16 Messrs. Richardson, Nicol, and Parnell on the 
dissociation being that given by k)=0°64. In order to 
compare the two we have plotted the values of log Q on 
Winkelmann’s theory against log P taking the dissociation 
constant 4y;=0°64. The result is any line on the diagram 
parallel to curve A (fig. 4). It will be seen that the two 
cases are practically identical at pressures greater than 
10 cms. It is only at lower pressures that any discrepancy 
between Winkelmann’s view and our results becomes evident. 
The discrepancy is not due to the experiments having been 
made at different temperatures ; for we find, on calculating 
back from Winkelmann’s value of the amount of hydrogen 
diffusing through each sq. em. of his tube per second, that 
his experiments were made as nearly as possible at 1000° C., 
whereas our results hold from 717°C. to 1136°C. As a 
matter of fact Winkelmann’s observations, which only extend 
from about 10 cms. to 70 ems., are all on the flat part of 
the curve and therefore agree with ours. It is only the 
interpretation which is different. 
In order to substantiate this point it was thought advisable 
to make further experiments at low pressures, where alone 
the difference between the two views becomes apparent. 
This was more especially the case, as the low pressures in 
the previous experiments were obtained by subtracting the 
readings of the mercury manometer from the baromeni 
height, and were therefore liable to considerable errors at 
pressures less than a centimetre. In these experiments the 
same apparatus (fig. 1) was used as before, but a new mode 
of procedure was ‘adopted. The apparatus was filled with 
pure hydrogen in the manner previously described, and was 
then exhausted with the water-pump till only about 3 ems. 
pressure of hydrogen remained. The water-pump and the 
tap A were then shut off, and the parts to the left side of 
the sulphuric-acid gauge exhausted with the mercury-pump 
to a pressure of about ,; mm. During this operation the 
sulphuric-acid column on the left side of the gauge B rose 
until the difference in the levels on the two sides was about 
200 mms. ‘Thus, since the pressure on the left side of the 
gauge is for all practical purposes zero, we have a volume 
of g gas whose pressure is given by the difference of level in 
the sulphuric-acid gauge. In this way the readings of the 
acid gauge were made to give both the rate of escape and 
the actual pressure of the gas. This arrangement was found 
to be very satisfactory and | naturally very sensitive. 
Before giving the observations it is necessary to obtain a 
formula connecting the gauge-readings with the rate of 
escape of the gas. “Using where not otherwise explained the 
