278 CHEMICAL DISCOVERY AND INVENTION 



In modern factories a certain proportion, from 1 to 2 per 

 cent, of the mantles are tested for resistance to shock by being 

 burnt off and mounted on the burners illustrated with the 

 shocking machines. At the foot of these burners is fixed a bar 

 at right angles to the burner stem, and the machine is arranged 

 so that a little stamp battery (Fig. 76), having weights of 2 to 5 

 oz. on each side, can be run at different speeds upon this bar, and 

 submit the mantle to the same sort of vibration as it might 

 experience when mounted in a lamp-post adjacent to a road 

 gully over which a heavy lorry is passing. 



The rings for support of the inverted mantles are composed 

 of a mixture of china clay and silica with a small proportion of 

 magnesia. For rings which have to resist a very high tem- 

 perature, as when high-pressure gas is used, from 10 to 40 per 

 cent of carborundum is added. The mixture of powders, mois- 

 tened with a special oil to make it cohere when pressed, is squeezed 

 into a metal die, made of three or four pieces which move together 

 in a machine contrived for the purpose. After pressing into 

 shape the rings are weathered for a period from two days to a 

 week, and are then scraped to remove rough edges and give them 

 their finished shape. They are then packed in fire-clay boxes, 

 called saggars, which are stacked together in a pottery furnace 

 or kiln where they are baked. The baking which requires slow 

 heating up and cooling down occupies about two days. The 

 rings are then examined, brushed to remove dust, and the 

 perfect ones packed in layers to be sent to the mantle factories. 



The manufacture of the rings has been chiefly in the hands of 

 one firm having branches in Germany, France, England, and 

 America. It is estimated that in the year 1914 about 25 to 30 

 million rings were manufactured in this country, and probably 

 another 15 millions were imported from Germany. 



The works of the Volker Corporation in Wandsworth were 

 established in 1895 to work a patent of Dr. Voelker's, who 

 seasoned the mantles in an electric furnace. Very beautiful 

 mantles appear to have been made, but they were far too ex- 

 pensive, and ultimately the Auer von Welsbach patents, in which 

 the thorium-cerium process already described were put into 

 operation, were adopted. At the present time the Company 

 employs about seven hundred hands and turns out from 18 to 

 20 million mantles per annum. 



With regard to the total world production the best way to 



