240 SMITH'S INTERMEDIATE CHEMISTRY 



example, would entirely stop the reproduction of these substances, 

 and so would enable the forward action (4HC1 + 2 2H 2 O 

 + 2C1 2 ) to ru-n to completion. 



This might be realized by causing one end of a sealed tube 

 charged with the substances, after the contents had settled down 

 to a condition of equilibrium, to project from the bath in which the 

 whole had been kept at 345 (Fig. 66, which is simply diagram- 

 matic). By cooling this end, a 

 large part of the steam would 

 quickly be condensed in it to the 

 liquid form, while the other sub- 

 stances would remain gaseous. 



In other words, the concentra- 

 tion of the water vapor would be greatly reduced. In fact, only 

 the trace of vapor which cold water gives would then be avail- 

 able to interact with the chlorine, and reproduce hydrogen chlo- 

 ride. Meanwhile the decomposition of the latter would go on, and 

 thus, eventually, almost all the water would be found in one end of 

 the tube, and the chlorine, all free, would occupy the rest. By 

 this purely mechanical adjustment the chemical change would 

 therefore be carried from 80 per cent completion to almost absolute 

 completion : 



4HC1 + O 2 * 2C1 2 + 2H 2 (vapor) * 2H 2 O (liq.) 



If, on the other hand, arrangements were made to have pow- 

 dered marble, in a sealed bulb of thin glass, enclosed in the tube, 

 we might imagine the very opposite of the above effect to be pro- 

 duced. The breaking of the bulb of marble, when equilibrium 

 had been reached, would provide means for the removal of all the 

 hydrogen chloride,* while the other three substances would still be 



* The hydrogen chloride would be destroyed by interaction with the 

 marble: 



2HC1 + CaCO 3 -> CaCl 2 + CO 2 + H 2 O. 



The calcium chloride is a solid. The gas, carbon dioxide, does not interact 

 with the other substances, and would not, therefore, interfere with the forma- 

 tion of fresh hydrogen chloride. 



