88 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



filled with water and supported near the surface of the 

 trough. 



The gun-barrel, 7 (second plate : compare Hales, Fig. 

 19^), with a tobacco-pipe stem, or glass tube luted to the 

 open end, was used to expel gases from solid substances. 

 To reduce the volume of air, the gun-barrel was filled up 

 with dry sand. In the figure, the gas is being collected 

 over mercury ; Cavendish had transferred to a basin of 

 mercury, a bottle full of fixed air which was too soluble to 

 be stored long over water, but this is the first use of 

 mercury for collecting gases ; a carefully-shaped MERCURY 

 TROUGH was introduced by Lavoisier (Fig. 33). An 

 apparatus for . collecting over mercury gases expelled from 

 liquids by heat, or set free by the action of acids, is shown 

 at 8, where a is a basin of mercury, b a tube filled with it, 

 c the tube from which the gas is expelled and d a glass trap 

 to condense out moisture ; the apparatus used for generating 

 and collecting gases not freely soluble in water is shown at e 

 in the first plate. 



The bladder 9, provided with funnel and delivery tube, 

 was used to receive gas from a jar standing in water and 

 then to transfer it free from water to a vessel standing in 

 mercury. A bladder is also shown in the apparatus, 10, 

 used for impregnating liquid with gas, e.g., water with fixed 

 air ; the gas generated in c was collected in the bladder and 

 then squeezed out through the flexible leathern tube, d, into 

 the flask a. 



The candle, i2<z (first plate), mounted on the end of a 

 wire b, was used to test the air in the narrow tube 1 1 ; the 

 candle, c, was used in the wider cylinders, from which it could 

 be withdrawn through the water as soon as it was extin- 

 guished. 



The syphon, 13 (second plate), was used to suck air out 

 from a cylinder as at /(compare Hales, Fig. iqa). 



The receiver, 14, exhausted by an air-pump, was used 



