564 Dr. J. T. Bottomley on Radiation of 



shows that a solid body first becomes luminous at a very 

 much lower temperature than 535° C, which was given by 

 Draper as the temperature of the lowest visible red heat. 

 Weber found traces of luminosity at a temperature as low as 

 391° 0. for platinum, and about 378° C. for iron. 



He also describes very carefully, and in an extremely in- 

 teresting manner, the dawn of a dusky gray light at these 

 temperatures, and its transformation at higher temperatures 

 into the light of low red heat, and subsequently into bright 

 red light and white light. 



Following Weber I have commenced by bringing up the 

 platinum strips to the very lowest temperatures at which 

 luminescence could be perceived, and 1 then measured the 

 electrical power required to maintain the temperature of 

 each strip, one of them polished and the other covered with 

 lampblack. This being done, I passed on to very dull red, 

 then to dull red, and subsequently to cherry-red, and to a 

 white heat; and in each case I made similar measurements 

 with the voltmeter and amperemeters, to ascertain the power 

 in watts required in each case. The two strips were matched 

 as to light-giving quality as closely as possible. 



After some practice my assistant, Mr. W. T. Evans, whose 

 accuracy I cannot sufficiently praise, attained great skill in 

 matching the pairs of strips. The results of our experiments 

 are shown in the following table (p. 565) and in the corre- 

 sponding curves 1, 2, and 3, Plate VI. 



Column 2 of the table describes the condition of the strips. 

 The specification gray light, dull red, &c. is purely arbitrary, 

 and mainly indicates the opinion of the experimenters, Mr. 

 Evans and myself, as to the light-giving condition of the 

 surface. The gray light referred to in experiments 1 and 2 

 is the colourless light of Weber ; though in our experiments, 

 at the lowest point of temperature at which he observed the 

 beginnings of light, we could not see anything *. The nearly 

 white light of experiment 13 is not nearly so white as the light 

 produced from an incandescent electric lamp. It has been 

 explained that the two strips were matched in appearance in 

 our experiments, and Column 9 shows approximately the 

 temperature of each strip, inferred from the resistance of the 

 strip which is given in Column 6. The resistance is easily 

 obtained as the ratio of the potential-difference to the current. 

 The current in amperes passing through each strip is shown 

 in Column 4, and the potential-difference in volts between 

 the points EE, in each strip, is shown in Column 5. 



By multiplying together the amperes and the volts and 

 dividing by JS, the product of Joule's equivalent J, and S 

 * Doubtless through want of practice. 



