OPTICAL PROJECTION 



fro on a spindle, B, at the edge, through which the gas enters 

 at A, and which has a valve so constructed, that when the 

 bellows opens to a certain point, the gas is cut off entirely. 

 It passes to the jets by the nozzle E. 

 This regulator only measures about 3 

 inches each way, works very well, and 

 costs about 25s. 



Mr. Beard has since brought out a 

 small regulator, costing 80s., shown in 

 fig. 52, which is about one-third the 

 actual size. This also works with a 

 central valve, but, instead of a screw, 

 has a curious sort of * lazy-tongs ' ar- 

 rangement, which need not be particu- 

 larly described. The material fact is 

 that there are now at least two regulators 

 in the market, measuring only a few 

 inches each, which simply screw on to the 

 cylinders, and then keep the pressure 

 practically uniform, and allow the jets to 

 be regulated just as usual, or even to 

 be turned off. Beard's is generally found most satisfactory. 

 Fig. 53 shows a cylinder of compressed gas with a pressure- 

 gauge, one of these regulators, and a screw-key, all in position. 

 47. Price of Oxygen. By a fortunate coincidence, no 

 sooner had efficient regulators been provided, than the price 

 of compressed oxygen was cheapened by the introduction of 

 Brin's process for manufacturing it. In this process retorts 

 are charged with pure anhydrous barium oxide, which is 

 raised to a low red heat, when common air carefully washed 

 and purified is pumped into the retort. The oxide under 

 these circumstances takes up more oxygen from the air and 

 becomes dioxide, when the nitrogen is conveyed away ; after 

 which, on raising the dioxide (Ba0 2 ) to a still higher tempera- 

 ture and reversing the air-pump, the dioxide is reduced again 



FIG. 53. Cylinder with 

 accessories 



