﻿The Rev. J. M. Heath on Athermogenic Condensation. 127 



each individual particle, and this can only be effected by a high 

 temperature. 



If, on the other hand, we observe very simple and faintly 

 luminous spectra where, in spite of the marvellously tenuous 

 distribution of the substance (as in the aurora borealis, the co- 

 rona, the zodiacal light, and nebulae), considering the enormous 

 thickness of the radiant layers it must be admitted that a great 

 number of luminous particles are effective, the temperature of 

 the gases here incandescent may in general be relatively low. 

 These considerations satisfactorily corroborate the conjecture of 

 Lockyer *, that the simple spectra of nebulae do not belong to 

 high, but to relatively low temperatures. 



Remark. — Probably the admissibility of the comparison above 

 instituted between the aurora borealis and a Geissler's tube 

 might be doubted, because in the latter the gas, on being heated, 

 cannot expand, as in the aurora, and hence its density remains 

 constant. Yet, as regards the above considerations, but very 

 little importance can be ascribed to this circumstance, because 

 the diminution of the density is only a secondary effect of the 

 increase of temperature, and occurs relatively much later than 

 the optical effect of the incandescence ; and even if it were taken 

 into account, it would necessitate only unimportant alterations 

 in the example treated. On the other hand, I consider it pro- 

 bable that at least a part of the remarkable phenomena of motion 

 observed in the aurora borealis are to be referred to the disturb- 

 ances of equilibrium arising from such important differences of 

 temperature in the extremely rarefied strata of our atmosphere. 



XVIII. On Athermogenic Condensation. 

 By the Rev. J. M. Heath f. 



HPHE proposition which I understand is good thermodyna- 

 JL mics, though to my mind very questionable mechanics, 

 may be thus stated. If P and Q are the forces tending respec- 

 tively to contract and expand the volume V of a unit of gas, 

 and if (P — Q)8V = 0, and if actual contraction of V does take 

 place by the uniform descent of the piston through the space SV, 

 then the internal energy of the gas will be augmented by the 

 energy due to the action of P through the space 8V. 



Those who accept this proposition do so in the belief that it is as 

 true in mechanics (that is, when the energy spoken of is regarded as 

 molecular motion) as it is in thermodynamics, where such energy 

 is identified with heat. And they admit, as a mechanical prin- 



* Proc. Roy. Soc. 1869, No. 1 12. 

 t Communicated by the Author. 



