702 PRELIMINARY OBSERVATIONS. 



sorption of the carbonic acid produced in the combustion, 

 This instrument consists of five balls, of which m is larger than 

 the others ; no more of the potash ley is put into it than fills 

 the three central bulbs, leaving a bubble of air in each. One 

 corner is elevated a little by a cork placed under it, and the 

 whole supported upon a folded towel ; the potash apparatus, 

 when filled with ley, commonly weighs from 750 to 900 grains. 

 This apparatus is also weighed before and after the combustion, 

 and the increase ascertained. 



Before introducing the combustion tube into the furnace, it 

 must be tapped smartly in a horizontal position, so as to pro- 

 duce a vacant space above the oxide of copper through the 

 whole length of the tube, by which the gaseous products may 

 escape (Fig. 85). The same precaution must be taken in the 

 preparatory operation of drying the oxide of copper, otherwise 

 it often happens that a portion of the oxide is thrown forwards 

 out of the tube. Before beginning the combustion, it is neces- 

 sary to ascertain that all the joinings are tight, by sucking out 

 a bubble or two of air from the apparatus, by means of the 

 FIG. 90. suction tube (Fig. 90), applied by means of a per- 

 forated cork not fitting very tightly to the open end 

 of the potash apparatus. The slight exhaustion 

 causes the ley to stand I \ or 2 inches higher in 

 the inner limb m of the potash apparatus than in 

 the outer limb. This elevation will be maintained 

 if no air enters by the cork or caoutchouc joint, 

 and the apparatus is then certainly tight, but not so if the 

 level changes and the liquid falls back into the middle part of 

 the apparatus. 



In conducting the combustion, the anterior portion of the 

 tube, containing only oxide of copper, is first surrounded by 

 red hot charcoal. The fragments of charcoal are kept in their 

 place and the heat prevented from spreading, by a screen, 

 FIG. 91. (Fig. 91) of sheet iron, of the same width as the 

 furnace. This screen is slowly moved backwards, by 

 half an inch to an inch at a time, and the fire space 

 immediately filled up with red hot charcoal, so as to 

 raise rapidly the portion of tube newly exposed to 

 a red heat. A. screen (Fig. 92) should be placed upon the front 



