142 



REPORT — 1900, 



The other photograph is one of aluminium containing ten per cent, of 

 nickel. One sees that an opaque body has again crystallised first The 

 varying thickness of some of the crystals gives them an effect of solidity 

 which is absent from a surface photograph. 



Unfortunately the alloys have to be very slowly cooled in order to 



obtain large crystals, 

 during 



Magnification 



the process of taking 

 the skiagraph is clearly 

 impossible, and as the 

 figure on the plate is 

 not a real optical image, 

 but a shadow from a 

 radiant point of finite 

 size, it is not sharp 

 enough to bear great 

 magnification after- 

 wards. The method, 

 however, is capable of 

 doing more than it has 

 done yet. It was first 

 successfully carried out 

 by Mr. Heycock and 

 shown by him in a 

 lecture at the Royal 

 Institution. 



Electrical Methods. 



Herschkowitz (^'), 

 following Laurie (^*), 

 has compared the elec- 

 trical potential of a 

 number of binary al- 

 loys with that of the 

 more positive metal 

 contained in each alloy, 

 the electrolyte being 

 always a salt of the 

 more positive metal 

 dissolved in water. 



From theoretical 

 considerations he con- 

 cludes that when the 

 solid alloys contain only 

 separate crystals of the 

 two metals a and b, the 

 potential of all the alloys will be that of the more positive metal A 

 (a point practically proved by Laurie's experiments), so that if we plot the 

 composition of the alloys horizontally, and their potential referred to that 

 of A vertically, we shall get a horizontal straight line ; this is realised 

 in the pair Cd-Bi. If, however, the two metals mutually dissolve each 



The numbers below the curve give the Centigr.ide temperature; the 

 numbers above the curve give the atomic percentage of tin in 

 the alloy. 



