284 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of an electric current passing tlirough a wire of suitable size to afford the 

 necessary resistance. After numerous experiments that need not be 

 described iu detail it was found that the heat from a coil of wire sur- 

 rounding the glass cup containing the substance, even with a sheet of 

 asbestos interposed, caused such unequal expansion of cup and prism 

 that the lute cracked, causing leaks at the joint. On trying to maintain 

 the desired temperature by a coil of wire placed within the glass cup it 

 was found difficult to regulate the temperature at just the point desired. 

 Finally it was found that a coil of wire surrounding the glass cup could 

 be relied on to raise the temperature nearly to the point desired, and by 

 means of an inner coil the temperature just required for melting could 

 be obtained. German silver wire was found to be more suitable for the 

 coils on account of its rigidity, and the size selected was No. 25 English 

 standard gauge. The arrangement of outside and inside coils, prism, 

 glass cup, and attachment for stirring the melted solid, are shown in the 

 figure. 



The outer coil of wire W is drawn over a thin sheet of asbestos A 

 which surrounds the glass cup and another thicker sheet is placed out- 

 side to prevent radiation. German silver wire of the same size is used 

 for the inner coil and connected above the glass cup to longer copper 

 conductors, which are bent twice so as to dip into mercury cups. This 

 coil is kept in place by means of a cork support K, and the asbestos 

 cover B, which also serves to retain the heat. The inner coil also serves as 

 a stirrer by moving it up and down, which is possible on account of the 

 deep mercury cups. In this manner the solid after melting is very evenly 

 heated. The solid is placed in the glass cup on the face of the prism N, 

 melted by the heated wire, and the temperature kept constant by the 

 inner coil as indicated by the thermometer. 



The rays of light L enter the prism through a slit in the asbestos, 

 0.5 cm. square. A current of 3-5 amperes was sufficient to maintain the 

 temperatures desired in these determinations. With this combination 

 of internal and external heating a temperature of 100° may readily be 

 kept constant without fear of breaking the joint between the prism and 

 glass cup. 



The accuracy of this method of heating was determined with water 

 with the following results : — 



20° 30° 40° 50° 60° 70° 



Angle, 67° 38' 67° 51' 68° 10' 68° 10' 68° 48' 70° 9' 



Index of refraction, 1.3313 1.3304 1.3291 1.3276 1.3241 1.3208 



Molecular refraction, 3.692 3.692 3.692 3.692 3.691 3.692 



Theoretical molecular refraction of water, 3.692. 



