MK. \V. N SHAW OX HYGIM.MKTKIC MKT1K !>> 121 



favour of that suggestion ; attributing, in fact, the diminished pressure of vapour to 

 the molecular action of the glass side of the vessel upon the saturated vapour contained 

 in the air, producing a condensation upon the glass, the slowness of diffusion preventing 

 the pressure reaching its maximum value by consequent evaporation.* 



The second and third questions may likewise be disposed of theoretically, or, 

 perhaps I should say, hypothetically, provided aqueous vapour may be regarded as 

 behaving like a perfect gas when its pressure and temperature are made to vary. 



In that case the specific gravity of steam referred to hydrogen may be determined 

 from its molecular weight, and the specific gravity referred to air may then be 

 calculated from the known specific gravity of hydrogen ; this theoretical value would 

 of course be constant for all temjwratures and pressures, and equal to 0'622. Several 

 series of experiments to ascertain if such were the case are detailed in the paper we 

 are now considering. The first two series of experiments were made on water vapour 

 in vacuo, and showed that the number quoted, '622, was, within the limits of error of 

 experiment, the true value of the specific gravity, provided the fraction of saturation 

 of the vapour experimented on did not exceed 0'8, but the specific gravity is sensibly 

 greater when the state of saturation is more nearly approached. The third series of 

 experiments was made by applying the chemical hygrometric method to air artificially 

 saturated with moisture at a known temperature. A volume of this air was made to 

 pass through two drying tubes of sulphuric acid and pumice.t by means of an aspirator 

 whose capacity was accurately determined ; the gain in weight of the drying tubes 

 gave the quantity of water vapour contained in unit volume, and this could also be 

 calculated from the known pressure of saturation at the temperature of the saturating 

 vessel, assuming the theoretical value '622 for the specific gravity of the vapour. A 

 comparison of the results obtained serves to show whether this assumption of '622 as 

 the value of the specific gravity is justifiable or not. 



The series comprises 68 experiments on air saturated at temperatures varying 

 between C. and 27 C. The following table shows the mean percentage difference 

 between the observed and calculated values of the mass of moisture per unit of volume 

 at the different temperatures : 



' This method of accounting for the discrepancies between hypothesis and experiment in reference to 

 DALTON'S law has been since confirmed by an experiment of HKKWIO (PoouKN DO BIT'S ' Annalen,' vol. 137, 

 1869, p. 592) upon compression of vapours. He found that the pressure of the vapour could be increased 

 beyond the point at which a deposit was first formed on the sides of the vessel, and that the vacuum 

 saturation pressure was the increased pressure, and not the pressure at which the deposit is first formed. 

 (See also p. 96.) 



t A number of preliminary experiments were made to verify that the sulphuric acid was efficient as an 

 absorbent. 



MDCCVI \\XVIII. A. R 



