ON ODU KNOWLEDGE OF SPECTRUM ANALYSIS. 313 



vaponrs which might become diffused in the tube was provided. The 

 metal employed to pi-oduce the spectrum has varied according to circum- 

 stances, but for most purposes nickel was preferred. In many cases, 

 especially where absorption-bands occur, it is desirable to photograph 

 with the slit wide open ; in this way a continuous and more uniform 

 spectrum is obtained. Wet-plate photography did not give satisfactory 

 results ; the more refrangible end of the spectrum either not photograph- 

 ing or being very weak, and in a small room the ozone generated by the 

 electric discharge causes a deposit of silver on the plate directly the de- 

 veloper is applied. Accordingly, recourse was had to dry-plates ; gelatine 

 plates wei'e found to be the most generally serviceable. The lines of 

 zinc, cadmium, and aluminium were employed to define the region of 

 absorption. 



After examining a very large number of specially purified substances, 

 and recording the results by diagrams which accompany the report, the 

 following generalisations were arrived at : 



(1) The normal alcohols of the series €„ Ha^.i OH, are remarkable 

 for transparency to the ultra-violet rays of the spectrum, pure methylic 

 alcohol being nearly as much so as water. 



(2) The normal fatty acids exhibit a greater absorption of the more 

 refrangible rays of the ultra-violet spectrum than the normal alcohols 

 containing the same number of carbon atoms. 



(3) There is an increased absorption of the more refrangible rays 

 corresponding to each increment of CH2 in the molecule of the alcohols 

 and acids. 



(4) Like the alcohols and acids, the ethereal salts derived fi'om them 

 are highly transparent to the ultra-violet rays, and do not exhibit absorp- 

 tion-bands. 



Exatninatwn of Suhstances containing the Benzene Nucleus. 



In the examination of substances represented by a formula containing 

 a closed chain of carbon atoms doubly linked together, it was shown that 

 all such bodies are highly adiactinic, the hydrocarbons being least so. 

 Prof. Stokes has pointed out that one of these substances, salicine (a 

 glucoside of saligenin) during the process of dilution causes an absorption- 

 band in the spectrum of the transmitted rays. It was thought worth 

 while to examine allied substances, such as phenol, salicylic acid, etc., and 

 ascertain whether they also produce absorption-spectra. The following 

 points of interest were made apparent by the results of this examination 

 of benzene and its derivatives : — 



(1) Benzene and the hydrocarbons, alcohols, acids, and amines 

 derived therefrom, are remarkable — first, for their powerful absorption 

 of the most refi-angible rays ; secondly, for the absorption-bands made 

 visible by dissolving them in water or alcohol ; and thirdly, for the 

 extraordinary intensity of these absorption-bands even in very dilute 

 solutions. 



(2) Isomeric bodies containing the benzene nucleus exhibit widely 

 different spectra, inasmuch as their absorption-bands vary in position and. 

 in intensity. 



(3) The photographic absorption-spectra can be employed as a means 

 of identifying organic substances, and as a most delicate test of their 

 purity. The curves obtained by co-ordinating the extent of dilution, or 

 in other words the quantity of substance, with the position of the rays of 



