Pyrometer on the Melting -points of Gold and Silver. 221 



ments on the melting-points of gold and silver at his laboratory 

 at the Royal Mint. Professor Roberts- Austen himself sug- 

 gested most of the experiments, and verified some of the 

 observations. The results are interesting, not only as con- 

 firming his experiments, but also as showing the applicability 

 of the platinum pyrometer to the accurate observation of small 

 changes in the freezing-points of alloys at high temperatures. 



The pyrometer used was constructed as described in my 

 previous paper. It had a scale of 1 centimetre to the degree, 

 and the galvanometer was so sensitive that it was possible to 

 read the scale with certainty to a tenth of a degree at 1000° C. 



The first experiment was an observation of the freezing- 

 point of a specimen of gold containing as impurity a small 

 percentage (0*05) of silver, which Prof. Roberts- Austen has 

 shown to have very little effect in lowering the freezing-point, 

 as it possesses very nearly the same atomic volume*. It 

 probably tends to crystallize out, when present in small 

 quantities, isomorphously with the gold. 



About 10 ounces of gold were melted in a clay crucible in a 

 small Fletcher oxygen furnace and allowed to cool. The 

 temperature of the metal was observed to fall rapidly till the 

 freezing-point was reached. It then remained steady to a 

 tenth of a degree for a minute or two at pt = 902°*2 (on the 

 platinum scale) while the metal was solidifying. The gold 

 was then again melted and well stirred and another reading- 

 was taken. The result found was pt = 902 o, 3, differing by 

 only a tenth of a degree from the preceding. 



In order to test the effect of the addition of silver in lower- 

 ing the freezing-point the mass was well heated, and 0*5 per 

 cent, of silver was added to it. A rough observation taken 

 while cooling gave pt = 901 c, 5. The metal was again 

 cautiously melted and continuously stirred while cooling. 

 An observation of the freezing-point gave pt =901 o, 8. This 

 was repeated with the result ^>£=901 o, 9. This seems to show 

 that the effect of silver in lowering the freezing-point, though 

 very slight, is still distinctly appreciable. 



Some observations of the melting-point of the metal were 

 taken as well as the freezing-point, but it was not found 

 possible to make the conditions during heating as uniform as 

 during cooling. It was observed that the temperature rose 

 rapidly up to near the freezing-point and then very slowly 

 for 2 or 3 degrees till the whole was melted, when the tem- 

 perature again rose rapidly. The solid metal cannot be 

 stirred, so that one part melts before the other, even if the 



* Proc. Roy. Soc, March 1891, p. 355. 



