Chemical Absorption of Gases. 355 



raised to allow the potash to run into A. b is a small ground 

 glass stopper, the opening to the connecting-tube being very- 

 narrow ; this was raised simultaneously with a to afford an 

 exit for the displaced mixture of gases. Hydrogen or air, as 

 the case might be, thoroughly dried and purified, was introduced 

 by the tube A, and the absorbable gas by d. In preparing the 

 latter gas, all the air in the generating and drying apparatus 

 was expelled by passing down £, having a mercury or oil-of- 

 vitriol trap, and finally through a potash tube attached to k : the 

 completed expulsion of the air was indicated by no gas passing 

 through the potash. C is a small Sprengel-purup for exhausting 

 the apparatus, the mercury-fall tube being about 200 millim. 

 long. B is the manometer ; the tube, which was carefully 

 selected and calibrated, had a very uniform internal diameter 

 of 1*46 millim. ; the scale, a very fine one engraved on glass, 

 was divided into 500 millim., and inlaid into the wooden sup- 

 port with a backing of white paper. 



As it was important that the manometer should be perfectly 

 reliable in its indications, considerable time was expended in 

 testing its accuracy. At first an air-manometer was employed, 

 having a comparatively large air-chamber inserted in a water- 

 bath to keep the temperature constant, the index being a 

 globule of mercury moistened with oil-of-vitriol. Although 

 it worked admirably, it had to be abandoned owing to the 

 difficulty of maintaining the index at the zero-point, due to 

 fluctuations of barometric pressure and temperature. Oil-of- 

 vitriol was next tried ; but this was found to be utterly useless, 

 owing to the viscosity of the liquid, for measuring variable 

 pressures ; for on exhausting the whole apparatus the column 

 of acid would continue to rise from 5 to 15 millim. after the 

 exhausting had ceased, whereas when air or water was em- 

 ployed the stoppage of the pump and the movement of the 

 index were practically simultaneous. Mercury could not con- 

 veniently be used in the manometer, as the experiments were 

 designed to be made at comparatively low pressures. Finally, 

 it was decided to use water in this part of the apparatus ; and 

 to prevent ingress of aqueous vapour into the absorption- 

 vessel, a small tube c, containing fused calcic chloride, was 

 attached between the two vessels. 



To perform an experiment, A was disconnected and tho- 

 roughly dried and placed in position; a current of air — passed 

 through strong potash, oil-of-vitriol, and about two feet of 

 tubing packed with fused calcic chloride — was swept through 

 the apparatus for about one hour. When hydrogen was em- 

 ployed as the atmosphere, the gas, generated from zinc and 

 dilute hydric sulphate, was passed through strong potassic 



