402 Mr, Herapath on the Causes, Laws, and principal [June, 



ture in the medium, in the same manner as the excess of velocity 

 would cause an increase. In both these cases, the excess or 

 defect of temperature will be proportional to the celerity of con- 

 densation or rarefaction, supposing the time occupied by the 

 change of volume to be of sufficient length for the stroke to be 

 propagated to all parts of the medium. Therefore, if the celerity 

 of the stroke be great, a great change in the temperature will 

 ensue ; and if it be small, there will be Httle or none. 



Our theory of gases likewise explains another phsenomenon, 

 which has usually been attributed to a different cause. It is 

 well known that if two gases be placed in any position with 

 respect to each other, and have ever so small a communication, 

 they will intermix, and become equally diffused the one through- 

 out the other. This has been accounted for on the principle that 

 all gases have a slight affinity for one another, or in other words, 

 that though the parts of all gaseous bodies individually consi- 

 dered mutually repel each other, yet the parts of different gases, 

 however great or however small that difference may be, mutually 

 attract each other. It is repugnant to my intention to attempt 

 any refutation of this idea, or to make any observations on the 

 confusion that would result from thus burthening nature with 

 hypotheses ; I shall, therefore, merely show that the phaenome- 

 nonin question is an easy consequence of our theory. Suppose 

 that portions of the surfaces of two media are exposed to each 

 other, and that parallel to these portions the media are divided 

 into strata. Then because a difference in the numeratoms, a 

 difference in the motions of the particles of the two strata, or a 

 cornered irregular figure in the particles, would render it impos- 

 sible for the particles of one stratum to be so reflected by 

 the particles of the other, that each stratum would retain its 

 particles entire and unmixed with those of the other, the two 

 strata would intermix ; and would not arrive to a state of equili- 

 brious action, until the particles were uniformly and equally 

 disposed in each stratum. For the same reasons, these newly 

 compounded strata would mix with their next ; and thus it 

 would go on stratum intermixing with stratum, until an equili- 

 brious action throughout was restored, by the equal and uniform 

 intermixture of the two gases. 



By the same principles it is easily perceived, why gases so 

 readily acquire the temperature of the surrounding bodies ; why 

 they transmit temperature so rapidly and so feebly in right fines ; , 

 and why, according to the manner in which Mr. Leslie, Sir H*. 

 Davy, and MM. Dulong and Petit, have made their experi- 

 ments, it is owing to the gravitation of the gases alone, that the 

 fighter gases cool bodies immersed in them quicker than the 

 heavier. When I first turned my attention to the cooling power 

 of gases, I was surprised to find from my theory, that all gases, 

 under the same circumstances, ought to cool equally fast. This 

 inference for some time perplexed me very much. As soon, 



