A PLATINUM SURFACE AT HIGH TEMPERATURES. 5CT5 



A large number of experiments were made, but only the most typical results have 

 been recorded here. The observations are given in the tables according to the order 

 in which they were taken. In Table HA will be found an illustration of the way in 

 which all the results were worked out. 



The curves relating to air, hydrogen, carbon dioxide, oxygen, and steam are given 

 in figs. 2, 3, 4, and 5. The pressure in centimetres of mercury is indicated on each 

 curve. Observations for each of the first three gases are given at three distinct 

 pressures, for steam and oxygen the emissivity is only determined at a pressure of 

 76 and 228 centims. respectively. 



The degree of purity of the hydrogen used was 97 per cent., the remaining 

 3 per cent, being, in all probability, air. The carbon dioxide, which was prepared 

 from calcium carbonate, contained less than 1 per cent, of impurities. 



During the course of each experiment the exact height of the barometer and the 

 temperature of the room, the standard resistance, and the potentiometer, were 

 recorded. The temperature of the enclosure was also read at short intervals of time 

 on a thermometer divided in tenths of a degree. These factors have too little effect 

 on the main point at issue for each of the readings to be recorded here. 



In the course of the preliminary experiments it was found that the emissivity, as 

 far as these results are concerned, is practically independent of the condition of the 

 glass surface (whether perfectly clean and freshly polished or covered with a thin 

 layer of lamp-black), and of the exact position of the radiator in the enclosure. 



The emissivity does not seem to bear any simple relation to the specific gravity 

 of the gas. In hydrogen, and to a lesser degree both in carbon dioxide and in 

 oxygen, the emissivity is greater than in air. 



In fig. 2 the rise in emissivity caused by saturating the air with moisture will be 

 seen. The difference in emissivity is nearly constant throughout the entire range of 

 temperature, averaging '0001 therm, per second per square centimetre of surface, per 

 degree above the enclosure. This is all the more remarkable as the average tempe- 

 rature of the air, and therefore the quantity of water vapour present, naturally 

 increases with the temperature of the radiating wire. 



In fig. 6 some curves will be found giving the relation between the temperature of 

 the wire and the mean temperature of the gases in the enclosure. These curves were 

 obtained by using the enclosure itself as a rough form of air thermometer, the 

 measurements being made at atmospheric pressure. The three gases are here in the 

 same order as when classed with regard to the experiments on emissivity. For all 

 the gases included in this study, the rise of emissivity due to any given increase of 

 pressure is not proportional to the initial value of the emissivity, but is more nearly 

 constant at all temperatures. 



For facility of reference some parts of the hydrogen curves given both in fig. 2 and 

 fig. 3 (Plate 18), and the curve representing the emissivity in air at three atmospheres, 

 will be found again in fig. 4 (Plate 19). 



VOL. CXCI. A. 3 T 



