absorption spectra of highly concentrated solutions, etc. 207 



whenever they were removed for any purpose. The strongest 

 solution was contained in a thin glass cell of exactly 5 mm. 

 internal width. The issuing light was concentrated by a lens 

 of 3 in. focal length, which was fixed at that distance from the 

 slit, whilst at the other end of the tube a lens of similar focal 

 length was fixed, and beyond this again there was a fixed screen 

 with a hole of about a of an inch diameter, and still further 

 beyond, was the source of light. The latter was a Welsbach 

 incandescent light with no chimney, and it was placed several 

 inches from the screen in order that the mantle of the burner 

 should not be focussed in the slit. The photographs were taken 

 on spectrum plates made by Messrs Neal and Cadett. 



Method of experiment. 



The solutions used were the nitrates and chlorides of 

 Didymium and of Erbium. These two earths had been care- 

 fully separated from Gadolinite and Euxenite by a long and 

 laborious series of operations, and the only impurity is a 

 trace of Yttria. Weighed quantities of the oxides, DL0 3 and 

 Er 2 3 , were dissolved in the two acids, and the solutions 

 evaporated to drive away excess of the free acid. Each salt 

 was then dissolved in a measured quantity of water. The most 

 concentrated solutions of Didymium contained 606'3 grams of the 

 anhydrous nitrate dissolved in a litre, and 459"7 grams of the 

 anhydrous chloride in one litre. These numbers represent about 

 1*834 gram-molecules per litre. In the Erbium solutions the 

 most concentrated solutions were 785'6 grams of the anhydrous 

 nitrate to the litre, and 609'6 grams of the anhydrous chloride to 

 the litre. These numbers represent about 221 3 gram-molecules 

 per litre. The dilute solutions were made from these by taking 

 measured quantities and adding water to the required amounts. 

 As the width of the cell containing the strongest solutions was 

 5 mm. and as the lengths of the three tubes were 610 mm., 

 1224 mm., and 3599 mm. respectively, one c.c. of the strongest 

 solutions was diluted 122, 245, and 719"8 times respectively. 



The method of experiment was to fill one of the tubes with 

 distilled water which had previously been boiled and filtered, and 

 then to place it in position in the V-grooved blocks fixed in front 

 of the slit of the spectroscope. The strongest solution in the 

 5 mm. cell was then firmly held by a clamp and stand against the 

 end of the tube nearest the slit end of the spectroscope, and an 

 exposure to the photographic plate of 10 or 15 minutes allowed. 

 When this was completed, the tube and cell were replaced 

 by one of the duplicate tubes of equal length containing the 

 dilute solution, and exposed to the photographic plate for exactly 



14—2 



