of the Laws of Elastic Fluids. 63 



This theorem, combined witli another which he immediately 

 deduces, includes, he says," les lois generales des fluides elas- 

 tiques." 



" Let us imagine," proceeds M. Laplace, "that the envelope 

 and the contained gas have a common temperature t. It is 

 manifest that any molecule whatever of this gas will every 

 instant be struck by some of the calorific rays emitted by the 

 siu-rounding bodies. A part of these rays it will stifle ; but 

 to maintain the temperature unchanged, it must radiate as many 

 rays as it stifles. In any other space of the same temperature 

 the molecule will be struck by the same quantity of calorific 

 rays ; the same part of which as before it will absorb and re- 

 place by its radiation. The quantity of calorific rays therefore 

 which any given surface at every instant receives, is some func- 

 tion of the temperature alone, and independent of the surround- 

 ing bodies : I shall denote it by /7 {t). Hence the extinction 

 will be qn (/), q being a constant factdr depending on the 

 nature of the molecule or of the gas. I will here observe that 

 the quantity of rays emitted by the surrounding bodies, and 

 which constitutes the free caloric of space, is, on account of 

 the extreme velocity we must necessarily assign those rays, 

 but a very insensible part of their whole caloric ; which is 

 otherwise manifest from the experiments made to condense it. 

 Now in whatever manner the caloric of the siirrmtncUiig mole- 

 cules acts by its repulsioji on the caloric of any particular niole- 

 culeof gas, to detach a part of this caloric and make die molecule 

 radiate, it is evident that this radiation will be in a ratio com- 

 pounded of the density of the gas surrounding the molecule, 

 or of qc and the caloric c contained in the molecule. It will 

 therefore be proportional to qc': which is consequently jiro- 

 portional to die extincton (/H (/) ; so diat we may suppose, 



qr = q'n{t); (2) 

 q bemg a constant factor depending on the nature of die gas, 

 and n (/) a function of the temperature independent of this 

 nature." 



These are die arguments by wliich M. Laplace attempts to 

 establish his ccjuation 2. If lor the sake of brevity we pass 

 over die first conclusion, namely, that the radiation to the 

 molecule is independent of the surrounding bodies and some 

 function of /, which if rigidly considered is probably not so 

 evident as M. Laplace seems to think it ; philosophers will, I 

 presume, hardly then grant the latter conclusion, that the 

 radiation of a molecule is proportional to qcxc. We might 

 also easily show that surrounding molecules tend rather by 

 their repulsion to compress Uie caloric of an inclosed molecule 

 closer towards Uie centre, dian to disperse it ; but this too we 



will 



