1894.] On the Liquation of Silver-Copper Alloys. 265 



the midst of which there was no regular effect to be seen ; but as the 

 experiment went on, and the medium improved, the expected effect 

 came out distinctly : a quick downward jump of the fringes at or im- 

 mediately after the instant of the spark. Under good optical con- 

 ditions, and at potentials high and low, the effect was perfectly 

 regular, and was distinct and pure as that in oil of colza, though 

 apparently not quite so large. 



(2.) Plane of polarisation of the pencil BF horizontal : Rise of 

 fringes indicates relative retardation of BF. The disturbance of the 

 fringes was greatly reduced as the experiment went on, till at last 

 there was nothing left but a set of slow movements, very irregular 

 and very small, sometimes invisible. In the midst of these, as in 

 their absence, and in a long set of observations, taken at different 

 potentials, from low to highest, there was no trace ever seen of a 

 jump of the fringes at the instant of the spark. It appears, there- 

 fore, that in this negative dielectric, as in oil of colza, the total 

 optical effect of electric strain is an acceleration of the vibration 

 which is directed along the line of force. 



The conclusion to be drawn from the preceding experiments has 

 been stated already by anticipation j but I repeat it finally in other 

 terms as follows : 



If light pass through an electrostatically-strained medium at right 

 angles to the lines of force, and be represented by tux) component lights 

 whose planes of polarisation are respectively parallel to the lines of force 

 and perpendicular, then the proper and immediate optical effect of the 

 electric strain is a chang-e of velocity of the latter component.* 



The use of the words proper and immediate in this statement may 

 be thought objectionable ; but some such words are required for the 

 purpose here chiefly intended, which is to exclude those undoubtedly 

 remote effects of electric action that appeared as disturbances in all 

 the experiments. 



IV. " On the Liquation of Silver-Copper Alloys." By EDWARD 

 MATTHEY, F.C.S., Assoc. Roy. Sch. Mines. Communicated 

 by Sir G. G. STOKES, F.R.S. Received February 16, 1891. 



It is a well-known fact that daring the solidification of certain 

 alloys groups of the constituent metals fall out of solution, giving 

 rise to the phenomenon called " liquation." The molecular arrange- 

 ment which results from this behaviour of alloys has been investi- 

 gated by many experimenters, notably by Devol, Roberts- Austen, 

 and Guthrie. The author has also studied the behaviour of a large 



* The change of Telocity in the case of any positive dielectric ia of course a 

 decrease. 



