ON THE PRESENT STATE OF THE THEORY OF POINT-GROUPS. 131 



where Q=2j9-2-R and (j\ stands ior ^^^~l^ ' ' ' ^^"^."^^^ 



but the rigorous proof only appeared in the year 1890. For particular 



values of q, however, viz. q=l, 2, 3, and assuming the rigorous proof of 



formula (A), formula (B) was proved in the previous papers. In fact, we 



-see that (a) and (6) on p. 130 are particular instances in which E,=A;, q=i, 



Report on the Chemical Compounds contained in Alloys. 

 By F. H. Neville, F.R.S. 



Past I. 



TLOa 



Methods of Discovery of Compounds ......... 131 



Chemical Methods of Isolation .......... 131 



Freezing -Point Curves ............ 132 



Microscopical Study ............ 141 



Montgen-ray Photography 141 



Determination of iHectrical Potential atid other Physical Methods . . . 142 



Part II. 



Table of Intermetallic Compounds and JDiscussion of it ..... 144 



Molecular Weights of the Metals 146 



Tables of Depression of the Freezing-point caused by dissolving other metals in 



Tin, Zinc, Bismuth, Cadmiuvi, Lead 147 



Table of References 149 



PART I. 



Although most students of alloys are now convinced that they often 

 contain definite chemical compounds, yet these 'intermetallic' com- 

 pounds are still passed over in silence by the authors of books on descrip- 

 tive chemistry. The cause of this omission lies in the difficulty of isolating 

 these bodies in a pure state, and in their resemblance to the metals. It 

 must be acknowledged that just as the metals resemble one another more 

 than do the non-metals, so their compounds often present a gi-eat 

 superficial resemblance to their constituent elements. Intermetallic 

 compounds might well be compared to the somewhat intangible bodies 

 formed by the union of the halogens with each other and with sulphur. 

 Many of these bodies show marked dissociation — that is to say, they readily 

 form systems in true equilibrium with their components ; it is almost 

 certain that ' intermetallic ' compounds present the same phenomenon when 

 in contact with liquid alloy. 



Methods of Studying Intermetallic Compounds. 



The method that naturally suggests itself to a chemist is that of ex- 

 tracting the pure compounds from an alloy by filtration, by volatilisation 

 of excess of a volatile metal, or by removing the excess of metal by 

 means of a suitable solvent. Each of these methods has been employed 

 with some success. 



Filtration methods are very difficult at high temperatures, but if the 

 difficulties can be overcome so that the first solid separating from a liquid 

 as it freezes is isolated, we shall get invaluable information. By the 

 filtration of a partly solidified solution of gold and cadmium in tin, Heycock 

 and Neville (') obtained a crystalline residue approximating to the formula 



K 2 



