1879.] 



Properties of Mattel^ in the Gaseous Slate. 



305 



with hydrogen on both sides of a porous plate, the pressure on the 

 one side being that of the atmosphere, a difference of 160° F. 

 (from 52° to 212°) in the temperature on the two sides of the plate 

 secured a permanent difference in the pressure on the two sides equal 

 to an inch of mercury ; the higher pressure being on the hotter side. 

 With different gases and different plates various results were obtained, 

 which, however, as will be seen, are connected by definite laws. 



3. Again, although Graham found that he obtained not only very 

 different results, but also very different laws of motion with plates of 

 different coarseness, or with plates and capillary tubes, neither he nor 

 any subsequent observer appears to have followed up this lead. As 

 regards Graham, this appears to me to be somewhat surprising. For 

 although he may have considered the mere difference in the results to 

 have been analogous to the difference found by Poiseuille for liquids, 

 it would seem as though the difference in the laws of motion should 

 have excited his curiosity ; and then, as he was avowedly of opinion 

 that gas is molecular, he could hardly have failed to observe that so 

 long as the mean distance separating the molecules in the gas bore a 

 fixed relation to the breadth of the openings in his plates, he should 

 have had the same laws of motion. This view, however, appears to 

 have escaped him as well as all subsequent observers, otherwise it 

 would have been seen that with a simple gas such as hydrogen, similar 

 results must be obtained so long as the density of the gas is inversely 

 proportional to the lateral dimensions of the passages through the 

 plates. 



By experiments to be described I have now fully established this 

 law. I find that with different plates similar results are obtained 

 when the densities of the gas with the different plates bear certain 

 fixed ratios, and that this is the case whatever may be the cause of 

 transpiration, i.e., a difference of temperature or a difference of pres- 

 sure ; a difference of gas I have not investigated, as it was obviously 

 unnecessary to do so. Thus, with two plates, one of stucco and the 

 other of meerschaum, similar results of transpiration caused by pres- 

 sure were obtained when the densities with the . two plates were 

 respectively as 1 to 5*6 both with hydrogen and air, and at pressures 

 ranging from 30 to 2 inches of mercury ; also with the same two 

 plates similar results of thermal transpiration were obtained when the 

 densities were respectively as 1 to 6'5 both for hydrogen and air, and 

 through a range of densities from 80 inches to '25 of an inch of 

 mercury, the discrepancy between 5'6 and 6*5 being in all probability 

 owing to a slightly altered condition of the plates. 



This correspondence of the results at corresponding densities holds 

 al though the law of motion changes. Thus, with air at 30 inches 

 through the stucco plate, the law of motion was the same as that 

 found by Graham for a stucco plate, while at the smallest pressure 



z 2 . 



