1879.] 



Properties of Matter in the Gaseous State. 



309 



time, also for measuring the pressure of gas in the chambers, for 

 exhausting the chambers, and for bringing the chambers into direct 

 communication when required. The different temperatures were 

 secured by a stream of steam on the one side, which gave a tempera- 

 ture of 212° F., and a stream of water on the other side, which gave a 

 temperature constant for the time, but which ranged during the 

 investigation from 47° in February to 70° in July. 



The porous plates tried were of biscuit- ware, stucco, and meerschaum, 

 and ranged in thickness from '06 (1/5 millims.) to '44 inch (11*2 

 millims) . The pressures of gas within the chambers and the difference 

 of pressure on the two sides of the plate were measured by mercury 

 gauges. A special instrument used for reading the differential gauge 

 read to the T ^0Q-th of an inch ("0025 millim.). 



Several weeks were spent on this apparatus in getting it tight, 

 getting the gauges to work, and getting rid of the disturbing effects 

 of moisture, before any definite results were obtained, but finally the 

 instrument answered extremely well. 



The Experiments on Thermal Transpiration. 



The streams of steam and water having been kept going for several 

 hours, long enough for the condition of temperature in the instrument to 

 be perfectly steady, the tap which established communication between 

 the chambers on the opposite sides of the porous plate having been open, 

 so that the pressure in these chambers was equal, this tap was closed^ 

 so that the sole communication was through the porous plate. Any 

 difference of pressure between these chambers was then read on the 

 differential gauge. 



Supposing that on the first reading the gas (whatever it might be) 

 within the instrument was at the pressure of the atmosphere, a certain 

 quantity of gas was then drawn out and the experiment repeated. 

 This was done until the pressure within the instrument was as low 

 as '25 inch of mercury. 



According to the theoretical deductions, it had appeared that when 

 the sole communication between the two chambers was through the 

 porous plate, and the gas in these chambers was at the same pressure, 

 the difference of temperature would cause the gas to pass from the 

 colder chamber to that which was hotter, until a certain difference of 

 pressure was established, after which there would be no further change 

 as long as the same difference of temperature was maintained, so that 

 the result to be expected as giving evidence of thermal transpiration 

 was a difference in the pressure on the two sides of the plate. 



This difference was first obtained with air at the pressure of the 

 atmosphere and a biscuit- ware plate, the difference being 1 inch 

 (2*54 millims). 



It further appeared from the theory that the difference which would, 



