550 Mr. P. L. Gray on the Minimum 



In the same paper Draper arrives at the conclusion that all 

 solid bodies become visible at the same temperature', a conclusion 

 which is fully borne out by the observations given below, as 

 to the equality of temperatures in the case of bare and lamp- 

 blacked platinum. 



Draper's value being, as shown above, somewhat doubtful, 

 the point seems worthy of more exact experiment, both from 

 a physical and a physiological point of view ; and as I had, 

 in beginning a series of investigations on Radiation, a means 

 of keeping a platinum surface at any desired temperature, I 

 made a number of observations on the subject, while waiting 

 for other apparatus for use with which the platinum strip 

 was primarily intended. 



The instrument in question is Wilson and Gray's modified 

 form of Joly's Meldometer, and is described in their paper, 

 "Experimental Investigations on the Effective Temperature 

 of the Sun," read before the Royal Society on March 15th, 

 and shortly to be published*. It consists essentially of a strip 

 of very thin platinum, about 10 centim. long, 1 centim. broad, 

 and ^o millim. thick. The plane of the strip is vertical. 



It can be heated by an electric current, and its linear 

 expansion is indicated by an optical method, by which an 

 alteration in temperature of 1° can easily be noticed. The 

 method of calibration is described in Joly's paper f, and in 

 that already mentioned, so that it is unnecessary to do more 

 than briefly refer to it here, Minute fragments of substances 

 of known melting-points are placed on the strip and watched 

 through a microscope, while the temperature is very slowly 

 and cautiously raised until, in any case, melting is seen to 

 take place, when the position of the spot of light which indi- 

 cates the expansion of the strip is noted. In these experiments 

 the substances used were K 2 N0 3 (339°), AgCl (451°), KBr 

 (699°), and gold (1041°) J. From these observations a curve 

 showing the relation between temperature and scale-readings 

 is obtained. One point may, however, be mentioned, viz. as 

 to the difference of temperature between the middle of the 

 strip and its surface. Joly says § that the difference is prob- 

 ably measured by hundredths of a degree only. The following 

 figures will show the correctness of this assumption. 



At a temperature of about 500° C, amount of energy lost 

 per second per square centimetre ="3 calorie. 



* Phil. Trans. 1894. 



t Proc. R. I. A. vol. ii. 3rd series, 1891-92, p. 38. 



X See the same two papers already quoted. 



§ Proc. R. I. A. vol. ii. 3rd series, 1891-92, p. 49. 



