286 Dr. T. Ewan. [June 21, 



2. The spectrum is different in different parts of the nebula. 



3. The spectrum bears a striking resemblance to that of the 

 planetary nebulae and bright line stars. 



4. The suggestion, therefore, that these are bodies which must 

 be closely associated in any valid scheme of classification, is con- 

 firmed. 



5. Many of the lines which appear bright in the spectrum of 

 the nebula appear dark in the spectra of stars of Groups II and III ; 

 and in the earlier stars of Group IV, and a gradual change from 

 bright to dark lines has been found. 



6. The view, therefore, that bright line stars occupy an inter- 

 mediate position between nebulae and stars of Groups II and III is 

 greatly strengthened by these researches. 



XIV. " On the Absorption Spectra of Dilute Solutions." By 

 THOS. EWAN, B.Sc., Ph.D., 1851 Exhibition Scholar in 

 Chemistry in the Owens College. Communicated by Pro- 

 fessor H. DIXON, F.R.S. Received April 7, 1894. 



(Abstract.) 



The measurements recorded in the paper were made in the hope of 

 obtaining some information as to the molecular condition of salts in 

 dilute solution. 



In order to obtain exact quantitative results it is necessary to 

 measure the extinction coefficients of the solutions. For this purpose 

 a new spectrophotometer was devised, by means of which it was 

 possible to work with very dilate solutions. In this instrument, by 

 the advice of Professor A. Schuster, F.R.S. , a Lummer and Brodhun 

 photometric prism was used, and the photometric measurements were 

 made by means of Abney's rotating sector. 



The absorption spectra of solutions of cupric sulphate, chloride 

 bromide and nitrate, containing generally 0'003 to O004 gr. mol. 

 per litre, were measured and found to be, within the limits of experi- 

 mental error, identical. The solutions of cupric acetate absorb, for 

 the same amount of copper, much more light than those of the other 

 salts used. The difference tends to disappear as the solutions become 

 more dilute, and it is increased by the addition of acetic acid. These 

 facts point to the conclusion that the difference is due to the incom- 

 plete electrolytic dissociation of the salt, and to the undissociated 

 part having an absorption spectrum differing from that of the dis- 

 sociated part. 



Dilute solutions of the potassium and ammonium salts of -dinitro- 

 phenol (1 ' 2 ' 4) were found to possess very nearly the same absorp- 



