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PROFESSOR 0. REYNOLDS ON CERTAIN DIMENSIONAL 
Section III.— Experiments respecting the rate of Transpiration. 
33. The experiments to be described in this section, besides being necessary for 
the verification of the Laws V., VI., and VII., Art.. 9, were necessary to complete the 
verification of Law I. In the last section no direct notice was taken of the rate of 
thermal transpiration when unprevented by the difference of pressure on the two sides 
of the plate, and for this reason. 
Although the thermal differences of pressure indicate in a general way the manner 
in which transpiration would have taken place had the pressure been equal, yet in 
order to examine the results strictly, as regards the various rates of thermal transpi¬ 
ration to which they correspond, it is necessary to know the exact law of transpiration 
for gases under pressure. The comparative rates of transpiration for different gases 
and the rates of transpiration of each gas for different pressures are not sufficient. So 
far, the laws established by Graham are all that can be desired, but these laws say 
nothing about the variation in the rate of transpiration consequent on a large variation 
in the density of the gas. Thus, Graham has shown that, through a fine graphite 
plate, the time of transpiration of a constant volume (measured at the mean pressure) 
will be exactly proportional to the difference of pressure, and will diminish slightly 
with the density, but his experiments were not carried to pressures many times less 
than the pressure of the atmosphere ; whereas, for the purpose of this investigation, it 
was necessary to compare results at pressures as low as '01 of an atmosphere. Nor is 
this the only point in which Graham’s results appeared insufficient for the present 
comparison. Graham had found that the law of transpiration for a fine graphite 
plate differed essentially from the law for a stucco plate; his experiments having been 
made in both cases at pressure not many times less than the pressure of the atmosphere. 
Thus, for the stucco plate, the comparative times of transpiration of air and hydrogen 
were as 2'8 to 1, while for the graphite plate they were as 3'8 to 1. He had also 
shown that for plates of intermediate coarseness an intermediate ratio would maintain; 
but he had given no law that would enable us to predict the result with any parti¬ 
cular plate. 
In order, therefore, to effect my comparison, it was necessary, by actual experiment, 
to ascertain the rates of transpiration through my particular plates with the same 
gases as those used for thermal transpiration, and at similar pressures. It was this 
consideration which mainly determined the manner of making the experiments. 
The apparatus. 
34. The thermo-diffusiometer, without the streams of steam and water, after having 
undergone certain slight modifications, lent itself very well to this part of the 
investigation. 
By means of extra branches from the tube KK, fig. 2. two 8 oz. flasks were con- 
