PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 247 



vapor. Should the experiment rccpiire too much, time, the tube may be at first 

 filled ^vith warm water. 



Sir. II. Davy succeeded in meltin<^t\vo pieces of ice l)y rubbing- them together in 

 a space exhajisted of nir and cooled ]>eh)W the freezing point, while Mayer first 

 showed (1842) that heat is developed by the friction of water against solid bodies, 

 having, by simple agitation, rais'.'d its temperature from 12° C. to 13° C. {Annal. 

 der Chem. und Pliarm., May, 1842.) 



II. — TUE NATUKE OF HEAT. 



As regards the explanation of the phenomena of heat, two contrary Inqootheses 

 have stood, from an early period, in opposition to one another. According to the 

 one, these phenomena ])roceed from an imponderable element, which, filling up 

 the intervals between the separate atoms of matter, operates as a repulsive prin- 

 ciple. Through an augmentation of the particles of heat in a body, its tempera- 

 ture is raised, its constituent atoms still fm-ther separated from one another, and 

 thus its volume increased, while cohesion becomes more and more enfeebled and 

 the conditions of aggregation are changed; solid bodies melting, and fluids 

 passing into vapor. This mode of explanation has, till the most recent times, 

 formed the basis of the doctrine of heat as presented in most popular works ou 

 the subject, without any positive assertion however, as to the correctness of such a 

 theory. It was employed, in the interim, for want of a better, in order more easily 

 to combine the various phenomena of heat under a common point of view. 



The hypothesis of which we speak, namely, that the phenomena of heat result 

 from the quiescent presence of an imponderable calorific element, and which, on 

 that account, we will call briefly the 'material theorij, stands opposed to another, 

 according to which heat is the result of a vibratory motion of the minutest par- 

 ticles of bodies, and which thus refers the explanation of the phenomena to 

 mechanical principles; on this account we shall designate the latter in our further 

 discussion of those principles as the mechanical tlieonj of heat. It was h^ng ago 

 said by Locke that '' heat is a most active concussion of the imperceptibly small 

 particles of a bod\', whi(;h produces in us the feeling we term warmth ; the cause 

 of our perception of heat is, in reality, therefore only a motion." There is nothing, 

 in fact, which argues more conclusively in favor of the mechanical explanation 

 of the jdienoniena of heat than its production through mechanical forces, as 

 exhibited in preceding paragrajdis. Certainly, neither the experiment of Rumford 

 nor that (d'lJavy gives the smallest countenance to the conduction of caloritic 

 matter from without. 



The adherents of the material theory sought to explain the development of 

 hesit by the agency of compression, on the assumption that the capacity of bodies 

 for heat decreases with their density, whence a body, when its density is increased, 

 must give out heat. Tlie difference between the specific heat of gases under 

 constant pressure and with constant volume seemed to argue in favor of this 

 hypothesis till Ilegnault had proved that the specific heat of a given weight of gas 

 is independent of its densitij. "With regard to the development of heat by friction, 

 the material theory endeavored to account for it by assuming that friction is always 

 attended by a corresponding compression, and by the diminution of specific heat 

 thereby occasioned. But more exact investigation showed that the specific; heat 

 of the shavings, which fall away from the cannon in boring, ditlercd not sensibly 

 from that of the metal before th(! boring ; while in the exiteriment of Davy a body 

 is formed, namely, water, whose specific heat is not only not smaller than that of 

 ice, but is actually twice as great. Here, then, the development of heat in 

 nowise admits of being referred to a diminution of the specific heat. 



Ituinlbrd, as well as Davy, had instituted their experiments with a view to 

 prove the necessity of having recoiu-se to a mechanical explanation of heat. But 

 though they had certainly indicated the right course for answering the question 

 as to the natm-e of heat, that course was, for the time at least, not followed up; 



