448 Scientific Proceedings, Royal Dublin Society. 



black oxide is visible. If coi^per be heated, some particles melt at the fusion- 

 point of cuprous oxide, and at the same time molten oxides may be seen 

 flowing from smaller specks, representing partial oxidation ; and, as the 

 temperature rises, the copper may be observed to melt suddenly, and to 

 become coated with a film of black oxide wliioh rapidlj' liquefies. The true 

 melting-point of copper cannot be obtained in this manner, owing to the 

 formation and mutual solution of cuprous oxide and copper to form 

 Cu - CtioO alloys with melting-points at temperatures below that at which 

 copper melts.' 



Surface-tension effects : — Melted copper oxide shows remarkable surface- 

 tension effects with change of temperature ; the edges of the fusion darting 

 about in every direction in streamers which attach the platinum, leaving 

 arborescent patterns visible in tlie cold. These effects are seen to a less 

 extent with other substances and with some minerals. 



Brightness and Emissivitj/ : — Some powders e.g., cassiterite and monazite, 

 approximate in colour to the hot ribbon so as to be rendered nearly invisible' 

 when it is difficult to perceive indications of melting. Other substances, e.g., 

 palladium, while difficult to see before melting, flash into view on assuming 

 the molten condition, and so facilitate accurate measurement. 



The loss of emission-power which often occurs when a bright fusion is 

 further heated is described in the table (p. 450) as "loss of body." 



Glowing :— When certain sulphides are heated quickly to a temperature of 

 about 700° C, the rapid oxidation which occurs causes incandescence. This 

 does not occur with a slow rate of heating of over a few seconds' duration, 

 as when the powder is heated in a crucible, nor j'et in the absence of 

 free oxygen. Suoli glowing was observed in some earthy uraninites; and 

 when such particles are spread through a powder tliey may be seen as 

 individual momentary flashes of light. 



IVte melting-point : — Some substances such as silver melt " easily," that 

 is, relatively large particles flash into the molten condition with obvious 

 indications of the change. Others which melt " with difficulty " assume the 

 fluid state, even with small particles, so slowly as to defy accurate observation. 

 In these cases the iridescence of the liquid may be the first indication that 

 melting has occurred. Such bodies show no rounding of the corners which 

 may represent either softening or free melting of extreme angles and edges. 

 Tlie fusion may, as the temperature rises, flow more freely, or collect 



' This phenomenon has been observed Tritli nickel (Day, Sosman, and Allen, Amer. Jour. Sci., 

 [4], "vol. xxix, 1910, |). 137), where it was found that a sharp cliange in the melting-curve was obtained 

 at a point 10° C. below tlie true melting-point of nickel in an atmosphere of hydrogen, and which may 

 represent the formation of a Ni — NiO eutectic. 



