these experiments are in such accord that 

 we may consider the above value of the mechanical equivalent of heat 

 a settled question. 



We have arrived at the knowledge that heat is a state of motion of 

 the ultimate molecules. The system adopting this proposition is called 

 the mechanical theory of heat (thermo-dynamics, thermo-mechanics). 



Various opinions may be held in regard to the nature of this state 

 of motion of molecules called heat: indeed, up to the present time, dif- 

 ferent views have been expressed in regard to this question. For the 

 mathematical basis and treatment of the mechanical theory of heat the 

 verification of the fact that heat is a state of motion is sufficient: while 

 for the purpose of explaining and calculating the relation of gases in 

 regard to heat, as well as to render the conception and comprehension 

 of the corresponding mathematical deductions of the mechanical theory 

 of heat, or thermo-dynamics, generally more easy, the leaders in this 

 branch of science have attempted to set up a certain hypothesis relative 

 to the inner structure of substances, as also to the manner of motion of 

 their ultimate molecules. 



Let us first turn our attention to the conception of the inner structure 

 of matter. Even Leucippus, as early as 510 years B. 0., and after him 

 Democritus (470 P>. C.) were of opinion that matter was composed of the 

 minute indivisible particles which they called atoms (an indivisible thing). 

 Their atomic theory was only developed further when physics com- 

 menced to be subjected to mathematical treatment, as material points or 



"[Joule's unit of the mechanical equivalent of heat is more popularly expressed 

 in Great Britain and in this country in foot-pownda: being 772 pounds lifted one foot,— 

 tho equivalent of one pound of water heated one degree Fahrenheit. — Ed.] 



