*36 ELECTRIC LIGHTING. 



tain an arc of 10 to 15 millimetres long. The following 

 results were obtained : 



i. The phosphate of lime was decomposed, and the re- 

 duced calcium burnt in the air with a reddish flame ; the 

 light measured by the photometer was double that given by 

 retort-coke carbons of the same section. The lime and the 

 phosphoric acid were, however, diffused in the air, producing 

 some smoke. 



2. The chloride of calcium, the borate, and the silicate, 

 were also decomposed. But the boric and silicic acids 

 seemed to escape the action of the electricity by volatilizing. 

 The light was less than with phosphate of lime. 



3. Silica made the carbon worse conductors, and dimi- 

 nished the light; it also fused and was volatilized without 

 being decomposed. 



4. Magnesia, borate and phosphate of magnesia were de- 

 composed, and the magnesia vapour passing to the negative 

 pole burnt in the air with a white flame, which much in- 

 creased the light 'less so, however, than the lime salts. 

 Alagnesia, boric, and phosphoric acid were diffused in the 

 air as smoke. 



5. Alumina and silicate of alumina were difficult to de- 

 compose ; a very strong current and a considerable arc were 

 required. Under these conditions the aluminum was seen 

 to issue in vapour from the negative pole like a. jet of gas, 

 and to burn with a bluish little luminous flarne. 



Archereau found that the introduction of magnesia into 

 the carbons increased their illuminating power in the pro- 

 portion of i '3 4 to i. 



According to Carre it would seem 



i. That potash and soda doubled at least the length of 

 the arc, rendered it silent, and, by combining with the silica 

 always present in retort-coke, they eliminated it in the state 

 of transparent, vitreous, and often colourless, globules at 6 

 or 7 millimetres from the points. The light was increased 

 in the proportion of 1*25 to i ; .. 



