During the combustion of a mixture of two volumes of hydrogen with one of oxygen, an enormous amount of heat is produced, which is usefully applied in the arrangement of the oxy-hydrogen blowpipe. The flame of the mixed gases produces little or no light, but when directed on various metals contained in a small hole made in a fire brick, a most intense light is obtained from the combustion of the metals, which is variously coloured, according to the nature of the substances employed. With cast-iron the most vivid scintillations are obtained, particularly if after having fused and boiled the cast-iron with the jet of the two gases, one of them, viz., the hydrogen, is turned off, and the oxygen only directed upon the fused ball of iron, then the carbon of the iron burns with great rapidity, the little globule is enveloped in a shower of sparks, and the whole affords an excellent notion of the principle of Bessemer's patent method of converting cast-iron at once into pure malleable iron, or by stopping short of the full combustion of carbon, into cast-steel. The apparatus for conducting these experiments is of various kinds, and different jets have been from time to time recommended on account of their alleged safety. It may be asserted that all arrangements proposed for burning any quantity of the _mixed_ gases are extremely dangerous: if an explosion takes place it is almost as destructive as gunpowder, and should no particular damage be done to the room, there is still the risk of the sudden vibration of the air producing permanent deafness. If it is desired to burn the _mixed_ gases, perhaps the safest apparatus is that of Gurney; in this arrangement the mixed gases bubble up through a little reservoir of water, and thus the gas-holder--viz., a bladder, is cut off from the jet when the combustion takes place. (Fig. 120.) This jet is much recommended by Mr. Woodward, the highly respected President of the Islington Literary and Scientific Institution, and may be fitted up to show the phenomena of polarized light, the microscope, and other interesting optical phenomena. [Illustration: Fig. 120. Gurney's jet. A. Pipe with stop-cock leading from the gas-holder. B. The little reservoir of water through which the mixed gases bubble. C. The jet where the gases burn. D. Cork, which is blown out if the flame recedes in the pipe, C.]