130 HELMHOLTZ ON THE CONSERVATION OF FORCE. 



considerable in quantity and intensity only in the actions on the 

 green portions of plants. 



IV. The Force-equivalent of Heat, 



The mechanical processes in which an absolute loss of force 

 has been heretofore assumed, are — 



1. The collision ofunelastic bodies. — The loss is mostly con- 

 nected with the change of form and the compression of the body 

 struck ; hence with the increase of the tensions, we also find, by 

 the repetition of such shocks, a considerable development of 

 heat ; for example, by hammering a piece of metal ; finally, a 

 portion of the motion will be communicated as sound to the 

 contiguous solid and gaseous bodies. 



2. Friction, both at the surface of two bodies which move 

 over each other, and also that arising in the interior from the 

 displacement of the particles. In the case of friction certain 

 small changes in the molecular constitution of the bodies take 

 place, especially when they commence to rub against each other ; 

 afterwards the surfaces generally accommodate themselves to each 

 other, so that this change becomes a vanishing quantity in the 

 further course of the motion. In many cases such changes do 

 not at all appear, for example, when fluids rub against solid bo- 

 dies or against each other. Besides those already mentioned, 

 thermic and electric changes always take place. 



It is customary in mechanics to represent friction as a force 

 which acts against the existing motion, and the intensity of 

 which is a function of the velocity. This mode of representing 

 the subject is only made use of for the sake of calculation, and 

 is evidently an extremely incomplete expression of the compli- 

 cated process of action and reaction of the molecular forces. 

 From this customary manner of regarding the subject, it was 

 inferred that by friction vis viva was absolutely lost ; and the 

 same was assumed in the case of unelastic collision. It is not 

 however here taken into account that, disregarding the increase 

 of the tensions caused by the compression of the body rubbed or 

 struck, the heat developed is also the representant of a force by 

 which we can develope mechanical actions ; the electricity deve- 

 loped, whose attractions and repulsions are direct mechanical 

 actions, and the heat it excites, an indirect one, has also been 



