76 OPTICAL PROJECTION 



and, if necessary, an iron rod passed right through it, and 

 any manganese that may have collected cleared out. By at- 

 tending to this, I used for years delivery-tubes composed of 

 half-inch gas-pipe, but three-quarters would be a better size, 

 and even this should be examined. 



FiG. 46. Oxygen Retorts FIG. 47. 



The third kind of retort is of cast iron, made of various 

 shapes, such as fig. 47. I have also known a small Papiii 

 digester fitted as one. Cast iron is most durable of all, and 

 such are usually fitted with very large delivery-tubes ; but the 

 greater thickness of metal requires more heat. 



41. The Mixture and its Use. Such a retort, with the 

 requisite quantity of mixture, is placed on either a fire, or, 

 what is far better, a Fletcher's ring gas-burner, whose flame 

 can be regulated according to the evolution of gas. The 

 mixture often advised, of two parts chlorate to one of man- 

 ganese, is pretty steady in evolution of gas, but apt to carry 

 much manganese into the delivery-tube. Four parts of 

 chlorate to one is much faster, but requires watching, or the 

 gas will come with a rush and blow out the cork, wasting the 

 oxygen. Till the gas begins to come, the flame may be full 

 on, but directly it begins to come pretty fast, must be turned 

 down considerably, or this * rush ' will ensue. That is the 



