PROPERTIES OF MATTER IN THE GASEOUS STATE. 
741 
difference of pressure, were very much disturbed, the first definite and consistent 
results were obtained with a porcelain plate No. 3, ^-th inch (2'5 millims.) thick, on 
February 22. 
Table I.—Thermal transpiration of air by biscuit ware plate No. 3 (‘1 inch or 
2-5 millims. thick). Temperature of steam, 212° F. or 100° C.; temperature of 
water, 47° F. or 8° C. 
Mean pressure by vacuum 
Difference of pressure by 
Ratio of mean pressure to 
gauge. 
siphon gauge, February 22. 
difference of pressure. 
inches. 
millims. 
inch. 
millims. 
30 
762 
•l 
2-54 
300 
This result was found to remain constant over a period of 8 hours, during which the 
steam and water were kept constantly flowing. It was also found to be the same 
whichever side of the diffusiometer was heated. During the experiment the tap 
bringing the hot and cold chambers into direct communication was frequently opened, 
and the differential gauge then indicated equal pressures. After each of these open¬ 
ings on the tap being again closed the same difference was re-established in a few 
seconds. 
The next experiments were made with a somewhat thinner plate of meerschaum 
No. 1. 
Table II.—Thermal transpiration of air by meerschaum plate No. 1 ( - 06 inch or 
1 '5 millims.). Temperature of steam, 212° F. or 100° C.; temperature of water, 
47° F. or 8° C. 
Mean pressure by vacuum 
Difference of pressure 
Ratio of mean pressure to 
gauge. 
by siphon gauge, March 12. 
difference of pressure. 
inches. | millims. 
inch. millims. 
30 762 
•08 | 2-03 
350 
As it seemed highly probable that the meerschaum plate was of finer texture than 
the porcelain plate previously tried, the fact that the difference of pressure with the 
meerschaum was not larger than with the porcelain was a matter of some surprise. 
There appeared, however, to be a possible cause for this in the thinness of the meer¬ 
schaum. It was possible that there was some flaw in the plate, or more probably that 
the thinness of the plate allowed a considerable equalisation of temperature by the 
conduction of heat. It was therefore resolved to try a thicker plate of meerschaum, 
and a plate ’25 inch (6'3 millims.) was introduced in place of that previously tried. 
