The high-pressure torch, which has largely taken the place of the low-pressure one in France, and which we also see most frequently in this country, has a different method of mixing the gases, due to the fact that they both are under pressure. According to many authorities the tip where the gases are mixed is by far the most important factor in the success or failure of the tool. In the high-pressure torch the oxygen enters the tip from a hole in the centre, while the acetylene enters it from two holes, one on each side. They meet under high pressure at the upper end of the tip, and have the length of the hollow tip in which to mix, before they strike the air. The long, narrow hole in the tip is called the mixing chamber. Those who are interested in the high-pressure torch declare that it is the fact that the gases are positively mixed in proper proportion in the detachable tip, that so greatly adds to the efficiency of the tool. They declare that by allowing the acetylene to enter the tip laterally, at right angles with the oxygen, the blast of the oxygen is broken as it mixes with the acetylene, and the tendency of an oxygen flame to oxidize any metal with which it comes in contact by reason of an excess of oxygen in the flame is largely done away with. This, with the small diameter of the mixing chamber and the friction with the walls, gives a perfect mixture, according to the claims of the high-pressure torch enthusiasts. Moreover, the small hole which is the mixing chamber, effectually prevents serious accidents by flash-backs of the highly explosive acetylene, and also provides a much easier method of control. Each outfit has several different sizes of tips for various kinds of work. The pressure under which the two gases are used is the other big difference between the high-pressure and the low-pressure torches, as said before. In the the high-pressure tool the oxygen is compressed about the same as in the low-pressure torch, while the acetylene is under several pounds pressure, just in accordance with the size of the tip used. In the low-pressure torch the pressure on the acetylene is only about ten ounces to the square inch, or only enough to keep it flowing. On account of this difference in the pressure making the big difference in the mixture of the gases, scientists have chosen to call the low-pressure torches injector mixture types, from the fact that the acetylene is sucked into the tip by an injector system, while the high-pressure torches are called positive mixture types, because the gases are mixed directly by pressure. In the latest high-pressure tool the mixture of gases is 1.14 parts of oxygen to 1 part of acetylene, while the low-pressure torch takes a proportion of 1.7 parts of oxygen to 1 part of acetylene.