48 OPTICAL PROTECTION 



is unscrewed with a pair of gas-pliers, when a gas-nozzle of 

 one or the other pattern shown in fig. 29 (each of them being 

 provided in both standard sizes of screw-thread) can be screwed 

 in or on in a moment, and will give a good supply, with a 

 nice descending start for the rubber-tube. A gas-flame being 

 provided in this way, a jet of oxygen is then blown either 

 through its centre, or across it. This jet must, therefore, 

 have two gas-tubes, and be controlled by two taps. 



The oxy-gas jet is used in four forms, the principles of 

 which are shown in fig. 30. That marked A is most usual, a 

 small stream of oxygen being blown through the centre of the 

 flame from an open tube of gas, which it will be convenient 

 to call ' hydrogen ' henceforth. No explosive mixture can 



n 

 Fm. 30. Oxy-gas Jets 



take place with this jet, but when the oxygen has been turned 

 off for a little time, the hydrogen may creep down the oxygen 

 orifice a little, and cause a slight ' snap,' if the oxygen is 

 turned on again suddenly. Very gradual turning on of the 

 oxygen will avoid this, or a very small quantity of oxygen left 

 on will do so. 



The B form is the same, except that the oxygen terminates 

 inch or more below the top edge of the hydrogen tube. 

 Hence there is a little better mixture, and rather more light. 

 This form is, however, a little more apt to ' snap ' out the light, 

 if, after being off some time, it is turned on suddenly. A small 

 supply of oxygen left on, or an oxygen bye-pass ' (see 58), 

 is almost necessary to avoid this with certainty. The general 

 form of both these jets is shown in fig. 31, but the lime-turn- 



