1821.] PImnomena of Heat , Gases, Gravitation, S;c» 281 



3. Let it be granted that gaseous or aeriform bodies consist 

 of atoms, or particles, moving about, and among one another, 

 with perfect freedom. I 



4. Let it be granted that what we call heat arises from an 

 intestine motion of the atoms, or particles, and is proportional 

 to their individual momentum. 



5. Let it be granted that a gaseous- body of very great tenuity 

 in its parts fills all space, and extends to its utmost limits. 



I have purposely put these hypotheses (if indeed we can call 

 those things hypotheses which have been deduced from the 

 analysis of phsenomena) into the form of postulata, to avoid 

 being obliged to establish them by direct demonstration. It is 

 not my intention, for the reasons I have already given in the 

 beginning of this memoir, to make any comparative remarks on 

 their relative simplicity and probability, I shall only say a few 

 words for the purpose of explaining the difference between my 

 views on certain points and those which have been taken by others. 



One of the sublimest ideas of the ancients was, that there is 

 but one kind of matter, from the different sizes, figures, and 

 arrangements of whose primitive particles, arises all that beau- 

 tiful variety of colour, hardness and softness, solidity and 

 fluidity, opacity and transparency, 8cc. which is observed in the 

 productions of nature. Our first two postulata do not necessa- 

 rily require that there should be but one kind of matter ; there 

 may be several kinds. But since it seems possible to account 

 for all the phsenomena on the supposition of one kind only, and 

 since nature is always disposed to employ the simplest machi- 

 nery, probabiUty is strongly in favour of the ancient idea. In 

 fact it does not seem to be impossible, from some of the phseno- 

 mena of light and other circumstances, to show that nature has 

 embraced the simplest means, and has likewise, if not in the 

 size, at least in the figure of the atoms, confined herself within 

 certain limits. Bat these things are too recondite to be pursued 

 in this memoir; and experiments have not yet furnished us with 

 sufficient data to be able to exhibit the exact line and rule with 

 which nature has laid out her work. 



Philosophers, since the time of Newton, have taught us that 

 the elasticity of gases is owing to a mutual repulsion between 

 their particles, by which they endeavour to fly from one ano- 

 ther ; but by our third postulatum we have divested matter of 

 this repulsive property, and nevertheless, as it will be seen, the 

 laws of gaseous bodies, investigated under this point of view, 

 agree mathematically with phsenomena. 



The advocates for the theory of heat by intestine motion have 

 usually considered the temperature as measured by the velocity 

 of vibration ; and I am not aware that any of them have defined 

 it otherwise. This will do very well for different temperatures of' 

 the same body ; but it seems to require the theory I have given ' 

 in the fourth postulatum to enable us, under all circumstances, ; 

 to compare the temperatures of different bodies. • ^^ ijo 



