With an 'Injector' jet there is no need for the acetylene gas to be under pressure at all, and a simple generator such as described on page [12] will answer perfectly, though in practice very seldom used. With such a generator the pressure is so low that in many cases the jet will not even burn until some oxygen is turned on; but this introduces no real difficulty, as with a good 'Injector' a snap is practically impossible, provided the generator is large enough to evolve sufficient acetylene. It is far better in every way, however, to use the acetylene from a cylinder, just as with coal gas. Only in this case the cylinder is completely filled with a porous material, and this again filled with liquid acetone or other suitable fluid, in which the acetylene is dissolved as rapidly as it is pumped into the cylinder.
To compress acetylene in the ordinary way is neither safe nor practicable; but these 'dissolved' cylinders are now used extensively for both oxy-acetylene welding and motor car lighting, and may be entirely relied upon.
The D.A. (Dissolved Acetylene) Company were the pioneers in this country of the industry, and their methods of business are peculiar and ingenious. The user is requested in the first place to purchase a cylinder, and he then becomes the owner of a cylinder, but not of one particular cylinder. A list is supplied to him of various depots in the country where the Company's cylinders are stored, and when empty he can, on payment of a fixed sum, exchange his empty cylinder for a full one, which then becomes his cylinder pro tem.
This saves the delay and expense of returning a cylinder to London, and incidentally clears the customer of any question of deterioration, this being obviously covered by degrees with each individual exchange. The system was first introduced in connection with the lighting of cars and only applies to the standard size for this purpose, viz. 20 cubic feet capacity,
but as this is, on the whole, the most convenient size for lantern work also, the limitation is not a disadvantage. The arrangement is also in vogue to a less extent with cylinders of 6 feet capacity (a size sometimes used for motor cycles), but the depots of exchange are at present far fewer for this size.
The oxy-acetylene blast is much hotter than the ordinary oxyhydrogen, and therefore produces a more intense light. I have therefore used it with success on occasions when even the ordinary limelight would fail, and the choice has lain between an oxyhydrogen jet of enormous bore (and, of course, corresponding consumption of gas), and the oxy-acetylene.
For this very reason great care must be taken only to use the hardest limes, and even then to use the lime-turning movement frequently, or the lime will pit or crack and a broken condenser follow.
The Fallot Acetylene Light.—This light consists of a jet of acetylene under pressure, without oxygen, but producing its own air blast from the atmosphere by suction, much as the 'Injector' jet does, but the reverse way round.
The light is better than with an ordinary acetylene jet, though not quite so good as with a 'blow-through' jet; but as it only requires a cylinder of dissolved acetylene, or even a 'Pressure' generator, it is fast coming into favour.
The peculiarity of the Fallot apparatus is that, instead of providing a direct beam of light in the direction of the screen, it projects the beam backwards on to a concave mirror, and it is the reflected light from this that is used (Fig. 23).