1879.] Properties of Matter in the Gaseous State. 313 



give absolutely consistent results is because the ratio of the correspond- 

 ing abscissae is not exactly the same as that of the corresponding 

 ordinates, the difference being found on examination to be owing to 

 a discrepancy in the temperatures, which affected the ratio of the cor- 

 responding ordinates, but not the ratios of the corresponding abscissas. 

 These, therefore, give 5 as the ratio of the coarseness of these parti- 

 cular plates. 



The woodcut, fig. 3, merely illustrates the result. The logarithmic 

 homologues of the curves for both air and hydrogen, plotted with the 

 greatest care, the points marking the experiments being so close 

 together that it was scarcely necessary to draw a curve, have been 

 compared, and the agreement is very remarkable, the only slight 

 deviation being that shown in fig. 3, which was found to be owing to 

 some impurity in the hydrogen at pressures below an inch of mercury.. 



In order fully to appreciate the force of this agreement, it must be 

 uoticed that it is not only the portions of the curves which overlap 

 that agree in direction, but also the distances between the curves for 

 hydrogen and air, which are shifted in pairs. 



Nothing could prove more forcibly than this fitting that the differ- 

 ence in the results for different plates depends on a relation between, 

 the density of the gas and the coarseness of the plates. 



Experiments on Transpiration under Pressure. 



According to the theoretical deductions the rate at which gas would 

 be forced through a tube or porous plate by a difference of pressure 

 bearing a fixed ratio to the mean pressure of the gas in passing, would 

 vary with the mean density of the gas according to a law which would 

 hold with different plates, the corresponding results being obtained at 

 pressures inversely proportional to the diameters of the tubes. The 

 differences in the laws of transpiration which Graham found with 

 different tubes and plates are, so far as they go, in fair accordance with 

 the law as deduced from this theory, but the range of densities over 

 which Graham's results extend is too small to allow a very complete 

 verification, and the chief object in these experiments was to extend 

 this range of densities. The apparatus used was the thermo-diffu- 

 siometer, slightly modified, and without the streams of steam and 

 water. The instrument lent itself very well to this part of the investi- 

 gation. It allowed of the measurement of the time of transpiration 

 of a definite volume of gas, measured at whatever might be the pres- 

 sure of the instrument, through the porous plate, under a difference of 

 pressure bearing a fixed ratio to the pressure within the instrument. 



The times of transpiration of equal volumes of air and hydrogen 

 through plates of stucco' and meerschaum were determined at pressures- 

 varying from that of the atmosphere to a fraction of an inch of 

 mercury. 



