PROPERTIES OF MATTER IN' THE GASEOUS STATE. 
773 
conviction was spreading that what I suggested contained the germ of the explanation, 
I set to work in earnest to complete the explanation, and ascertain by an extension of 
the dynamical theory of gases what effect the hot molecules receding from the surface 
should produce on the number and temperature of those approaching. . 
My first attempts to accomplish this were altogether unsuccessful. When contem¬ 
plating the phenomena it seemed to me that I could perceive a glimmering of the 
method of reasoning for which I was in search, but as soon as ever I attempted to 
give definite expression to it this glimmering vanished. 
The reason for this I now perceive clearly. When contemplating the phenomena, I 
had a surface of limited extent before me, and I considered the effect on such a surface 
without recognising the fundamental importance of the limit to size. 
On the other hand, when I came to definite reasoning, for the sake of what appeared 
to be a simplification of the conditions of the problem, I assumed the surface to be 
without limit, thus introducing a fundamental alteration into the conditions of the 
problem without perceiving it. 
The importance of this limit only became apparent to me when I found, by simple 
dynamical reasoning, that with surfaces of unlimited extent such results as those 
actually obtained would be impossible. This appeared as follows :— 
No force on unlimited surface. 
54. If we had two plane plates of unlimited extent, H and C, the surface of H 
opposite to C being hotter than the surface of C which was opposite to H, the outside 
surfaces of both plates being at the same temperature, then in order to produce 
results similar to those obtained with limited plates, the gas between the two plates 
must maintain a greater steady pressure on the plate H, than that which it exerts on 
the colder plate C. Whereas it is at once obvious that such a condition is contrary to 
the laws of motion, which require that the gas between the two surfaces should exert 
an equal and opposite pressure on both surfaces. 
Having once perceived the force of this reasoning, it became clear to me that if, as 
I had supposed, the results obtained in the experiments were due to gaseous pressure, 
then they must depend on the limited extent of the surfaces. 
This gave me the clue, in following which I have not only had the satisfaction of 
finding the explanation complete as regards the phenomena from which it originated, 
but I have also found that the theory indicated the phenomena of thermal transpira¬ 
tion, and explains much that hitherto has been considered anomalous respecting the 
laws of transpiration of gases through small ohannels—suggesting the experiments by 
which might be established the relation between these actions. 
The manner in which the force arises in the case of a limited surface was at first 
rendered much clearer to me by considering an illustration, which I introduce here, 
although it forms no part of the proof which will follow. 
