The mixed-gas jet is intended then to be used with both gases under pressure, and is the only jet to be seriously
considered in cases where a really powerful light is required. The power of this jet is indeed almost unlimited, and those made with large bores, such for example as used for cinematograph work, provide a light amounting often to some two or three thousand candles, and consume an enormous amount of gas; but the ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an inch bore, and using some 5 feet of each gas per hour, or perhaps slightly more for the coal gas, will suffice for all ordinary work.
The mixed-gas jet, like the 'blow-through,' is made in many forms, but these may be roughly divided into two main types, viz. those with small mixing chambers immediately below the nipple (Fig. 17), and those with larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern (Fig. 18).
The construction of the mixing chamber itself varies also, but that advocated by my father, the original author of this work, is generally followed, the chamber being packed with alternate discs pierced as in Fig. 19, which ensures a thorough mixture of the gases. A layer or two of gauze is often introduced as well by way of further improvement. The distance between the lime and nipple is much less than with the 'blow-through' jet, and the adjustment has to be more exactly made. About ⅛ inch is approximately correct for a jet of moderate power, and rather more for a bigger bore; also care must be taken to turn the lime frequently, as the latter 'pits' pretty quickly with these jets, and if it is neglected the jet may spurt back out of the hole, which is gradually formed, and crack the condenser.
There is still an erroneous opinion extant that these jets are dangerous, and if the operator is working with the now obsolete gas-bags it is certainly a fact that an accident in careless hands is possible; but with cylinders there is, so far as I know, no possibility even of an accident under ordinary conditions.
It is true that if too much oxygen is turned on the jet may suddenly go out with a loud snap or pop, and this is in reality a miniature explosion in the mixing chamber; but it can in any case hardly be serious enough to matter, though I have found after such a snap that the gauze packing, inside the chamber above referred to, has been pierced right through, and, when first lit afterwards, the jet has for a few minutes burnt with a characteristic green flame, denoting the presence in the gas of fine copper or brass particles.
To obtain a good light with these jets, and in fact with all jets, great care must be taken that the nipple is absolutely smooth, otherwise the flame is bound to hiss. The simplest plan is to slightly roughen a suitable sized needle with emery paper and to burnish the inside of the nipple from time to time with this. Especially if there has been one of the 'snaps' referred to is it desirable to see that the inside of the nipple is thoroughly smooth and polished.
Manipulation of the Mixed-Gas Jet.—On this point there is not much to be said. A good hard stone lime must be used—'soft' limes are useless for this jet—and the coal gas flame should be lit first, and the lime thoroughly heated with this before the oxygen is slowly turned on. As the oxygen increases the flame will gradually disappear and the light increase, until it is at a maximum for that particular amount of coal gas. This latter can then be turned on a little more, and more oxygen passed to balance it until the jet begins to 'roar,' when we are getting the maximum light for that particular sized nipple. When the two gases are, however, in the proper proportion to give the best light, there will always be a slight excess of coal gas flame visible playing about the lime.