EFFECT OF MOLECULAR WEIGHT. 



6l 



50% 



derivatives of benzene showed a shift of the maximum toward the short 

 wave-lengths. An examination of two petroleum distillates, C 8 H 18 and 

 C 24 H 50 , showed no shift of the maxima, and on the whole the evidence of 

 a shift, due to a change in molecular weight, was inconclusive. 



In Carnegie Publication No. 65, p. 56, the sulphates showed absorption 

 and reflection bands which seemed to shift toward the long wave-lengths 

 with increase in molecular weight. This data will be discussed in the 

 present paper. 



Ever since the announcement of "Kundt's Law" various observers 

 have tried to find relations among the absorption bands, viz, such that 

 with an increase in molecular weight the band shifts toward the long 

 wave-lengths. The results have just been discussed in the first part of 

 this chapter. There is a physical basis for expecting a shift of the band 

 with change in molecular weight. For example, the sulphate of the metals 

 K, Rb, and Cs have been compared by Tutton, 1 who has shown that both 

 as regards crystalline form, specific gravity, thermal expansion, and corre- 

 sponding refractive indices the Rb salt lies between the K and Cs salts. 



In Carnegie Publication No. 65, the sulphates of Mg, Ca, Sr, Cd, and 

 Ba were found to have the maximum of 

 their absorption at 4.5, 4.55, 4.6, 4.6, and 

 4.63 , respectively. The same shifting of 

 the maximum was observed in the reflec- 

 tion bands of SrS0 4 and BaS0 4 ; in the 

 former the maxima are at 8.2, 8.75, and 

 9.05 fi } while in the latter the maxima are 

 shifted to 8.35, 8.9, and 9.1 //. 



In the present paper all the available 

 data have been compiled, to show this shift. 

 In fig. 42 is given a composite of the re- 

 flection curves of the carbonates [(Mg, Ca, 

 Zn, Ba, Pb) C0 3 ] described on a previous 

 page. The maxima are evidently double, 

 although it is not very apparent in some 

 curves. As a whole, however, the location 

 of the maxima of PbC0 3 is so much farther 

 in the infra-red than in MgC0 3 that there 

 can be no doubt that the shift is a real one. 



In fig. 43 are given the graphs of the 

 maxima of the reflection band plotted 

 against atomic weight of the basic atom in the molecule. The carbon- 

 ates of Mg, Ca, Zn, Sr, Ba, Fe, Cu, Pb are included. In this figure the 

 two maxima of the complex band at 6.4 to 7.2 [x are plotted. When the 



Fig. 42. Reflection curves of carbonates. 



See Miers's Mineralogy. 



