Ml! W. N. SHAW ON HTGROMETRIC METHODS. 



83 



protect the thermometer from external radiation, as well as to ensure the complete 

 saturation of the air. 



5. A series of observations was first made, with the view of testing the drying 

 tubes and other parts of the apparatus. There were two points to be determined : 

 first, whether the desiccating substances used could be regarded as completely drying 

 the air passed over them ;* and secondly, whether one drying tube was sufficient for 

 the purpose, or two or more were necessary. For this purpose four drying tubes 

 were mounted, two being filled with phosphoric anhydride in the ordinary form of 

 white powder ; the other two were filled with pumice moistened with commercial 

 sulphuric acid (sp. gr. T84). The pumice was broken into course fragments, and 

 before being used was saturated with sulphuric acid, and heated to redness. 



The saturated air was then sent through all four tubes, and usually the gain in 

 weight of each tube determined. From the gain in weiyht of flic first tube alone, 

 the pressure of vapour in the saturator was calculated by the formula 



e = 



760 (1 + at) 



tc B [ e 

 V" B-E 



L*L"? 



1 + at 



and this was compared with the pressure as given by REGNAULT'S table for the 

 temperature indicated by the thermometer in the saturator, corrected by the table of 

 Kew corrections, and a determination of the freezing-point during the course of the 

 series of observations. The subjoined tables give the results obtained. Table I. is a 

 specimen of the observations as they were taken, and Table II. gives the collected 

 results. 



TABLE I. Experiment 10. Aspirator A. 



Tension of vapour calculated froui Tube I., II., 16'66 mm. 



See note at end (p. 148). 

 M 2 



