478 Correspondence — B. B. Woodicard. 



In the case of the copper-tin series of alloys tliey find that, 

 according to the temperature and coustitution of the liquid, crystals 

 •belonging to no less than six different types may separate, namely : — 



a, a solid solution of Cu with less than 9 per cent, of Sn. 



/3, a solid solution of Cu with less than 27 per cent, of Sn. 



7, of which the constitution is not known. 



c, which probably has the composition Cu^ Sn. 



1], which probably has the composition CujSn. 



H, which probably has the composition Cu Sn. 

 Both /3 and 7 are unstable at ordinary temperatures. The compound 

 o crystallises out of /3 or 7 while they are already in the solid state, 

 when the temperature falls sufficiently. 



A glance through the 101 photographs of chilled and etched 

 ingots which accompany Heycock and Neville's paper on this series 

 of alloys shows how impossible it would be from the final com- 

 position of the solid alloy to ascertain the various stages through 

 which it has passed during cooling ; as the authors remark, it is of 

 the nature of a palimpsest. For example, the alloy, containing 

 14 atoms of tin to 86 of copper, consists at 800° of a crystals in 

 a ground-mass which probably contains fi ; it solidifies at about 

 775° ; at 675° there are only /3 crystals ; at 600° there are a and y3 

 crystals, but here a has crystallised out of ji after it became solid ; 

 at 530° there is a much larger proportion of a ; at 470° there are 

 a crystals immersed in a mixture of a and o into which the residual 

 ft has broken up on cooling. 



If the course of events is so complex in an alloy of only two 

 metals, how much more difficult must it be to decipher in the case 

 of a mass of complicated silicates which are even more prone to form 

 isomorphous mixtures, such as we have in a solid rock, not to 

 mention the additional presence of aluminates, oxides, and sulphides. 

 And yet geologists are accustomed to speculate freely about the 

 crystallisation of rock constituents from the magma without taking 

 account of anything save the final stage. 



I cannot help thinking that the experimental method of Heycock 

 and Neville will have to be applied to the study of slags and fused 

 silicates if we are to trace successfully the evolution of rock species. 

 The value of their work to geologists is not only that the results are 

 skilfully interpreted by the light of modern physical chemistry, but 

 primarily that it is experimental work upon actual crystallising 

 materials. 



{To be concluded in our next member.) 



coI^I^JBs:po35^D"jE3^c:E. 



THE CHALK BLUFFS AT TEIMINGHAM. 

 Sir, — Students of East Anglian geology will read with some 

 surprise, in the concluding paragraph of the paper by Professor 

 Bonney and Mr. Hill on the Trimingham Chalk Bluffs, that a hypo- 

 thetical interpretation of these masses was out of place in a Survey 

 memoir. Surely in a work dealing with this district some attempt 



