Absorption Spectra of Dilute Solutions. 119 



bodies which are only electrolytically dissociated, and it will be better 

 to consider them along with the experimental numbers. Changes of 

 colour may also be due to the breaking up of -polymeric molecules into 

 smaller ones. It was found that solutions of colloid ferric hydroxide, 

 obtained by dialysis, absorb less light than those obtained by the 

 hydrolytic decomposition of ferric chloride, and in this case there is 

 some reason to suppose that the dialysed hydroxide consists of larger 

 molecular groups than that formed in the very dilute solution of 

 ferric chloride. 



Apparatus Employed. 



The measurement of the quantity of light of given wave-length 

 which is absorbed by a substance in very dilute solution, is not 

 very easy. In order to compensate for the small absorbing power of 

 the solutions, it is necessary to employ long layers of them, and this 

 makes it almost impossible to use a spectrophotometer such as that 

 of Vierordt. The apparatus shown in tig. 1 was therefore put 

 together.* Both for advice and assistance in its construction, and 

 for the loan of valuable apparatus, I am much indebted to Professor 

 A. Schuster, F.B.S. My thanks are also due- to the Government 

 Grant Committee of the Royal Society, who provided me with the 

 means of obtaining several pieces of apparatus. 



Fig. 1 is a diagramatic representation of the arrangement of the 

 parts of the apparatus as seen from above. 



It consists of a collimator A, with two lenses, a and 6, of 20 in. 

 focal length, which are both filled with light from the slit c. The 

 source of light was an incandescent " Auer " lamp ; and in order to 

 distribute the light more evenly a plate of ground glass was intro- 

 duced between the lamp and the slit. When it was desirable (as in 

 the violet and extreme red ends of the spectrum) to obtain a stronger 

 illumination this was removed. The parallel beam of light from the 

 lens a passes through the solution ; that from b was used as the 

 standard of comparison ; the object of using only one source of light 

 being to keep the ratio between the .brightness of these two beams 

 constant. The solutions were contained in tubes which rested on 

 Y-shaped supports capable of sliding along the bar C, D, which was 

 clamped to the board on which the apparatus rested, and which 

 allowed of a horizontal adjustment of the position of the tube. The 

 Y-supports allowed of a vertical adjustment. The ends of the tubes 

 were ground as nearly as possible parallel to each other, and closed 

 by plates of glass which were simply pressed against the ground ends, 

 no cement or washer being necessary. The tubes were 34 5 mm. 



* A description of a very similar instrument by Lummer and Brodhun appeared 

 in the ' Zeitschrift fur Instrumeutenkunde,' 1892, p. 133, when my apparatus was 

 nearly completed. 



VOL. LVI1. K 



