It is worth observing that although injector burners are satisfactory in practice, and are in fact almost the only jets yet found to give prolonged satisfaction, the method of injecting air below the point of combustion in a self-luminous burner is in some respects wrong in principle. If acetylene can be consumed without polymerisation in burners of the simple fish-tail or bat's-wing type, it should show a higher illuminating efficiency. In 1902 Javal stated that it was possible to burn thoroughly purified acetylene in twin non-injector burners, provided the two jets, made of steatite as usual, were arranged horizontally instead of obliquely, the two streams of gas then meeting at an angle of 180°, so as to yield an almost circular flame. According to Javal, whereas carbonaceous growths were always produced in non-injector acetylene burners with either oblique or horizontal jets, in the former case the growths eventually distorted the gas orifices, but in the latter the carbon was deposited in the form of a tube, and fell off from the burner by its own weight directly it had grown to a length of 1.2 or 1.5 millimetres, leaving the jets perfectly clear and smooth. Javal has had such a burner running for 10 or 12 hours per day for a total of 2071 hours; it did not need cleaning out on any occasion, and its consumption at the end of the period was the same as at first. He found that it was necessary that the tips should be of steatite, and not of metal or glass; that the orifices should be drilled in a flat surface rather than at the apex of a cone, and that the acetylene should be purified to the utmost possible extent. Subsequent experience has demonstrated the possibility of constructing non-injector burners such as that shown in Fig. 13, which behave satisfactorily even though the jets are oblique. But with such burners trouble will inevitably ensue unless the gas is always purified to a high degree and is tolerably dry and well filtered. Non-injector burners should not be used unless special care is taken to insure that the installation is consistently operated in an efficient manner in these respects.