1896.] On the Superficial Colour of a Silver-Zinc Alloy. 223 



the red colour in an atmosphere of pure hydrogen. To effect this 

 some filings of the alloy were placed in a glass tube about 18" 

 long. Pure diy hydrogen was then passed through the tube for 

 an hour or more, the tube being heated throughout its length to 

 at least 300° C. The tube was finally sealed by the blow-pipe at 

 both ends, the arrangements throughout being such that no air 

 could possibly enter the tube containing the alloy. To avoid the 

 danger of the hydrogen containing oxygen it was prepared, in one 

 experiment from palladium which had been charged at a lower 

 temperature, and in another experiment, the hydrogen, from zinc 

 and sulphuric acid, was passed through a foot of red-hot copper 

 gauze before entering the alloy tube. All important joints were 

 made by sealing the glass tubes to one another with the blow-pipe. 



We thus obtained two closed tubes containing a little of the 

 alloy in an atmosphere presumably free from oxygen. The alloy 

 in the tube was then brought to one end and that end heated to 

 300° C. while the other end was cold. We then threw the hot 

 alloy to the cold end of the tube and shook it there to promote 

 rapid cooling. In both cases it turned red. We have also obtained 

 the red in an atmosphere of carbon dioxide. 



Hence the formation of the red colour does not depend on the 

 chemical nature of the surrounding atmosphere : it is a change 

 for which the alloy is itself sufficient. We find that in a low 

 vacuum of anything above a millimetre the red comes as easily 

 as in air, but Mr Skinner, who was kind enough to get a good 

 vacuum, say ^ mm. in a tube of the alloy, found that it did not 

 change colour. We have since verified this repeatedly but we 

 find that if such a high vacuum-tube be heated above the 

 temperature at which it was sealed the red can be obtained in it. 

 The behaviour of these vacuous tubes is explained we think by 

 Mr Skinner's suggestion that it is due to variations in the power 

 of cooling by convection. In low vacua the convection cooling of 

 the particles of alloy by the gases in the tube will be almost as 

 rapid as at the atmospheric pressure, but in high vacua this mode 

 of cooling will be absent and hence the alloy cools too slowly to 

 turn red. If we heat such a tube to a higher temperature gas 

 is given off from the walls and the vacuum falls, hence the red 

 can be obtained. 



The other way of getting the red colour is to turn up in a 

 lathe or to file the white alloy. The fine particles thus obtained 

 have a pink tinge, but in the course of a few days or weeks they 

 become a bright pink. This gradual change is not as one might 

 think due to the air but to a latent power communicated to the 

 turnings as they are separated from the main mass of metal — 

 the temperature of each fragment being very high at the moment 

 of its separation. 



