Disintegration of Metal & at High Temperatures. 289' 



Thus it seems as though too little iridium has been lost ; 

 this is due to the deposition of a slight film o£ oxide on the 

 cool ends of the wire, which diminishes the observed loss of 

 weight. 



JBlack Deposit upon the Walls of Vessel containing 

 Gloicing Wire. 



The oxide formed deposits itself upon the walls of the 

 glass vessel as a soft black or greyish- black powder. The 

 manner in which the deposit is disposed about the vessel 

 gives every indication that it is the result of the condensation 

 of a vapour ; it gives no support to the view that particles- 

 are ejected from the wire. For the deposit is not distributed 

 equally round the vessel nor is it mostly upon the bottom or 

 top. It is frequently found that the vapour has to be cooled 

 considerably before it deposits. Thus although the thin 

 water-cooled glass must be at practically the same tempera- 

 ture at all parts, the vapour rising upward with the con- 

 vection currents is not sufficiently cooled to deposit on first 

 striking the upper surface, but on flowing over the surface 

 for a short distance it begins to form the deposit at a sharply 

 defined line ; this shows that it has to be cooled to a definite 

 temperature before condensation takes place; the greatest 

 amount of deposition is usually found to take place where 

 the cooling vapour is on its downward journey over the 

 sides of the vessel *. 



* Description of Plate II. 



The plate shows photographs of the small glass vessels containing the 

 glowing wires after the deposits had been formed. Nop. 1 to 5 show 

 the deposits due to an iridium wire, Nos. 1 and 2 being after five minutes 

 heating of the wire, Nos. 3, 4, and 5 after half an hour. As the iridium 

 wire remained straight when heated, and coincident with the axis of the 

 tube, the convection currents in the heated gas would be symmetrical 

 with respect to the cylindrical vessel. As the vessels were 1 cm. in 

 radius and were water-cooled, the convection circulation was probably 

 very vigorous. No. 1 shows the view from above, and it is seen that the 

 black deposit begins at a fairly sharply defined line on the upper surface - r 

 as the cooling vapour circulates round the tube the two streams meet 

 together and the deposit at the bottom of the vessel shows a sharp line 

 of demarcation vertically below the wire : this is well shown in Nos. 1, 

 2, and 4, which are all views from above, the deposit showing this line of 

 demarcation being on the surface of the vessel furthest away from the 

 reader. In Nos. 1 and 2, which are photographs of the same vessel, it ia 

 seen that the side tubes have interfered with the symmetry of flow. 

 Nos. 3, 4, and 5 are different views of the same vessel, and it is seen 

 that the deposit is greatest near the centre of the wire, which was the 

 hottest part. The most important point in all the figures, however, is 

 that the deposit is not formed on the upper surface of the vessel, with 

 which the ascending vapour first comes into contact, but the deposit 



