334 



Mr. W. N. Shaw. 



[Jan. 26, 



the value 0*622 gave results which were less than the tabulated 

 pressures, the errors being always of the same sign, but so small in 

 amount that he neglected them in his subsequent work. 



The ultimate object of the experiments described in the report was 

 to examine the behaviour of dew-point instrumeDts in air of known 

 state, and for this purpose air was saturated at a known temperature 

 and drawn by an aspirator through vessels in which the dew-point 

 instrument could be placed when required, and subsequently through 

 drying tubes of special pattern. The vapour-pressure was thus 

 obtained at the two extremities of the train of apparatus and the 

 results compared. 



The following questions are raised and discussed : — 



i. Were the drying tubes used as efficient as Regnault's ? 



ii. Does the pressure of vapour in the air become changed by passing 



through the apparatus designed to contain the dew-point 

 instrumeuts, or by the mere presence of those instruments 

 themselves ? 



iii. Do the results of the chemical method agree with the tabulated 



vapour-pressures in vacuo when the air is more or less heated 

 after being saturated ? 



iv. Can the observed differences between the results be obviated by 



assuming a value for d (other than 0*622), which is com- 

 patible with values obtained by other methods ? 



v. Can any reason be assigned for the differences observed by Reg- 



nault in the case of saturated air ? 



(i.) The answer to the first question is given in an account of a 

 series of twelve experiments practically repeating Regnault's obser- 

 vations with saturated air. The tabulated results show divergences 

 in the same direction and of the same order of magnitude as those in 

 Regnault's paper. Some incidental points are also discussed namely, 

 the comparative efficiency of phosphoric anhydride, sulphuric acid, 

 and calcium chloride, and the effect of india-rubber and glass con- 

 nexions between drying tubes. It is shown that the sulphuric acid 

 and phosphoric anhydride tubes are efficient, that as a rule one 

 tube is all that is strictly necessary, but that two should be used to 

 provide for the case of exhaustion of the first tiibe or too rapid flow 

 of air, and further, that the glass and mercury connexions between 

 the tubes employed in the second series of experiments cannot be 

 regarded as producing any effect. 



(ii and iii.) The answers to the second and third questions are fur- 

 nished by the results of eighty-two experiments with the chemical 

 method upon air saturated at known temperatures by a specially designed 

 " saturater " in a water bath. The temperatures of saturation lay 

 between 1° C. and 21° C, and, with one exception, were below the tern- 



