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LXIV. — NOTE ON THE SPECIFIC HEAT OF THE ETHEE. 

 By GEOEGE FEANCIS FITZGEEALD, M.A., F.E.S., 

 F.T.C.D. ; Erasmus Smith's Professor of Natural and 

 Experimental Philosophy in the University of Dublin. 



[Eead, May 18, 1885.] 



It is not quite accurate to describe the quality of the ether that is 

 here called its specific heat by that name. The energy of the ether 

 per unit volume is not in exactly the same form as heat-energy in 

 matter. It is in the form of vibrations transmitted continually 

 through it, and is more nearly analogous to sound vibrations in 

 matter. As this energy of the ether is, however, intimately con- 

 nected with the temperature of the matter that is in it ; and as 

 its amount depends on that temperature, it is quite intelligible to 

 speak of the temperature of a volume of ether, meaning that it 

 is on all sides in temperature equilibrium with surrounding matter : 

 on these accounts it is intelligible, and, with proper restrictions as 

 to its meaning, quite right to speak of the specific heat of the ether. 

 There is no danger of the words being mistaken as referring to 

 anything else than what they are intended to describe. It must, 

 however, be clearly kept in view that the energy per unit volume 

 of the ether is not exactly like that of matter. We can also only 

 calculate the energy per unit volume, for we don't know whether 

 any perfectly intelligible meaning can be attached to the mass of 

 the ether. 



The problem to be solved in determining the specific heat of 

 the ether is. What is the increase in the quantity of energy per 

 unit volume of the ether that corresponds to its temperature being 

 raised one degree ? If we consider the temperature equilibrium 

 of a surface of lampblack, which is assumed to be approximately a 

 perfect absorber and radiator when it is one degree hotter than its 

 surroundings, it is seen that it is radiating energy outwards at a 

 certain rate, and receiving it at a certain less rate. Now, the 

 energy per cubic centimeter of the ether considered as transmitting 

 radiations outwards is greater than its energy considered as trans- 

 mitting radiations inwards, and it is this difference that causes the 



