504 MR. J. B. PETAVEL ON THE HEAT DISSIPATED BY 



The melting point of palladium calculated in this way from the formulae of 

 CALLENDAR and GRIFFITHS was for the thermometer B, 1489, as against 1500 as 

 determined by VIOLLE. 



A second confirmation of the temperature determinations will be found in the fact 

 that in alt the curves given below, the calculated temperatures agree with those 

 determined directly by the fusion of platinum or palladium. 



Finally, it may be well to state here that although every precaution was taken to 

 ensure the accuracy of the results, the absolute resistance of the wires used was too 

 small for me to lay claim to as great a degree of precision as that obtained by 

 HEYCOCK and NEVILLE in their valuable researches on the melting point of metals. 

 Up to about 500 I believe the results given to be correct to a fraction of a degree, 

 but above 1200 it is difficult to prove that they are absolutely exact. 



In Table I. will be found a determination of the volume specific resistance of 

 platinum and palladium. The values obtained for platinum show that the tempera- 

 tures above 1500 cannot be calculated by the ordinary equations ; these, for 

 instance, in the case of R 2 would give for the melting point of platinum a 

 temperature more than 100 too low. 



Part I. ON THE EMISSIVITY OF A BRIGHT PLATINUM SURFACE IN AIR 



AND OTHER GASES. 



Throughout the experiments, the results of which are given in Tables II. to XIII., 

 a cylindrical glass enclosure, 5 '8 centims. in diameter and 24 ceutims. in height, was 

 used. The straight platinum wire, which served at the same time as radiator and 

 thermometer, was placed in the axis of the cylinder. A platinum wire, 2 millims. in 

 diameter, bent into the form of a " J " and fused at its lower extremity on to the 

 thermometer, formed the return lead. This wire, as well as the potential leads, was 

 kept well to the side of the enclosure. The enclosure was always used with its axis 

 vertical. This arrangement forms the simplest geometrical disposition of the surfaces 

 of the radiator and enclosure that can be used with the method adopted, as for 

 obvious reasons a sphere enclosed in a sphere is out of the question. 



A sufficient length was left between the point of contact of the lower potential 

 lead and the terminal of the radiating wire, for the wire to acquire a constant 

 temperature. 



As pointed out by Dr. J. T. BOTTOMLEY, the low thermal conductivity of glass is 

 a very serious objection to its use as an enclosure in experiments on emissivity, it 

 being possible for the inner surface to rise to a temperature very appreciably above 

 that of the water circulation. Owing to this fact, it was not thought advisable to 

 take any observations with the radiator at much less than 100 above the enclosure. 

 At temperatures higher than this a slight error in the estimation of the temperature 

 of the enclosure would not alter the results. 



