OF ARTS AND SCIENCES. 215 



containing fresh anhydride. After drawing air through the system 

 at the same rate and during the same time as in a carbonic acid 

 determination, we reweighed this second tube and found that it had 

 gained 5.5 m.g. We next added to our system the U tube filled with 

 phosphoric anhydride, which is shown in Plate III. immediately before 

 the balloon, and repeated the experiment. There was then no gain in 

 weight perceptible. The observations were as follows. 



0.0055 " 1.4 



After interposing the U Tube. 



1st weight P 2 O s tube 25.9268 grams 299.3 19.6 306.7 

 2d " " 25.9268 " 299.2 21.6 304.6 



2.1 



In this table the column headed H contains the barometric pressure 

 at time of weighing in tenths of an inch ; that headed T° gives the 

 corresponding temperatures of the balance case in centigrade degrees. 

 By adding to the values H the quantities (27 — T)° we obtain the 

 values R, and these are the barometric heights which would give an 

 atmosphere of the same density, and therefore exerting the same buoy- 

 ancy, as the atmosphere at the time of weighing if the temperature 

 were uniformly 27° C. The correction that should be made for differ- 

 ences of temperature and pressure at successive weighings can at once 

 be calculated from these values. We found by experiment that for 

 the phosphoric anhydride tube used in these and in subsequent deter- 

 minations the correction amounted to 0.04 m.g. for each tenth of an 

 inch change of pressure, and hence in the above weighings the correc- 

 tion is too small to be appreciable. This simple method of correcting 

 weights for changes in the buoyancy of the atmosphere has been de- 

 scribed in a previous paper in these Proceedings.* In order to apply 

 it readily, the barometric heights, although usually read in millimeters, 

 were often also noted, as here, in tenths of inches, and the instrument 

 we used was provided with both scales. 



* Vol. xviii p. 55. 



