PROPERTIES OF MATTER IN THE GASEOUS STATE. 
767 
As previously explained with reference to the log. curves for thermal transpiration, 
the displacement O M in the direction of the abscissae represents the logarithm of the 
ratio of corresponding pressures, while the displacement O N in the ordinates represents 
the log of the ratio of the corresponding times for the two plates. This latter ratio 
cannot be made use of for the sake of comparison, as it involves the number of 
openings through the plate as well as their diameters. 
The relative coarseness of the plates. 
41. The ratio of the corresponding pressures, as given by the difference in the 
abscissae, is of the greatest importance. This ratio, according to Law V., Art. 9, 
corresponds with the ratio for the coarseness of the plates, and as these were the same 
plates as were used in thermal transpiration, it was to be expected that the results 
should agree ; that is to say, the displacement O'M, fig. 8, should be equal to the 
displacement O'M, fig. 6. The actual measures give 
For thermal transpiration, 0'M= '748=log 5’6 
,, transpiration under pressure, 0'M=‘819=log 6’5 
By this comparison the two independent and distinct experimental results check 
one another. 
The difference in the results, although too small to cast a doubt upon their agree¬ 
ment, is too large to be attributed to experimental inaccuracy. But it must be 
remembered that the conditions under which the plates are compared differs in an 
important particular. In the experiments on thermal transpiration the plates were 
heated, whereas in the experiments on transpiration under pressure they were at the 
normal temperatures, and it appears only natural to suppose that such a difference of 
temperature would somewhat alter the condition of the plate (see Appendix, note 3). 
Small densities. 
42. It appears very clearly from the curves, that as the pressure of the gas diminishes, 
the time of transpiration of equal volumes tends to become constant ; approximate 
constancy having been reached in the experiments. 
The ultimate ratio of the times of different gases was found by Graham to be as 
the square roots of the atomic weights of the gases, and the same ratio is obtained 
for air and hydrogen in these experiments. The square roots of the densities of dry 
air and hydrogen are 3'8 (3'79) to 1. The ratio of the times for air and hydrogen at 
the smallest pressures tried is 3'624, and as this is the result for both stucco and 
meerschaum the approximation is too close to be questioned, particularly when it 
is remembered that the smallest trace of impurity in the gases might cause the 
difference. 
MDCCCLXXIX. 
o F 
