,s24 Professor Sir James Dewar 



when studying the composition of the gases diffusing for an ex- 

 tended period. The first rough measures on the oil manometer of 

 the rate of water- vapour transmission, by comparing wet and dry air, 

 seemed to indicate that at ordinary temperatures water vapour had 

 at least forty times the rate of dry air reckoned at the same pressure, 

 l)ut that at high temperatures the proportion was much less. More 

 exact determinations were necessary to settle this, and the method 

 adopted was to weigh the diffusing water vapour after condensation 

 in a cooled U-tube in the exhausted space behind the membrane, the 

 cooling agent employed being liquid air or solid carbonic acid. 



A preliminary trial on these lines showed that when air from the 

 room was diffusing through a 25 sq. cm. membrane and the cooled 

 U-tube, into the exhausted Mclicod gauge, then a visible ice deposit 

 Avas obtained in the U-tube in less than 10 minutes, and that nearly 

 oO mgms. of water were obtained in a day. 



A mounted membrane of 12 J sq. cm. area was then immersed in 

 water at 15' C. In 18 hours oG'8 mgms. of w^ater were obtained, 

 which is a rate of o " 93 mgms. per day through each square centi- 

 metre. To compare with the values given above for air this may be 

 transferred into volume of water vapour reckoned at N.T.P., when 

 the rate becomes -t * 88 c.c. per day per sq. cm. 



Neglecting any effect of the water immersion on the character of 

 the membrane, and assuming that the diffusing water vapour was 

 merely that due to the tension of water at 15° C, then we have only 

 12 mm. pressure instead of 760 mm. for the air diffusion. Therefore 

 the proportional value for water vapour at an equal pressure would 

 be 1)4: times that given by air, reckoned weight for weight, or IQ'd 

 times in relative volumes, the figures then becoming 249 mgms., or 

 310 c.c. of N.T.P. water vapour. 



The effect of dissolving a salt in the water round the membrane 

 was next tried. Taking a 40 per cent solution of calcium chloride 

 at 15° C. as an example, the rate of transmission of water vapour 

 was reduced to 2*45 mgms. per sq. cm. per day, equal to 3 * 04 c.c. 

 of N.T.P. vapour, being about 70 per cent of the rate of pure water. 

 It may be added that the tension of water vapour from such a 

 solution at 15° C. was found to be one-half that of water. 



In contrast to the salt a colloidal substance was tried, as having 

 no effect on the vapour pressure. Gum acacia was used as an 

 example of this ; a distinctly viscous solution of 1*07 S.G. gave the 

 same rate very nearly as pure water on the same membrane. The 

 actual rates per sq. cm. per day were 4-89 mgms. for pure water 

 and 4 -70 mgms. for the gum, or not more than 2 J per cent reduc- 

 tion. 



A determination with the membrane in pure alcohol gave 

 5G mgms. condensed from U hours' diffusion, which is a rate of 

 24 mgms. per sq. cm. per day, equivalent to 11 "G c.c. of alcohol 

 vapour reckoned at N.T.P. ; this is about the same rate as hydrogen 



