Transpiration and Radiometer Motion. 391 



results in a number of laws, but there is no occasion for us to 

 follow these seriatim as they are all contained in the sym- 

 bolical statements of equations (17) and (14), which we have 

 already tested by the experimental results. 



An interesting result of thermal transpiration experiments 

 is that they enable us to calculate the mean radius of the 

 passages in porous materials, for in the case of hydrogen the 

 measurements must be but little complicated by molecular 

 force ; thus the values given for B ; in the case of hydrogen, 

 if multiplied by 760 to pass from the mm. of mercury to the 

 atmo as unit of pressure, and then divided by 1033'3 X 981 to 

 pass to the dyne per sq. cm. as absolute unit of pressure, give 

 us the values of 12av B/n (v 2 + v l ) 2 ; now as i' 2 + i?imay be 

 taken as relating to a mean temperature of 57° C, 



(r 2 + r 1 ) 2 = 4i' 2 330/273, 



and i\)= 184400 cm. per sec. and 770= 00009, while a is 

 nearly unity ; thus for the mean radius of the passages in 

 Reynolds's meerschaum II. we get '0000112 cm., in meer- 

 schaum III. '0000092, or say ]/10 D cm. for meerschaum, while 

 for stucco I. the value is '000047 cm. Thermal transpiration 

 gives no information as to the number of passages, but this 

 could be found from a measurement of the volume of air 

 transpired by a plate in unit time under a measured excess of 

 pressure on one side, or by other measurements relating to 

 transpiration under pressure such as those made by Graham 

 and Reynolds; for the delivery of gas could be calculated as 

 that due to N tubes of radius R, the discharge of each being- 

 calculated according to 0. E. Meyer's equation given in (7). 

 In this way thermal and pressural transpiration measure- 

 ments can be made to yield a measure of the average porosity 

 of any solid through which hydrogen can pass. By artifi- 

 cially altering the porosity in a series of preparations, as for 

 instance by hardening stucco under different pressures and 

 similar means, so that specific gravity would give a measure 

 of relative porosity, it might be possible to find a porosity 

 at which hydrogen just failed to pass, which would furnish 

 an independent measure of the diameter of the hydrogen 

 molecule. 



[To be continued.] 



2 F 2 



