SPECIFIC HEAT AT LOW TEMPERATURES 71 



carbon, whose molecules very probably consist of several 

 atoms. 



On the other hand, the highest values so far found are 

 about twice three, and only exceed that amount by a little, 

 while if the work of expansion were taken into account 

 they would be smaller still. 



With these considerations may be taken Richarz's as- 

 sumption that in solids the atoms move about centres, and 

 that rise of temperature consists partly in an increase of 

 the kinetic energy of this motion, requiring, as in gases, 

 three calories per kilogram atom. To this is to be added 

 the increase of potential energy, which if the rotation about 

 a fixed point followed the Newtonian law of attraction, 

 would be equal to the increase in kinetic energy, making 

 in all six calories, in accordance with Dulong and Petit's 

 result. 



5. CAPILLARITY AND SURFACE TENSION. 

 A. Observations. 



The molecular attractions which are apparent even in 

 gases, and there produce a deviation from Boyle's law in 

 the sense of a reduction of the pressure caused by molecular 

 collisions (i. e. a retardation of the movement outwards) in 

 liquids, stop this movement outwards altogether. Whilst 

 gases tend to increase their volume, in liquids an opposite 

 tendency occurs, or an internal pressure. This is the cause 

 of the relatively great density of liquids, and of their 

 assumption of the smallest possible surface, which in the 

 absence of external forces is spherical. In many respects, 

 therefore, a liquid is comparable to a gas enclosed in an 

 elastic membrane, such as a soap bubble, only that the 

 elasticity, here depending on the membrane, is in the liquid 

 caused by internal attractions. The tension thus existing 

 in the surface layer of a liquid is called surface tension, 

 and shows itself in the first place as a force tending to 

 reduce the surface of the liquid. Any force, then, that 



