156 Dr. Williamson on the 



the ray and that produced by quartz, sugar, oil of turpentine, 

 and such bodies. 



Either by independent or by conjoint observations of the 

 different images, it proves that the effect is proportionate to 

 the length of ray submitted to the magnetic force (Experi* 

 mental Researches, 2163) ; for the unit length and multiples 

 of the unit length may be observed at once, the intensity of 

 the magnetic force and other circumstances remaining un- 

 changed. 



It permits the attainment of a far greater degree of accu- 

 racy in the measurement of the amount of rotation of a given 

 ray, or in the estimation of the comparative degrees of rotation 

 of the different coloured rays. 



The form of the arrangement makes a short piece of any 

 given diamagnetic, as a crystal, &c, sufficient for an experi- 

 ment, which would not suffice if the ray were passed but once 

 through it. 



It allows of the concentration of the magnetic force by an 

 approximation of the poles, when a magnet is used, so as to 

 exalt the effect; or to render a weak magnet equivalent to a 

 stronger one, so that even good ordinary magnets may now 

 be made available. Or if a helix be used, a much shorter and 

 weaker one than that which before was necessary, may now 

 be employed. 



[Note. — A heavy domestic affliction having suddenly taken the author's 

 attention away from this paper, the remaining part must be deferred to the 

 next number of the Phil. Mag.] 



XXVIII. On the Blue Compounds of Cyanogen and Iron. 

 By Alexander W. Williamson, Ph.D.* 



IT is a well-known fact that the different substances which 

 pass by the general name of prussian blue, when pre- 

 pared in the usual way, are not pure combinations of iron, 

 but invariably contain potassium, of which the quantity varies 

 according to the circumstances under which they are formed. 

 It has been the subject of frequent experiment to decide 

 whether this potassium should be considered as an admixture 

 or as an essential constituent, and in the latter case to dis- 

 cover what part it plays in the constitution of the compound. 

 Among those various researches we may confine ourselves to 

 the consideration of those of Berzeliusf and Gay-Lussac J. 

 Gay-Lussac found that prussian blue retains potassium so 



* Communicated by the Chemical Society; having been read March 

 16, 1846. 

 f Poggendorff's Annalen, vol. xxi. p. 490. % Ibid. vol. xxv. p. 385. 



