174 SCIOPTICON MANUAL. 



than half full of water. Lute on the cap with moistened 

 plaster of Paris, and make the connections as shown in 

 the diagram, except that the outlet pipe,/, of the wash- 

 bottle may be left open for a moment or so, until the 

 flow of gas expels the air ; see that the stop-cock is open 

 and that the tubing is unobstructed. 



Apply sufficient heat to almost immediately melt that 

 portion of the charge in contact with the bottom of the 

 retort, then as the rest melts in turn the operation will 

 be gradual. A slow fire is to be avoided ; for it, after a 

 tedious waiting, raises the whole charge to about the 

 melting point, when the decomposition suddenly pro- 

 ceeds with frightful rapidity, perhaps choking the pas- 

 sages and parting the connections. The connections, 

 however, are so easily parted that there will be at the 

 worst only annoyance and loss of gas, but no danger. 

 It is a common recommendation to abate the heat if the 

 flow is too rapid, but with a good heat from the start, 

 the operation is expeditious and safe. 



"When the bubbling ceases and we conclude from the 

 quantity of gas that the charge is spent, we disconnect 

 the retort and remove it from the fire, and close the stop- 

 cock at the bag. 



It rusts the retort less to break up the residuum with 

 a rod, getting it out dry; but it is easier and perhaps 

 better to pour in water and rinse it out, drying the 

 retort directly afterwards. 



PREPARATION OF HYDROGEN GAS. 



Hydrogen, one of the constituents of water, is pro- 

 duced by decomposing that fluid with zinc and sulphuric 

 acid. A few hours before generating the gas, a mixture 

 of, say four pounds of strong sulphuric acid (oil of 

 vitrol) and four quarts of water is made. Consider- 



