1870.] BARKER — ON VITAL AND PHYSICIAL FORCES. 421 



by exactly the amount of work done. The motion of the piston's 

 mass is precisely that lost by the steam molecules which batter 

 against it. The conversion of heat into electricity, too, is also 

 easily effected. When the junction of two metals is heated, 

 electricity is developed. If the two metals be bismuth and 

 antimony, as represented in this diagram, the currents flow as 

 indicated by the arrows; and by multiplying the number of pairs, 

 the effect may be proportionately increased. Such an arrangement, 

 called a thermo-electric battery, we have here ; and by it the heat 

 of a single gas-burner may be made to move, when converted, this 

 little electric bell-engine. Moreover, heat and light have the very 

 closest analogy ; exalt the rapitity 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 electri- 

 city. 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 

 machines 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, the 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 ai^pearance simultaneously. While, for 

 example, the eonversion 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 production 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 easilv answered in all cases. The best 

 known of these relations is that between motion and heat, which 

 was first established by Mr. Joule in 18-19, after seven years of 

 patient investigation.*" The apparatus which he used is shown in 

 the diagram. It consists of a cylindrical box of metal, through the 



