VARIOUS SOLIDS. 43 



Carborundum (SiC). 

 (Artificial product; t = i.2"j mm. Curve c, fig. 26.) 



The specimen examined was a flat crystal which transmitted blue light, 

 and reflected yellowish-green light. It has remarkable optical properties, 1 

 especially a high value for the refractive index. It is opaque in the ultra- 

 violet (Jewell) and beyond 2 p. in the infra-red. In this respect the trans- 

 mission curve c, fig. 26, is very remarkable as compared with all the other 

 substances examined, of which only one, viz, mellite, is as opaque to infra- 

 red rays. The reflection curve was examined previously; it also shows 

 remarkable properties. 



Cryolite (3NaF A1F 3 ). 

 (From Ivigtut, Greenland. Massive, semitranslucent; = 2.3 mm. Curve b, fig. 26.) 



From the composition of the material it was hoped to find this to be 

 more transparent to heat rays than usually has been the case with minerals. 

 Possibly a pure crystal of cryolite would be different. From the absorp- 

 tion bands (curve b, fig. 26) it will be noticed that this specimen contained 

 water of crystallization. 



Thomsenolite (NaLiAlF 6 + H,0), previously examined, is an alteration 

 product of cryolite, and from the similarity of the transmission curves it 

 would appear that this sample of cryolite had already undergone some 

 alteration into the hydrated mineral. 



Nitrocellulose (Transparent "Celluloid"). 

 (Curve d, fig. 26; =0.138 mm.) 



Transparent celluloid is a mixture of nitrocellulose [C 12 H 16 (NO 2 ) 4 O 10 ] 

 and camphor (C 10 H 16 O). The transmission curve which was obtained 

 with a fluorite prism and bolometer is conspicuous for two regions of 

 great absorption, with maxima at 3 //, 3.43 /*, and 6 //. 



Camphor is a very complex carbohydrate. Its alcoholic properties, 

 OH groups, should cause an absorption band at 3 p. (2.95 /* in Carnegie 

 Publication No. 35, p. 58, for rock-salt prism). Its ketone properties, 

 CO groups (perhaps aldehyde properties, CHO groups), should cause a 

 strong band at 6 p.. Excellent examples of OH groups are (see Carnegie 

 Publication No. 35, p. 108) carvacrol and thymol, of CHO groups are 

 benzaldehyde, cuminol, eucalyptol, colophonium, and Venice turpentine, 

 and of both these groups is terpineol. The band at 3.43 y. is characteristic 

 of the CH 3 -groups. 



Excepting in intensity, the transmission curve is similar to that of col- 

 lodium (Carnegie Publication No. 65, p. 60), which is also a complex 

 compound of cellulose. It seems rather remarkable that the characteris- 

 tic vibration period of these groups of atoms is not affected by the com- 

 plexity of the compound. 



1 Jewell: Physical Review, 24, p. 239, 1907. 



