302 Messrs. W. Spottiswoode and J. F. Moulton. [Apr. 8, 



small to be experimentally appreciable ; but this was distinctly stated 

 by Professor Maxwell himself, at the foot of p. 256. 



As to the second limit, I must remark, in the first place, that I can- 

 not find that Graham made any assumption that porous plates act as 

 apertures in thin plates. The result that the time of passage varies, 

 ceteris paribus, as the square root of the density in the case of fine 

 porous plates, was obtained by pure experiment; and though he 

 could not fail to notice the accordance of this result with that of the 

 mere hydrodynamical passage through a small aperture, he has care- 

 fully distinguished between the two. Nor can I agree with Professor 

 Reynolds in regarding the explanation given by Professors Thomson 

 and Maxwell of the phenomenon of thermal transpiration or thermal 

 effusion, whichever it be called, afforded by assimilating a fine porous 

 plate to a thin plate pierced by apertures of ideal fineness as erroneous, 

 even though it should be shown that such assimilation is unnecessary. 

 Professor Maxwell did not profess to treat in his paper the inter- 

 mediate cases between the two extreme limits. 



Perhaps I should mention, that the foot-note at p. 281 in Professor 

 Maxwell's paper was added as the paper passed through the press. I 

 recollect noticing the thing as, in my capacity of Secretary, I looked 

 over the paper before sending it to be printed off, and considering 

 whether I should affix a date. As, however, it seemed to me to con- 

 tain merely an explanation of an expression in the text, and as Maxwell, 

 who had carefully added the dates of fresh matter in other parts, did 

 not seem to have thought it necessary to do so in this case, I left it as 

 it was. In a letter I received from him at the time, he informed me 

 that he felt very ill, and was hardly fit even to go through his own 

 paper ; though a subsequent letter, in which he entered into some 

 scientific .matters, was written in his usual cheerful style. No one 

 had, I believe, at that time any notion of the very serious nature of 

 his illness. 



March 13, 1880. G. G. Stokes. 



II. " On the Sensitive State of Yacuum Discharges. Part II." 

 By William Spottiswoode, D.C.L., LL.D., Pres. R.S., and 

 J. Fletcher Moulton, late Fellow of Christ's College, 

 Cambridge. Received March 11, 1880. 



(Abstract.) 



This paper forms a sequel to that published under the same title in 

 the " Phil. Trans.," 1879, p. 165. It describes a continuation of the 

 research into the nature and laws of the disruptive discharge, or elec- 



