( S 80 ) 
Aut. XIII. — Observations on the Effects qfHeat and of Mo^ 
tion. In a Letter to J. F. W. Heuschel, Esq. F.R. S. 
Lond. and Edin. from M. Seguin. Communicated by Mr 
Herschel^. 
Sir, An non ay, \Wi Sept, 18^2. 
I HAVE taken the liberty of submitting, through your means, 
to the learned Society of which you are Secretary, a new me- 
thod of considering the effects of heat and of motion, which my 
granduncle M. Montgolfier, with whom I spent a great part of 
my youth, had often discussed with me. 
The principle which he maintained was, that the vis viva 
could neither be created nor annihilated, and consequently, that 
the quantity of motion on the earth had a real and finite exist- 
ence. 
In order to obtain an explanation of this result, I have ob- 
served, that, as all known bodies are elastic, and susceptible of 
experiencing variations of volume, the limits of which are un- 
known, their molecules must necessarily be at a distance from 
each other. 
As even the most solid and dense bodies are capable of being 
evaporated, — a property which is indicated by their smell, we 
may conclude, that there escapes from them at each instant an 
infinity of molecules, which, from their tenuity, elude all the 
means of comparison which we can employ. 
In order to assign to them ' the condition either of a solid, a 
liquid, or a gaSj it is necessary to suppose the existence, and th^ 
combination of two forces, which are sometimes in equiVibrio, and 
sometimes predominate the one over the other. We shall ad- 
mit, then, the supposition, that these two forces may be the same 
as those which regulate our planetary system, and that the mo- 
lecules of bodies are subject to circulate round one another, so 
* This ingenious paper was communicated by its'authorto J. F, W. Herschel, 
Esq. as Secretary to the Astronomical Society of London ; but being on a subject 
not Astronomical, Mr Herschel was so kind as to transmit it to Dr Brewster, fop 
insertion in this Journal. 
