THE BANISHMENT OF NIGHT 



certain other metals having proved better adapted for 

 the purpose. Thus the Welsbach patent of 1886 covered 

 the use of thoria, either alone or mixed with other sub- 

 stances such as zirconia, alumina, magnesia, etc.; 

 thoria being considered as having a very high power of 

 light emission. Later it was discovered that pure thoria 

 emits very little light by itself, although it possesses 

 a refractory nature that gives a stability to the mantle 

 unequalled by any other material as yet discovered. 

 When combined with a small trace of the oxides of cer- 

 tain rare metals, however, such as uranium, terbium, 

 or cerium, thoria mantles have a very high power of 

 light emission, most modern mantles being composed 

 of about ninety-nine percent, thoria with one percent, 

 cerium. 



In the ordinary method of manufacturing such 

 mantles, a cotton-net cylinder about eight inches long, 

 more or less according to the size of mantle required, 

 is made, one end being contracted by an asbestos thread. 

 A loop of the same material, or in some cases a platinum 

 wire, is fastened across the opening, to be used for 

 suspending the mantle when in use. The cotton- thread 

 cylinder is soaked in a solution of the nitrates of the 

 metals thorium and cerium, and is then wrung out to 

 remove the excess, stretched on a conical mold, and 

 dried. The flame of an atmospheric burner being ap- 

 plied to the upper part at the constricted position, the 

 burning extends downward, converting the nitrates 

 into oxides, and removing the organic matter. Con- 

 siderable skill is required in this part of the process, as 

 the regular shape of the mantle is largely dependent 



