144 



Mr. J. E. H. Gordon on the 



[June 15, 



preparations the various conditions of a fimgus, to which he gave a 

 generic and a specific name, and although he professed to find the vari- 

 ous conditions of spore, m3^celium, and fructification occurring in their 

 natural sequence, and that natural sequence to correspond with the 

 regular advance of the pathological process, there is no doubt that this 

 circumstantial account rests on erroneous observation and on defective 

 evidence, and that the appearances found in the sldn of the sheep are 

 none other than those resulting from the coagulation of albuminoid fluids 

 under particular circumstances. 



XII. " Determination of Yerdet^s Constant in Absolute Units. By 

 J. E. H. Gordon^ B.A._, Gonville and Caius College^ Cam- 

 bridge. — 1st and 2nd Memoirs. Communicated by J. Clerk 

 Maxwell. Received June 5_, 1876. 



(Abstract.) 



[Note. — The -wkole of this work has been clone under Prof. Clerk MaxAveH's super- 

 intendence ; he suggested the method and nearly all the details. He is, however, in 

 no way responsible for any errors there may be in the numerical results.] 



Il^TEODrCTIO:^". 



In the year 1845 Earaday discovered that if plane polarized Kght passes 

 through certain media, and these media be acted on by a sufficiently 

 powerful magnetic force, the plane of polarization is rotated. 



About the year 1853 M. Yerdet commenced a long and exhaustive 

 examination of the subject, and his first result (pubKshed 'Ann. de Chimie 

 et de Phys.' 3 serie, tom. xli.) was that, for any given magnet and 

 medium, " the ratio between the strength of the magnet and the amount 

 of rotation is constant"*. 



The object of the present research is to determine this constant in 

 absolute measure — that is, in the C.G.S. system. 



In order that the measurements may be expressed in absolute units, it 

 is necessary to modify M.Yerdet's mode of proceeding in several respects. 

 Til particular, an electromagnet with, an iron core is unsuitable for this 

 investigation, for both the amount and the distribution of the magnetic 

 force between the poles depend on the properties of the iron core, and 

 cannot be deduced from the strength of the current in the helix. Eara- 

 day's heavy glass and other media having the highest power of rotating 

 the plane of polarization were also unsuitable to be used as standard 

 media, on account of the difiaculty of procuring specimens exactly alike. 

 The following method was therefore adopted : — 



The magnetic force was produced by means of an electric current in a 



^ This is expressed much more fully in Maxwell's 'Electricity,' vol. ii. p. 400, art. 808. 

 The coefficient mentioned in the last line of the article may be defined as Yerdet's con- 

 stant. In tlie author's larger paper the identity of the two definitions is shown. 



