288 MR. J. C. MAXWELL GARNETT ON COLOURS IN METAL GLASSES, ETC. 



temperature is slowly lowered. It is, however, probable that, as in the case of gold 

 and copper ruby glasses, the molecules of each metal first group themselves into 

 small spheres. If the temperature were rapidly lowered at this stage, this granular 

 structure would be fixed in the alloy. If, then, one metal that, suppose, of which 

 the larger volume is present were suddenly annihilated, the other metal would 

 remain in a granular form, possessing a colour* quite different from that exhibited by 

 the normal form of that metal. 



Now when an alloy of potassium and gold containing about 10 per cent, of the 

 precious metal is thrown on to water, the potassium is, in effect, annihilated, t and 

 the gold is released as a black or dark brown powder. It will be seen from fig. 12 

 that granular gold, with a density slightly over '6 of that of normal gold, would 

 reflect light of a brown colour, while the reflecting power would not exceed '5. A 

 granular structure is thus in accordance with the dull appearance and with the colour 

 of the powder. Similarly when a silver-gold alloy containing two parts of silver to 

 one of gold is treated with nitric acid the silver is removed, the gold remaining in the 

 form of a dull brown powder, which can be converted into bright metallic gold by 

 slight pressure or by heating to redness. It appears, therefore, that this brown 

 powder is probably granular gold, the component particles being small compared with 

 a wavelength of light ; so that, once more, the evidence J does not require us to 

 suppose this form of gold to be allotropic. 



Finally, it seems unnecessary to assert that iron released from its amalgam by 

 distilling away the mercury is in an allotropic form because it takes fire on exposure 

 to the air. For this burning of the iron would be the consequence of the large 

 surface exposed to the air by an extremely finely divided form of the metal. 



We conclude, therefore, that in none of the cases of supposed allotropy, which we 

 have examined in this section, has the existence of an allotropic form of metal been 

 established. 



* See 10 above. 



t Of. ROBERTS-AUSTEN, loc. dt., p. 91. The potassium does not catch fire, but combines with the water 

 to form KHO (which immediately passes into solution and is thus removed) and H which catches fire. 



t We must except that of the alleged formation of auric hydrate, but I have been unable to obtain any 

 confirmation of the existence of such a compound. 



