MR. W. N. SHAW ON HYGROMKTRIC MKTIK >IS. 



81 



One of these two gives the volume of water run out in any experiment. The water 

 would be replaced partly by air which had come through the drying tubes and partly 

 by the vapour of the water formed in the aspirator. We may assume that the air in 

 the aspirator at the end of the experiment in saturated with moisture. 



(ii.) The drying tubes, A specially constructed form was used. Instead of being 

 closed with corks perforated with glass tubes, glass connexions of wider bore were 

 used, which were thickened and ground into the U-tubes. These latter were of the 

 ordinary size, about 6 inches long and i^-inch internal diameter. The long tubular 

 stoppers were bent over, in the case of sulphuric acid and pumice tubes through two 

 right angles, and in the case 'of phosphoric acid tubes (shown in fig. 2, Plate 5), 

 through one right angle. The wide ends of these tubes could then pass over narrower 

 tubes coming vertically through the bottoms of small mercury cups, and thus forming 

 the connexions between the drying tubes and the other parts of the apparatus. The 

 connexions were thus made by means of mercury joints. These joints were tested, 

 and found to be quite tight for differences of external and internal pressure many 

 times greater than those occurring in the experiments. The arrangement is very 

 convenient, as the tubes can be simply lifted from their places and as easily replaced ; 

 they require careful brushing to remove the adhering mercury, and the ends are closed 

 for weighing with small india-rubber stoppers. The liability to error in consequence 

 of moisture on the surface of the mercury is probably not so great as that to which 

 the tubes would lie exposed by using india-rubber connexions. The drying tubes 

 were filled either with phosphoric anhydride, or with rather coarse fragments of 

 pumice saturated with the strongest sulphuric acid (sp. gr. T84). Experiments will 

 be detailed below to show that either of these substances is perfectly efficient for the 

 purpose of withdrawing all the moisture from the air passed over it. A number of 

 experiments have shown me that drying tubes filled with recently fused chloride of 

 calcium, although in many ways convenient, are not capable of extracting alt the 

 moisture from air.* 



Correction of the iceight of the drying tubes for weighing in air. The main part of 



* The experiments were of two kinds : ' 



(1.) Two chloride of calcium tubes were arranged in front of two sulphuric acid tubes and nn 

 aspirator- ful of saturated air passed through 11 four. The gain of weight in each of the four for three 

 observations is given below : 



(2.) Tho two aspirators were used to determine by two independent .simultaneous observations the 

 MDCCCLXXXVTII. A. M 



