Yol. 63.] CONSTANTS IN MINERALS AND IGNEOUS ROCKS. 



147 



was bent midway between the clamps into a loop, in which the 

 substance to be melted was placed. The wires conveying the current 

 from the switchboard were connected to two binding-screws which 

 were attached by copper-strips to the base of the pillars (see fig. 1). 

 The current, which was taken directly from the mains at 

 100 volts, was reduced to a suitable strength by means of German- 

 silver wire-resistances. These resistances were in the form of long 

 spiral coils, and were enclosed in a perforated-zinc case. The 

 latter allowed free access of air, as it was necessary to prevent 

 heating as far as possible : for, when the wires became hot, their 

 resistance increased, and consequently there was a diminution in 



Fig. 1. — Apparatus used for melting igneous rocks, by means of an 

 electric current. 



the strength of the current. German silver was used for the coils, 

 in the place of galvanized iron, owing to the limited space ; the 

 latter, however, is more suitable, as it becomes heated less readily 

 and is far cheaper. The total resistance in the circuit when cold 

 was about 12*5 ohms, and ranged down to 2-32 ohms. 



The switchboard was calibrated to give from 8| to 43 amperes, 

 the last few studs being graduated in fractions of an ampere, so that 

 a careful adjustment of the temperature was possible when nearing 

 the fusion-point of the platinum, the ribbon usually becoming white- 

 hot at about 40 amperes. 



The method of procedure was as follows : — 



The specific gravity of a fresh specimen of the rock to 



be 



