MR. \V. N. SHAW ON HYGROMETRIC METHODS. 



85 



It appears, therefore, that after the tube had been once dried no further increase of 

 weight of the sulphuric acid tube occurred, and hence that, with due precaution, glass 

 tubes may be used as connexions without any fear of error. 



II. The glass tube was replaced by an india-rubber tube six feet long, and similar 

 observations taken, fifteen experiments being made. The sulphuric acid tube always 

 gained in weight, although every precaution was taken to keep the india-rubber tube 

 dry between the experiments. The least amount of moisture was obtained when a 

 second observation was taken, immediately after the completion of a first. The 

 increase of weight of the sulphuric acid tube generally amounted to about 15 milli- 

 grammes. An india-rubber tube cannot, therefore, be used with any security for 

 connecting two drying tubes. 



7. We may accordingly conclude that the increase in weight of the drying tubes 

 after the first, in the table of results given, was due for the most part to moisture 

 derived from the india-rubber connexions. For the last three observations in the 

 table, p. 84, these connexions were made as short as possible, so that the amount of 

 india-rubber surface exposed to the dry air might be small. With the apparatus in 

 that form the drying tubes gained very much less in weight than before, and we may 

 give the results obtained from these three observations as instances of the accuracy 

 which may be expected by this method. 



THE METHOD OF SATURATION AND THE CHEMICAL METHOD. EXPERIMENTS WITH 



UNSATURATED AIR. 



8. With the experience gained by the experiments just described, I proceeded to 

 arrange the apparatus by which the result of the chemical method could be compared 

 with that of the saturation method on unsaturated air, and by which moreover dew- 

 point observations could also be taken during the progress of an experiment such as 

 those described. At first a REGNAULT dew-point instrument alone was introduced. 



It was fixed by means of its glass tube into the cork of a three-necked globe in a 

 manner which will be sufficiently indicated by a glance at the figure (fig. 3) ; on each 

 side of the globe, between its long horizontal neck and the rest of the apparatus, was 

 attached a coil of copper tube to ensure that the temperature of the internal air was 

 the same as that of the external, except in the vessel containing the hygrometer 



