66 INFRA-RED ABSORPTION SPECTRA. 



or not the effect of the atom in the molecule gave rise to selective 

 absorption, and especially whether this particular band in CSg was due 

 to sulphur. As the work progressed it was found that this band was 

 frequently wanting in sulphur compounds, and finally it was found in 

 chloroform, which does not contain sulphur. The sulphur compounds 

 are distinguished by their great transparency, except allyl sulphide 

 (03115)28, which has a predominance of hydrogen. This agrees with 

 the work of Friedel,^ who found that the transparency increases in a 

 compound if H, O, OH, or N are replaced by S or the halogens. 



Sulphur. S. (Fig. 49.) 



Sulphur is quite transparent throughout the spectrum. Nothing was 

 known about the direction of the optic axes. Since this is a problem 

 by itself, in pleochroism, it will be sufficient to add that a natural crystal, 

 1.06 cm. in thickness, transmitted on an average about 50 per cent, 

 where this plate of 3.6 mm., but not so highly polished, transmitted 

 only about 38 per cent. The absorption bands are few, the most impor- 

 tant one being at 11.8 fx.. 



The emission curve, b, for a sulphur flame is due to Julius,^ while 

 curve c is for a film of S melted between rock-salt plates. Its opacity 

 is due to its crystalline surface. The emission band at 7.9 jj. coincides 

 with the absorption band of HoS instead of the SO, band, as one would 

 expect. 



Carbon Bisulphide. CS2. (Fig. 50.) 



This compound is distinguished for its general transparency and its 

 large absorption bands. But few of the compounds studied have such 

 a great transparency at 9 to 11 [x, the general experience being a great 

 transparency from 4 to 6/Lt, then a sudden opacity to 12 jw. The trans- 

 mission minimum at 12 /x has already been mentioned. The rest of the 

 curve shows no similarity to the sulphur curve, except a possible slight 

 depression at 7.9 [x. The emission curve, c, of CS, is due to Julius. It 

 shows well the behavior of emission and absorption. It should also be 

 noted that here the 4.6 /x band has a greater maximum than the 6.8 fx 

 band, which is just the reverse in the absorption spectrum. This, how- 

 ever, is what one would expect from our knowledge of the distribution 

 of the energy of a black body at the same temperature. 



The curve of the iodine solution coincides with that of the pure CSo. 

 In connection with the CS, curve the benzene bands at 6.75 fi and 11.8 /x 

 are to be noticed. The curve as a whole is in excellent agreement with 



^Friedel : Ann. der Physik, 55, p. 453, 1895. 

 ^Julius, loc. cit. 



