90 OPTICAL PROJECTION 



sufficient to do so. It is most convenient to arrange a pair 

 of cylinders upright in a little packing-case, which stands 

 firmly on the floor behind the lantern, and is also convenient 

 for carriage ; but when one cylinder only is used, it is often 

 laid flat on the table, if one is used as a stand for the lantern. 

 It should, however, be tied to prevent rolling off. 



For a blow-through jet the pressure is better adjusted to 

 only 5 or 6 inches. 



49. Dangers of Compressed Gas. The use of gas in 

 cylinders was said by some persons to remove all danger. 

 Experience has not quite borne this out, though what danger 

 there may be, appears to have attached to the persons engaged 

 in pumping it, and not to the consuming public. Years ago, 

 a cylinder exploded at the Eoyal Institution whilst being 

 filled, and examination showed that the intense heat caused 

 by the rapid compression had ignited some particle of 

 carbonaceous matter, and that this had in turn ignited the 

 iron itself, which readily burns in oxygen. This is now 

 guarded against. Another bottle burst when sent to have a 

 * stuck ' valve loosened, because the workman put it on the 

 fire ! No vessel would, of course, resist such an idiotic 

 proceeding, which is scarcely likely to happen again. I have 

 heard of no cylinder failing in England, whilst in the hands 

 of the public ; in testing, a certain portion of course do so, 

 and are accordingly rejected. A pressure-gauge has burst, 

 with serious, though not fatal results ; and it is safest to have 

 such gauges well secured against, at all events, broken glass. 



The only practical danger is in mixture. A terrible and 

 fatal explosion at Dublin, in the early part of 1889, was 

 caused by oxygen being pumped into a hydrogen cylinder for 

 an emergency, the cylinder on being sent back being filled up 

 again with hydrogen. It was the vendor himself who was 

 killed ; and he knew, and had been actually warned of, the 

 state of affairs and the danger, running the risk rather than 

 lose a few cubic feet of gas ! And a similar accident also 



