142 Transactions. 



of heat by radiation left gravity partly unbalanced: now the 

 addition of heat leaves the expansive power partly unbalanced. 

 Each particle must therefore move outwards, but in doing so 

 it moves against gravitation work being done and heat absorbed 

 (" rendered latent " was the old expression) ; but, as a com- 

 pressed gas heats by the transformation of work into heat, so 

 an expanding gas cools by the transformation of heat into work 

 (both processes being made use of in our steam-engines, freezing- 

 engines, &c). The added heat then does work against gravity, 

 and is all absorbed in doing so. 



Now, if this were all, we should have simply expanded our 

 sun at the original temperature. But when expanded to the 

 extent referable to the added heat each particle is further 

 from the centre than it was before heat was added ; gravity is 

 therefote diminished ; and since, by the hypothesis, the amount 

 of heat was previously exactly sufficient to balance gravity, 

 there is now more than enough to do so, so that there is a residue 

 of expansive power still left : more heat than that added will 

 therefore be used up in expanding the body. This must be at 

 the expense of the heat already possessed by the sun : in other 

 words, the temperature will be lowered. This is the same ap- 

 parent paradox, in an inverted form, that we had before : by 

 putting heat into a gaseous star we lower its temperature. 



We may put the results alongside each other thus :— 



(1.) The body parts with heat by radiation: it shrinks in 

 consequence, and the temperature rises. We may add, the 

 potential falls to the exact extent of the heat radiated away, 

 plus the added temperature. 



(2.) The body has heat supplied to it : it expands in conse- 

 quence, and the temperature falls. We may add, as before, 

 the potential rises to the exact extent of the heat added, plus 

 the latent heat of expansion. 



Now, let us apply these considerations to a " Nova." Some 

 cause which we can only conjecture occasions an enormous 

 amount of heat to be added to a body. The only probable 

 cause we can think of is a collision of some kind — it may be of 

 two large bodies, or two meteor-streams, &c. If the body was 

 already gaseous the above reasoning would apply at once. If 

 it were not gaseous the added heat caused by the collision (in 

 the case of a " Nova ") is sufficient to make it so, and the reason- 

 ing will apply as soon as it is *o. If the heat were due to col- 

 lision, as is probable, the process of heating would be exceedingly 

 rapid. If the colliding bodies were both gaseous the generation 

 of the whole heat of collision would take a few hours only ; but 

 as the whole of that heat could not be converted in the same 

 time into motion of expansion, because of the inertia of the 



