23G TliANSAClIOXS OF TEE AMERICAN INSTITUTE. 



over, lieat and light liave the very closest analogy ; exalt the rapiditj 

 -with which the molecules move and light appears, the difference 

 being only one of intensity. 



Again, if electricity be our starting point, we may accomplish its 

 conversion into the other forces. Heat results whenever its passage 

 is interrupted or resisted ; a wire of the poorly conducting metal 

 platinum becoming even red hot by the converted electricity. To 

 produce light, of course, we need only to intensify this action ; the 

 brightest artificial light known, results from a direct conversion of 

 electricity. 



Enough has now been said to establish our point. What is to be 

 particularly observed of these pieces of apparatus is that they are 

 macl lines especially designed for the conversion of some one force 

 into another. And w^e expect of them only that conversion. We 

 pass on to consider for a moment the quantitative relations of this 

 mutual convertibility. We notice, in the first place, that in all cases 

 save one, tlie conversion is not perfect, a part of the force used not 

 being utilized, on the one hand, and on the other, other forces making 

 their appearance simultaneously. While, for example, the conversion 

 of motion into heat is quite complete, the inverse conversion is not 

 at all so. And on the other hand, when motion is converted into 

 electricity, a part of it appears as heat. This simultaneous produc- 

 tion of many forces is well illustrated by our little bell engine, which 

 converts the electricity of the thermo-battery into magnetism, and 

 this into motion, a part of which expends itself as sound. For these 

 reasons the question " How much ?" is one not easily answered in all 

 cases. The best known of these relations is that between motion and 

 heat, which was first established by Mr. Joule in 18i9, after seven 

 years of patient investigation.* The apparatus which he used ii 

 shown in the diagram. It consists of a cylindrical box of metal, 

 through the cover of which passes a shaft, carrying npon its lower 

 end a set of paddles, immersed in water within the box, and upon its 

 upper portion a drum, on wliich are wound two cords, which, passing 

 in opposite directions, run over pulleys, and are attached to known 

 weights. The temperature of the water within the box being care- 

 fully noted, the weiglits are then allowed to fall a certain number of 

 times, of course in their fall turning the paddles against the friction 

 of the liquid. At the close of the experiment the water is found to 

 be wanner than before. And by measuring the amount of this rise 



* Joulf, J. p., Philosophical TrunsuctiouH, I80O, p. 61. 



