This advantage is now shared by the more recently introduced 'Injector' jets, which give a far better light, and have therefore rendered the 'Blow-through' type nearly extinct.
The general construction of a 'Blow-through' jet is shown in Fig. 16, and it will be seen that a short vertical spindle is
provided to carry the lime cylinder, and that this can be rotated from the back by means of bevelled gear wheels, which at the same time screw the spindle up and down. A lime cylinder of the usual pattern being placed on this spindle can be rotated from time to time to expose a fresh surface, as that in use gradually becomes 'pitted' by the blast, while the screw provides sufficient vertical movement to ensure that a complete rotation does not bring round the same position again.
Some arrangement is also generally provided by which the distance between the lime spindle and the jet can be adjusted. The exact position of this does not matter within a reasonable margin, but limes vary in size, and 'Pastilles,' and other substitutes for limes, which will be referred to later, vary still more, at any rate as regards this adjustment. The average distance which gives the best result is usually about half an inch, and once set need not be altered with that particular jet unless a lime of different size is employed; minor variations due to limes being drilled slightly out of centre, &c., do not seriously matter.
There is no accepted rule for colouring jet-taps in accordance with the cylinders, and although jets are sometimes met with painted in this way, i.e. red for coal gas and black for oxygen, it is more usual to find coal gas taps black and oxygen bright, or sometimes both black or both bright. Care must therefore be taken that the right cylinder is connected to the right tap on the jet, but there should be no difficulty in telling which is which, and fortunately any mistake, even if it be made, is quite harmless.
The Mixed-Gas or Double-Pressure Jet.—This jet is fundamentally different from the 'blow-through' form, inasmuch as the two gases are combined in one mixing chamber before combustion, and burn in their correct proportions at one nipple.
It is usually stated that this jet necessitates both gases being under equal or approximately equal pressure, but this
is not literally accurate, and I have given many a lantern exhibition with one of these jets, using coal gas from the ordinary supply, and oxygen from a cylinder. To use a mixed jet in this way needs care, as a very slight excess of oxygen puts the light out with a 'pop' which, although not dangerous, is disconcerting, while the light obtained under these conditions is very little better than with a 'blow-through' jet, and far inferior to the 'Injector' jets to be described next.
