M. F. Mohr on the Nature of Heat. 113 



fraction of the immediately preceding volume, and not of that 

 at 0° C, which temperature has no definite relation to gases. 

 Now, since this fraction must itself become absolutely larger 

 with the already too much increased motion, a greater force 

 must be required to heat a body from 90° to 100° C. than from 

 0° to 10°. This is the already generally observed increase of 

 capacity for heat at high temperatures. All bodies which ex- 

 pand according to this law must show increasing capacity for 

 heat. In the neighbourhood of the melting-point bodies ex- 

 pand rapidly and irregularly ; thus much heat becomes latent — 

 most of it, however, during the melting itself. In most cases, in 

 melting, heat is rendered latent in two ways : — by expansion and 

 by melting respectively ; in the case of water, which contracts 

 in melting, some heat is by this means given out ; or, more 

 correctly, the quantity of that rendered latent is diminished. 



If any species of gas is heated more strongly, it strives not 

 only to increase the number of its vibrations, but also to en- 

 large their amplitudes. If one prevents this expansion, it 

 appears as increased tension. One would require therefore a 

 smaller quantity of heat to warm a gas shut in by firm walls 

 than a gas contained in yielding walls, since, if heat be the 

 cause of the expansion, just as much heat must become latent 

 as there would be cold developed if the gas were allowed to 

 expand as much as before but without the supply of heat. An 

 imprisoned gas, therefore, cannot show the phenomenon of 

 increased capacity for heat. Were we able so to compress 

 solids and liquids that they could not expand by heat, even 

 these would show no increase of capacity for heat ; and one 

 could thus save an amount of heat which would, according to 

 the previous calculation, be equal to the compressing force. 

 On this account also the notion of changing, by pressure, 

 liquids into solids is by no means impossible, although we do 

 not as yet know what force would be necessary. The com- 

 pression of water in a piezometer is in approximation to this. 



The increased power of absorption of rough bodies for radiant 

 heat is to be explained by the easily produced vibration of pro- 

 jecting points ; while on polished closed planes the very cohe- 

 sion is opposed to the assumption of vibrations. It would 

 take us too far were we to attempt to reduce all heat-pheno- 

 mena to this view. Those we have given suffice to show how 

 they are all much more easily and intelligibly deduced from 

 the assumption of vibrations than from that of heat-substance, — 

 and that it is now time to give up, in science and in text-books, 

 the indefinite nomenclature of the earlier view. 



[About the time when Cokliug and Joule took up the experi- 

 mental investigation of Energy at the poiut where it had been left 



Phil. Mag. S. 5. Vol. 2. No. 9. Aug. 1876. I 



