98 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



vapour in unsaturated air is greater than the tabulated pressure for the temperature 

 of the saturator by 2/622 of the saturation pressure, that is, by an amount varying 

 from '02 mm. to '06 mm. for vapour pressures between 4'94 mm. and 19'65 mm. In 

 other words, to obtain the true pressure the proper value to substitute for d in the 

 formula of reduction of the chemical method is '622, at any rate when the fraction of 

 saturation of the air under experiment is less than '8, but if the air is saturated the 

 value '624 must be substituted. 



NOTES. (November 11, 1887.) (l) In order further to verify the condensation of 

 moisture upon a surface when the air surrounding it is not fully saturated, I have 

 tried to detect the actual difference of weight produced by the deposit. For this 

 purpose 1 have suspended a glass globe from one arm of a balance, and have counter- 

 poised it, surrounding the globe by a cylinder with a lid formed of two closely-fitting 

 pieces of glass, with a small hole left for the suspension wire. Inside the cylinder 

 were placed alternately a vessel of water and a vessel of sulphuric acid. After making 

 all allowances for the density of the air displaced, the experiments show an excess of 

 weight in the moist air corresponding to the condensation of about 1 X 10~ 5 gramme 

 per square centimetre. More accurate experiments are, however, in progress. 



(2) In this discussion DALTON'S law has been assumed to be strictly accurate. If 

 the differences observed by REGNATJLT (p. 120) should prove on further investigation 

 to be real differences, the specific gravity of the saturated vapour must be increased 

 to about '645 for saturated air, and '643 for non-saturated air. 



15. In conclusion, it may be well to give a specimen of the observations from 

 which the tables are compiled, and some notes of the practical conduct of the work. 

 I take one experiment at random from my note-book : No. 68, August 12. 



Initial weight of tubes V., VI. 

 Final 



Gain 



grammes. 



142-8090 



143-3856 



5766 



III., IV. 



grammes. 

 147-9447 

 147-9638 



0191 



Barometer. 

 30-00 

 30'00 



Temp. Balance Case. 

 24' 70 

 25-00 



Total gain '5957 gramme. 



