J i Tendency of Elastic Fluids to Dijffusion. 



this case, I am far from being able to explain. Much Ies5 

 can I imagine how air should pass one way and vapour the 

 ot her in the same pores, and how the transmission of the 

 one should be necessary to the transmission of the other. 

 I am satisfied, however, that it is by means of such pores 

 as air may be forced through, that this curious process is 

 performed ; because the experiment never succeeds but in 

 such vessels as, by the air-pump at least, appear to be 

 porous, thou oh in all such." 



The truth is, these facts, so difficult to explain, are ex- 

 actly similar to those which are the subject of this memoir; 

 only instead of a great number of pores, we have one of 

 sensible magnitude — the bore of the tube. Let the porous 

 retort have the same elastic fluid within and without, in 

 the one case ; and the two phials contain the same elastic 

 fluid in the other, then no transmission is observable in 

 either : but if the retort have common air, or any other 

 gas, without, and aqueous vapour, or any other elastic 

 fluid, except the outside one, within ; then the motion in 

 and out commences, just as with the phials in similar cir- 

 cumstances. In fact, this last observation has since been 

 vmfied by Dr. Priestley himself, of which an account is 

 given in No. 2, of the American Philosophical Transac- 

 tions, vol. V. After alluding to his experiments above 

 mentioned, he observes : " Since that time I have extended 

 and diversified the experiments, and have observed, that 

 what \\ as done by air and water will be done by any two 

 kinds of air, and whether they have affinity to one another 

 or not ; that this takes place in circumstances of which I 

 was not at all apprised before, and such as experimenters 

 ought to be acquainted with, in order to prevent mistakes 

 of considerable consequence." 



The facts stated above, taken altogether, appear to me 

 to form as decisive evidence for that theory of elastic fluids 

 which I maintain, and against the one commonly received, 

 as any physical principle which has ever been deemed a 

 «uHject of dispute can adduce. 



III. On 



