82 OPTICAL PROJECTION 



again into a blaze, in which case the match is thrown safely 

 away, the end of the tube slipped over the nozzle of the bag, 

 and the bag- tap instantly turned fully open. Then the speed 

 of the gas must be noted, and the rest of the preparation 

 proceeds as described. 



The bags should be filled tightly, unless for immediate 

 use. When full, the tap is turned off, and the tubing 

 detached the very moment afterwards. The next step is to 

 detach the first piece of tubing from the delivery-pipe of the 

 retort, and this must be done before the heat under it is 

 diminished. The reason is, that if the flame were lowered 

 first, for any appreciable time, the cooling would reduce the 

 pressure, and so suck back a portion of fluid from the purifier. 

 Such might possibly result in a steam explosion, which has 

 been known to happen from such a cause, but of which there 

 is no business to be any risk at all. When the retort has 

 cooled down a bit, the top should be unscrewed, water poured 

 in, and the contents washed out, adding one or two waters 

 till there is no blackness left to speak of. It saves much 

 time and trouble to wash it out at once, while warm, and the 

 residue corrodes the iron if left in. When washed out, the 

 retort should also be dried out, which is readily done on 

 the hot plate of a kitchen stove. 



CHAPTER VI 



COMPRESSED GASES 



FOE years oxygen and hydrogen had been used for the lime- 

 light compressed in iron cylinders furnished with screw valves to 

 their nozzles, the gas feeding the jet by its own pressure until 

 this was gradually reduced to that of the atmosphere. Such 



Should a full bag ever be tested in this way, and the match fall in, the result 

 would be such rapid combustion as to be tantamount to an explosion. There 

 should pot be the slightest risk run of such an accident. 



