7oo 



PRELIMINARY OBSERVATIONS. 



its open extremity. The lengths occupied by the different 

 layers of pure oxide, mixture, rinsings of mortar and again pure 

 oxide are indicated by dotted lines in the figure. The weight of 

 the whole oxide of copper used generally exceeds 1200 grains. 

 In these operations, the oxide of copper inevitably absorbs a 

 quantity of moisture from the air, which may amount to 0.2 

 or 0.3 grain, and which coming off afterwards during the ig- 

 nition would vitiate the determination of the hydrogen. The 

 tube and oxide must therefore be dried by a heat which will 

 not decompose the sugar. This is done by placing the com- 



Fm. 86. 



bustion tube C, in a wooden trough D, (Fig. 86') and covering 

 it with sand of the temperature of 250 ; connecting it at the 

 same time by a perforated cork b, with a tube B, containing 

 fragments of chloride of calcium, and an exhausting syringe A. 

 By means of the latter, air and moisture are withdrawn from 

 the combustion tube, the moisture being retained by the 

 chloride of calcium in B ; air is then admitted to C by the 

 stopcock a, and withdrawn again by the syringe ten or twelve 

 times. The mixture may then be considered dry. 



The furnace for the combustion is made of sheet iron of a 



