COMPRESSED GASES 83 



a cylinder cost little more than a bag, while the gas in it did 

 not deteriorate by osmosis, and might be kept for months. 

 But these bottles were very heavy, and the charge of Sd. per 

 natural cubic foot for each gas was prohibitory as regards 

 general use with the mixed jet, amounting to 13s. d. for 

 20 feet of gas, against about 2s. Gd. to an exhibitor who 

 made his own oxygen, and filled the other bag from the 

 nearest gas-pipe. The gradually diminishing pressure also 

 necessitated a constant turning on of more at the screw 

 valves ; so that to work a mixed jet required a skilled hand 

 to manage the light alone, and dissolving was very difficult, 

 the least mal- adjustment of the dissolver generally causing 

 such accumulation of pressure behind as to blow off the 

 rubber tubes. The use of cylinders under these conditions 

 was almost nil for the mixed jet, though a cylinder of oxygen 

 would often be purchased to save the trouble of making it, 

 or to take it portably to the place of exhibition, where it 

 would be used to fill a bag, which would then be worked in 

 the ordinary way. 



43. Use of Cylinders alone. With the oxy-gas jet it was 

 different. Only one of the gases had to be compressed and 

 paid for with this system, and for it cylinders came into more 

 or less use. For such jets very precise regulation is not 

 required, and a little more can be readily turned on from 

 time to time. Even now such jets are often used in this 

 simple way, direct from the cylinders and without regulation. 

 In working thus, the cylinder is simply secured as near the 

 jet as possible (in order to use a short length of elastic tube), 

 and connected with it as if it were a bag. Some advise 

 tying on the tube at both ends with strong tape as strongly 

 as possible, but this is not advisable, as if too much pressure 

 is turned on, the tube itself is apt to be ripped up with rather 

 a loud pop, which might startle the audience. The best plan 

 is to turn back outside an inch or so of each end of the rubber 

 tube which will tighten the ends quite as much as is desirable. 



42 



