286 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Attempts were first made to ascertain the index directly by melting 

 the solid on the face of the prism ; but on account of lack of uniformity 

 in melting, a thin film of the solid persisted on the face of the prism and 

 obstructed the passage of light. The method of mixture was, therefore, 

 resorted to with perfect success. Readings of angles were taken at tem- 

 peratures above the melting points of the solids and the density of the 

 solids was taken at the same temperatures. The solvent selected was a 

 petroleum distillate with high boiling point, 202°-203°, 50 mm. that 

 had been fractioned many times. It was essentially pure C 18 H 38 ; on 

 account of its high boiling point there was no danger of loss by vaporiza- 

 tion during the observations. The index and specific gravity of the solvent 

 were determined at 60°, 70°, and 80°. The specific gravity was in all 

 cases referred to water at 4°. 



The following values were given by the solvent oil : — 



The results of the observations were calculated by the formula for 

 mixtures : — 



^100 = ^ + ^(100-,;) 

 d d x d 2 



n = index of mixture 

 n x = " " solvent 



no = 



a a 



solid 



d := density of mixture \ 

 di = " " solvent > all taken at t° 

 d 2 = " " solid ) 

 p = per cent of solvent 

 (100 — p) — " " " solid 



The mixtures were made of such proportions that each contained ap- 

 proximately 20 % of the solid, and two grams was necessary for each 

 determination, one half the full capacity of the cup, which was necessary 

 to maintain a uniform temperature. The index and specific gravity of 

 each solid were taken at two temperatures, 60° and 70°, or 70° and 80°. 



The following results were obtained with the hydrocarbons from 

 Pennsylvania petroleum : — 



