290 Mr. J. H. T. Roberts on the 



After different experiments the apparatus was broken up 

 and the deposit examined in the microscope. A high-power 

 immersion microscope revealed a vast number of particles of 

 various sizes, individually black, and looking exactly like 

 particles of soot. By means of a scale in the eyepiece of 

 the microscope, the particles were seen to range in size 

 from 10 fj, down to about ^ p. There was no evidence of 

 crystallization. 



In order to remove the black deposit from a piece of 

 glass, the glass was boiled in aqua regia, but this did not 

 remove the black powder by any means completely. Another 

 piece of glass from the same apparatus was heated in the 

 Bunsen flame, when the black deposit quickly assumed 

 the form of a metallic film. On treating this with aqua 

 regia, the film was almost immediately dissolved. Another 

 metallic film, obtained by heating the black powder in the 

 same way, was examined in the microscope, and found to 

 consist of a more or less continuous film, which, however, 

 had drawn up here and there, leaving a large number of 

 roughly circular holes ; the film had, in fact, behaved as if 

 it had possessed surface tension ; light was able to pass 

 through the film *. 



These black deposits are also obtained, whenever oxygen 

 is present, by heating iridium, rhodium, and palladium. 

 The effect with iridium is very striking ; an iridium wire 

 430 mgms. in weight lost 30 mgms. in 30 minutes. The 

 deposit upon the glass began to be perceptible two or three 

 minutes after putting on the heating current, and at the end 

 of 30 minutes was thick and almost opaque ; the same result 



begins to be formed as the vapour is cooled on its downward journey 

 along the sides of the vessel. This disproves at once airy theory of the 

 emission of solid particles from the hot wire, as, if this were the case, 

 there would be a deposit on the upper surface of the glass. Nos. 6 and 7 

 show the deposits from a platinum wire in the form of a spiral ; as the 

 wire bent on heating it was nearer to one side of the vessel than to the 

 other, and consequently the convection currents went up one side of the 

 vessel and down the other ; hence the vapour was cooled on its upward 

 journey and deposits were in this case obtained on the top surface. In 

 other experiments with the spiral wire, when the spiral has remained 

 along the axis of the tube, deposits of the same pattern have been 

 obtained, but in these cases, as in Nos. 1 to 5, the deposits were not 

 produced upon the upper surface. 



* In this connexion see the interesting papers by Stone (Pliys. Iiev., 

 July 1905), and by Turner (Proc. Roy. Soc. 1908), where it is shown 

 that thin films of other metals, such as silver and gold, on being heated 

 to very moderate temperatures, below 500° C, also draw up, leaving 

 holes ; the metallic parts are still opaque, but the light is transmitted 

 through the holes. The reflecting power of the film is diminished and 

 the electrical resistivity is greatlv increased. 



