82 PBACTICAL PHOTO-MICKOGKAPHY 



In the blow-through jet the ordinary house gas is used, 

 directly from any convenient source of supply, and the oxygen 

 alone is under pressure. It has a considerably larger area 

 of incandescence than the mixed jet, and its illuminating power 

 is not so high. The jets in which a volatile substance, such as 

 ether or gasoline, is used instead of coal-gas, are otherwise 

 similar to the blow-through type. 



Lime-light as a radiant has many excellent qualities. The 

 source of light is constant, and in the mixed jet is not of large 

 area. It is also now, as a rule, easily obtainable in out-of-the- 

 way places, where electric light is not available. It is cheap and 

 easy to work, and quite free from danger when carefully used 

 and its principles understood. 



If we refer to the illustration of a mixed jet (Fig. 23 a), it 

 will be seen that it is constructed as follows : 



The hydrogen is conducted in at one tube, usually on the 

 left side, and the oxygen passes through the other ; the gases 

 then pass into a chamber which is packed with iron gauze or 

 some similar material, so that they there become thoroughly 

 mixed. Then they pass out through the nozzle and impinge 

 on to the lime which is dropped on to the pin in front of the 

 nozzle in the position as shown. The gases, which are contained 

 in steel cylinders, must be under equal pressure, and on each 

 cylinder, therefore, some form of regulator must be placed, so 

 that neither gas issues forth at a greater pressure than is required, 

 and so that this pressure may be regulated and maintained. 

 The efficiency of the mixed jet depends wholly on the proper 

 mixing of the gases, on the maintenance of this equality, and 

 on regularity of pressure. 



In working the jet, the hydrogen, or compressed coal-gas, 

 must be allowed to pass through first, and should then be lighted 

 at the orifice of the jet. The flame should be sufficiently 

 high to rise well above the top of the lime cylinder, and should 

 not be high enough to flare. The oxygen gas should then be 

 carefully turned on so that only a small pressure of gas comes 

 through at the start. The immediate effect will be for the 

 hydrogen flame to go down ; some more hydrogen should then 

 be let in and next a further supply of oxygen, each being 

 successively turned on in small quantities until the point is 

 reached when a hissing or roaring sound is heard. The point 



