THE LIME-LIGHT 59 



' safety packing,' it is impossible to say what pressure may be 

 needed to drive the gas through them. 



31. Safety arrangements. This brings us to the most 

 important point. The one, the sufficient, and the only security 

 for safety with the mixed jet, is the certainty of pure unmixed 

 gases in the bags to start with, and next, a good and fairly 

 equal pressure on both bags, with sufficiently free vent at the 

 nipple of the jet to make sure the gases shall always be flow- 

 ing outwards. This is simple, easily understood, and, when 

 understood, easily carried out. No other safety arrangement 

 can be depended upon, and if it is depended upon, to the pre- 

 judice of proper management in these respects, may be even 

 worse than useless. Self-acting valves which allowed passage 

 outwards, but not back, were soon invented, and are still sold. 

 I am not prepared to say that they are inoperative, for there 

 is evidence that, experimentally, they have prevented ex- 

 plosions which otherwise must have occurred. But such 

 arranged * experiments ' rarely represent the conditions of 

 unforeseen accidents ; and such occurred more than once in 

 spite of these valves. They also obstruct the flow of gas, 

 which is one safeguard. But the real mischief is, that people 

 who use these things usually depend upon them ; and that 

 means real danger. Packings of gauze in the jet are absolutely 

 useless, if the gas behind is in a really explosive condition. 

 The packing of granulated pumice presently described is 

 indeed a real stop to explosion, so long as it is in order ; but 

 it checks pressure materially, and a few ' snaps ' in front of it 

 may so disarrange and alter it by repeated shocks, as to render 

 it useless. With the double pressure-boards, a cool and in- 

 telligent operator ensures that explosive conditions cannot 

 occur, and that alone is true safety. 



32. Forms of the Mixed Jet. This brings us to the forma 

 procurable of the mixed gas jet, its faults and difficulties, 

 and the construction which will give the best results. Daniel's 

 form, one of the first made, resembled the blow-through jet D 



