ON COLOURS IN METAL OLASSES, ETC. 287 



composition,* does not require those different states to have been allotropic. Again, 

 it is unnecessary to suppose that BOLLEY'S lead,t prepared by electrolysis, and similar 

 in composition to sheet-lead, is allotropic because it oxidises rapidly in air while sheet- 

 lead does not : for the electrolysis gives the essential fine division, and the consequent 

 large amount of surface exposed to the air greatly accelerates oxidation. 



Lastly, SCHUTZENBERGER| supposed that the copper deposited on the platinum 

 electrode of a copper- platinum cell was allotropic because it was very fragile, its 

 density was only about '9 of that of normal copper, it oxidised rapidly in air, and it 

 could be converted into normal copper by prolonged contact with dilute sulphuric acid. 

 Here, too, the supposition of allotropy is not required to account for the facts. For 

 the low density, the fragility and the rapid oxidation are all accounted for by the 

 loose structure which we should expect in such a deposit of copper, while CAREY LEA 

 found that his silvers, which, if our conclusion at p. 285 is correct, were only finely 

 divided silver, could be transformed to normal silver by contact with sulphuric acid. 

 Similar remarks apply to SCHUTZENBERGER'S silver.^ 



Consider now MATTHIESSEN'S important generalisation^ that metals may sustain 

 change in their molecular condition by union with each other in a fused state. 

 ROBERTS- AUSTEN points out|| that the evidence that metals ever assume allotropic 

 states, when they enter into union with each other, is difficult to obtain. When 

 obtained, the evidence is generally composed of the facts that the specific gravity of 

 the normal metal is greater than that of the metal in the state alleged to be allotropic ; 

 that the chemical activity is less in amount, although the same in kind, for the former 

 than for the latter state ; and that the appearance of the metal is different in the two 

 states. Reference is also sometimes made to a difference in physical properties which 

 is accounted for by lack of continuity, and consequently of electric conductivity, in 

 the supposed allotropic state. IT Occasional reference is also made to a readiness to 

 form hydrates which the metal in the latter state exhibits. Setting this last property 

 aside, as not yet established, the remaining evidence is not conclusive, for all the facts 

 in question are also characteristic of optically continuous granular (or amorphous) 

 pieces of metal. Increase of chemical activity, for example, is a consequence of the 

 enormous effective surface in a medium built up of independent granules. 



Further, when one metal is united with another in a fused state, a chemical 

 compound is not, in general, formed, but the molecules of the two metals freely mix. 

 Thus one metal is in solution in the other. So long, therefore, as the temperature 

 remains sufficiently high to permit the molecules to move about freely, the molecules 

 of each metal tend to segregate, and to group themselves into separate crystals as the 

 * JOULE and LYON PLAYFAIR, 'Memoirs of the Chem. Soc.,' vol. iii., p. 57 (1846). 



t EOBERTS-AUSTEN, loc. tit., p. 90. 



J 'Bull. Soc. Chim.,' XXX., p. 3 (1878). 



ROBERTS AUSTEN, loc. cit., p. 87. 



|| Loc. cit., p. 91. 



IF Of. PETERSEN on "Allotropic Forms of Metals" (' Zeitschr. f. Phys. Chem.,' 8, pp. 601, 1891). 



