PROPERTIES OF MATTER IN THE GASEOUS STATE. 
809 
For putting 
we have for the usual equation 
and comparing (78) and (79) 
The difference between equations (78) and (79) is, however, very important. For 
whereas p is usually supposed to be constant, i.e. , independent of the diameter of the 
tube, it appears from (78) that such can only be the case so long as c is large as 
compared with s : s being a distance measured across the tube which by no variation 
in the condition of the gas can be made larger than the mean diameter of the tube. 
This fact that s cannot increase beyond the diameter of the tube at once explains 
the anomalies (as they appeared to Graham) between the times of transpiration for 
tine and coarse plugs. 
The mean diameter of the interstices of Graham's coarse plugs were so large, that 
with gas in the condition in which he used it, s was less than this diameter, and not 
being limited to the diameter of the tube was different for different gases and for 
different conditions of the same gas ; whereas with the fine plugs, s being limited to 
the diameter of the tube, could no longer vary with the nature of the gas. 
The limit to the value of s also indicates, what has been verified by the experiments 
described in Part I. of this paper, that the results which Graham obtained with fine 
plates only, are to be obtained with coarse plates when the condition of the gas is such 
that s is limited by the diameter of the interstices. 
dp _ d / du 
dx dz\ ^ dz 
— c~ dp 
£1 — u c — —— ~ 
2>p dx 
1 V 
\/7r ci 
• ( 79 ) 
. (80) 
The relation between s and the other 'properties of the <ja$. 
94. The experiments made by Graham and by Maxwell in which the distances 
between the surfaces was such that there was no chance of s being limited by this 
distance, give consistent results, from which it has been found that 
p oc-.* 
P 
y) 
Hence taking p—V- and substituting in equation (80) we have 
r 
s=d?-i . ■ .(so 
2 p 
* Added Dec., 1879.—Subsequent observers Lave found that p oc so that Maxwei.Ps conclusions 
are not borne out.—See Phil. Trans., Part I., 1879, p. 240. This makes no difference to the subsequent 
part of this investigation, as no further use is made of equation (81). 
5 L 2 
