Appendix to Lecture of Jan. 22, 1915 823 



the neon be seen distinctly, and then the hydrogen is also just visible. 

 One may infer that the ratios of helium and neon diffusion are as 9 to 1, 

 and that hydrogen and helium are nearly alike. But considering 

 the partial condensability of the hydrogen in the charcoal, the actual 

 hydrogen rate would be greater, as indeed is shown by the direct 

 measurements. In the same way, having regard to the slight con- 

 densability of the neon, the above estimate of one-ninth the helium 

 rate would be a minimum. The spectroscopic method cannot, how- 

 ever, be regarded as strictly reliable under such circumstances, as the 

 electric discharge does not necessarily induce the emission of spectral 

 lines proportionally to the several constituents. 



It must also be noted that when the discharge- tube had outside 

 electrodes, the first helium line to be seen was the green ; and that it 

 showed in at least one-fifth of the time necessary to bring out the 

 neon lines in these conditions, compared to about two-fifths of the 

 l^eriod required by the tube provided with platinum wire poles, which 

 gives the yellow helium line first and not the green. The method, 

 nevertheless, is capable of useful extension. 



PrOPORTIOX of " UXCONDEXSABLE " GaSES IX THE PRODUCT OF 



Air Diffusiox through PiUbber. 



One experiment was made with the object of ascertaining the 

 proportion of the uncondensable gases —helium, neon and hydrogen 

 — in the total material diffusing through a rubber membrane in air. 

 The method of condensation at 2u" Abs. was employed as described 

 in the Discourse a year ago. 



From this it appeared that after two hours' diffusion through a 

 membrane of 48 sq. cm. surface, with an air rate at 15° C. of 0*74 c.c. 

 per minute, the amount of uncondensable present was 17 '8 parts per 

 million ; but that this value was increased to 120 after 4 hours, or 

 nearly 7 times as much. The value for atmospheric air being about 

 27 parts per million, there would thus appear to be a five-fold 

 increase of the uncondensable material in this diffused gas. 



The total pressure of diffused gas passing into the highly ex- 

 hausted McLeod gauge of 5G0 c.c. volume Avas 9*11 mm. ; so that 

 the partial pressure left at 20° Abs. was 0*0011 mm. However, no 

 precaution was taken to dry the air coming into contact with the 

 membrane, as described in the previous experiments. It is therefore 

 possible that the metal of the membrane support was attacked to 

 same extent by the water vapour, thereby giving an increased pro- 

 portion of hydrogen, so that the present result requires confirmation. 



Diffusion through Rubber Membraxes ix Liquids. 

 Water Vapour Transmission. 



The diffusion of water vapour has already been alluded to in 

 connection with the necessity of drying the air over the membranes 



