54 Mr. F. Trouton on Molecular Latent Heat, 



tinuous form of discharge into the discontinuous agrees 

 entirely with the results previously obtained by approaching 

 the positive to the negative pole. If we denote as the positive 

 direction of the glow-rays that which issues from the bright 

 kathode-layer, many discharges occurred so long as a union 

 of the positive electricity could take place in the negative 

 direction with the glow-rays ; but as soon as that was no 

 longer the case, much fewer discharges occurred. Exactly 

 the same is the case here, so long as the glow-rays have not 

 all been deflected from the axis of the tube to the side ; the 

 positive light can still unite with them in the negative direc- 

 tion. But as soon as a complete deflection has taken place, 

 we have exactly the same case as if the positive electrode 

 were behind the bright kathode-layer, and the discharge 

 becomes continuous. 



[To be continued.] 



IV. On Molecular Latent Heat. 

 By Frederick Trouton, Trinity College, Dublin*. 



0_T comparing the quantities of heat necessary to evaporate 

 at constant pressure quantities of different liquids taken 

 in the ratio of their molecular weights, it is found that the 

 amount of heat required by any body is approximately pro- 

 portional to its absolute temperature at the point of ebullition. 

 For example, the latent heat of bromine is 45*9, it boils at 

 63°, and the density of the body is 7 i ) * 7 5 . The latent heat of 

 butyric acid is 114*7, it boils at 162°, and its density is 44. 

 The quantities of heat required to evaporate quantities of the 

 bodies in the ratio of their molecular weights is obtained by 

 multiplying the latent heat by the density ; and the quan- 

 tities thus obtained have an approximately constant ratio to 

 the absolute temperatures of the boiling-points, thus : — 



45-9* 79-75 in _ 114-6x44 

 _____ -=10-89, =11-59. 



This ratio is nearly constant for most bodies, but is still more 

 nearly so for those bodies which are chemically related to each 

 other. 



The relation, then, may be put into this simple form. 

 The molecules of chemically related bodies, in changing from 

 the gaseous to the liquid state at the same pressure, disen- 

 gage quantities of heat, which may be called the molecular- 

 latent heat, directly proportional to the absolute temperature 

 of the point of ebullition. The similarity is very striking 



* Comniumcated by the Author. 



