MB. W. N. SHAW ON HYOROMETRIC MKTIIODS. 99 



The balance was a short-beam OERTLING, and the weights a box of OERTLINO'S, 

 compared with the Laboratory Standard. The weighings were carried to O'l mgm. 

 The drying tubes after an experiment were transferred to a desiccator to cool, and 

 then weighed. The barometer and temperature of the balance case were read at 

 each weighing, and recorded, so that a correction for weighing in air might be 

 introduced if necessary. (See 4.) For Experiments 1-27 the tubes were filled with 

 sulphuric acid and pumice, and those for 28-82 with phosphoric acid. The alterations 

 in the state of surface of the tube are sufficient to prevent the accuracy of weighing 

 to O'l ingm. being entirely trustworthy, but the error is less than 1 mgm. (See 

 p. 74.) This limit gives to the accuracy of the weighing operations in the chemical 

 method a limit of '6 per cent, for the smallest amount of moisture, viz , '1737 gramme, 

 and of '16 per cent, for the largest amount, viz., '6880 gramme. 



The duration of the experiments was generally about two hours, varying, in fact, 

 from 1 hr. 10 min. to 2 hr. 55 min. During that time 36 litres of air passed through 

 the tubes. The amount received by the second tube was very variable. In 29 experi- 

 ments out of the 82 it was less than 1 mgm. ; in one experiment, however, it reached 

 99 mgm. The amount was always added to that received in the first tube, for reasons 

 previously stated ( 6-8). 



Before an experiment was commenced dried air was drawn through the whole of 

 the apparatus, except the saturator and aspirator, by the use of an aspirating water- 

 pump for from a quarter to half an hour, to clear the tubes of deposited moisture ; 

 for this purpose the third aperture of the three-way tap was employed. After this 

 the plug of the tap was turned, and the air passed through the saturator for a 

 quarter of an hour or more to fill the vessels with air in the state required for the 

 observation. The pump was then cut off, the aspirator connected, and the drying 

 tubes put in their places ; the entry tube to the saturator was then closed, and the 

 tightness of the apparatus tested by reading the pressure gauge and observing the 

 cessation of drip from the aspirator. At the close of an observation the moist air was 

 swept out of the globe, &c., by dried air (using the water-pump), unless it was 

 intended to take a second observation immediately ; when left over night, the globes, 

 <&c., were connected with a calcium chloride bottle. 



The temperature readings were by the following thermometers, compared at Kew, 

 us described in 3 : 



Saturator ;. .- . . Exp. 1-14 GEISSLER, C.L.O. 21 graduated to 0'l C. 

 .*. . :,.,. 15-82 HICKS 79,916 0'2 C. 



Air ... ,<.,., o|, [.{, . . . . HICKS 87,400 O d '2 C. 



REOHAULT..,,!,, v., ^.. l ,. ir .^ .... . HICKS 79,915 0'2 C. 



DINES. Its own thermometer, by HICKS (not compared). 



Aspirator. An ordioary chemical thermometer compared in the Laboratory, . 



graduated to 0'5 C. 



Other small points of detail, employed to secure as close an accuracy as possible, 

 may be passed over. 



o 2 



