Influence of Atomic Weight. 469 



of the ultra-violet spectrum than the normal alcohols contain- 

 ing the same number of carbon atoms. (3) There is an in- 

 creased absorption of the more refrangible rays for each 

 increase of CH 2 in the molecule of the alcohols and acids. 

 (4) Like alcohols and acids, the ethereal salts derived from 

 them are highly transparent to the ultra-violet rays, and do 

 not exhibit absorption-bands. (5) Aromatic hydrocarbons, 

 phenols, acids, and amines derived therefrom are remarkable 

 for their powerful absorption of the ultra-violet rays. (6) Iso- 

 meric bodies containing the benzene nucleus exhibit widely 

 different spectra, both as regards the position and intensity of 

 the absorption-bands ; and so much is this the case that these 

 absorption-spectra may be used as a means of identifying 

 organic substances, and is a most delicate test of their 

 purity. 



Relative Sensitiveness of Photographic Papers. — MacDougall 

 (Chem. Journ. [2] iii. p. 183) has shown that the relative sen- 

 sitiveness of silvered papers salted with solutions of the haloid 

 salts of various metals is independent of the nature of the 

 metal with which the halogen is combined, and depends solely 

 on the quantity and nature of the halogen. Combining the 

 results of Wright (Chem. Journ. [2] iv. p. 33) and of Carey 

 Lea (Am. Journ. Sci. [3] ix. p. 269), we find that the photo- 

 graphic sensitiveness increases with the atomic weight of the 

 halogen. This, however, does not hold good for mixtures of 

 the haloid salts. 



Spectra of the Elements. — Several cases have been observed 

 which appear to show that the atomic weights of the elements 

 have a marked and definite influence on the nature of the 

 spectra which they yield ; thus : — 



Chlorine Group (CI, Br, I). — (1) In passing from CI to Br, 

 and thence to I, the spectra expand gradually towards the 

 ultra-violet. (2) Each spectrum has two maxima of light, 

 which approach each other as the atomic weight of the element 

 increases; and the brilliant lines seem also to become broader, 

 and the maxima move towards the violet, as we approach I 

 (Ditte, Compt. Rend. 1871, lxxiii. p. 738). 



Ditte also examined the spectrum of fluorine produced by 

 passing the induction-spark through SiF 4 , and found that 

 though the spectrum had two well-defined maxima, yet it did 

 not present the brilliant region peculiar to the spectrum of 

 the CI group, and the maxima are wide apart. He there- 

 fore concludes that, according to its spectrum, F ought not. 

 to be placed among the halogens. Nevertheless I think it 

 resembles the CI group in possessing two maxima, which are 

 wider apart than in the case of the other elements of the group. 



