80 Mr. E. P. Perman. [Feb. 4, 



It must be here remarked that the experiments at the lower 

 temperatures are by no means so reliable as those at the higher 

 temperatures, for the following reasons : 



(1) They were the first carried out, before experience in working 

 the experiment had been gained. 



(2) The amount of water carried over was always very small, 

 so that it is impossible for these experiments to be so accurate as those 

 in which the water is carried off more rapidly. 



(3) A rubber connection was used at J (fig. 1), whereas in the 

 later experiments this was replaced by a ground-glass joint. Also, 

 the absorption flasks were weighed with glass stoppers connected 

 by rubber tubes, and it was found that moisture from the air found 

 its way slowly through the rubber, although the glass tubes were 

 made to touch, The absorption apparatus was in this way found to 

 take up sometimes as much as 0'5 mg. in an hour, which is 

 quite sufficient to account for the highness of the numbers obtained 

 in these experiments. This may be, perhaps, included under 

 reason (1). 



At the higher temperatures there was no rubber connection through 

 which moisture could pass to the absorption apparatus, and the 

 apparatus was closed with small rubber stoppers inserted into the 

 tubes. With this arrangement the weight was found to be constant. 



Deductions from Experimental Results. 



The vapour pressures obtained are seen to agree very closely with 

 those obtained by direct measurement. Now, in the calculation, 

 certain assumptions have been made, viz., Dalton's law of partial 

 pressures, and that the density of aqueous vapour is normal. It 

 would thus seem that these assumptions are justified. It is highly 

 improbable that deviations from Dalton's law, and from the normal 

 density, should so balance one another at all these temperatures 

 as to give correct vapour pressures. Fortunately there is inde- 

 pendent evidence of the validity of Dalton's law under the con- 

 ditions of the experiments. Galitzine,* by some well-devised experi- 

 ments, showed that there was no appreciable deviation from Dalton's 

 law for air and aqueous vapour at moderate pressures below 100 C. 



It may be concluded, then, that the density of saturated water 

 vapour at the pressures and temperatures employed is very near 

 the normal value. This conclusion seems to the author quite justified 

 by the experiments described, although it is at variance with the 

 work of a number of other investigators, who have found the density 

 to be greater than normal, usually from 1 to 2 per cent., sometimes 

 much more. 



* ' Wied. Aim./ 1890, vol. 41, p. 588 



