176 SCIOPTICON MANUAL. 



blown from it into a saucer, or to the aperture itself if pro- 

 tected by a fine wire gauze thimble ; but such troublesome 

 precautions rather tend to incur a risk, where there 

 would be none without them. We can judge near enough 

 from appearances when to complete the connection. 



Where the precaution of diluting the sulphuric acid 

 and allowing the mixture to cool has been neglected, 

 and sufficient time cannot be allowed for the purpose, 

 the zinc and water may be placed in the generator, and 

 the concentrated acid slowly poured down the tube 

 funnel as it is required. 



It is equally important that, before collecting the gas, 

 the bag in which it is to be received should be pressed 

 quite flat, or rolled with the stop-cock open, so as to 

 exclude all trace of atmospheric air. The time when 

 pure hydrogen is coming off may be known by the rapid 

 rise of the bubbles to the top of the water, and by the 

 accompanying sound, which the ear will recognize, after 

 a little practice, as being unlike that of other gases. 

 The purifier e should be about half filled with water; 

 and connection being made between the exit tube / 

 aid the gas bag by means of india-rubber tubing, as 

 shown in the cut, bo careful to turn on the stop-cock s, in 

 order that the gas may have free entrance into the bag. 



The process here given is the simplest of the several 

 in common use, and the best for collecting hydrogen 

 gas in a not very largo quantity. The self-condensing 

 gas cylinder, to be next described, promises to super- 

 sede the more complicated methods, so that their inser- 

 tion here would be useless. 



It may be proper here to suggest, that after an exhi- 

 bition the bags, particularly the one marked "H," if 

 not to be soon used again, should be completely emptied, 

 not only to preserve them, but to insure having fresh 

 gas next time. 



