1879.] 



Properties of Matter in the Gaseous State. 



311 



for air, fig. 1). From this it was clear that if the thermal difference 

 reached a maximum it would be at some pressure greater than that of 

 the atmosphere, With stucco the thermal difference for air increased 

 as the pressure fell from that of the atmosphere, and reached a 

 maximum only at a pressure of about 8 inches of mercury (shown in 

 fig. 2). 



Fig. 2. 



A comparison of these results shows that the density at which the 

 thermal transpiration is a maximum depends on the coarseness of the 

 plate ; and that it depends on the nature of the gas appears at once on 

 comparing the results for hydrogen, air, and carbonic acid, which are 

 shown on figs. 1 and 2. 



Experiments with plates of various thicknesses gave the thermal 

 difference of pressures independent of the thickness of the plate, so 

 long as the difference of temperature was the same. 



Several minor deductions from the theory were also directly verified. 



The Law of Corresponding Results at Corresponding Densities. 



In order to establish this law it was necessary to compare the results 

 obtained with different plates. According to the law, the ratio of the 

 thermal difference of pressure to the mean pressure with a particular 

 plate and a particular gas should be the same as with another plate and 

 the same gas, as long as the densities (or pressures) are in a fixed 

 ratio, which is the ratio of the fineness of the plates. 



A simple numerical calculation sufficed to show that this conclusion 

 is approximately verified. On dividing the thermal differences by the 

 mean pressure for both the meerschaum and stucco plates, it appears 

 that the resulting numbers are approximately equal, so long as the 

 pressure with the meerschaum is six times as great as with the stucco. 

 This is so both for air and hydrogen, and through a range of pressures 

 from 30 to '35 inches with the meerschaum, and 5 to *2 inches with 

 the stucco. 



The numerical comparison does not, however, bring out the agree- 

 ment in nearly as strong a light as the comparison which has been 

 effected by a graphic method. 



