MEMOIR L] THE RADIATIONS OF IGNITED BODIES. 27 



cision to any degree, notwithstanding slight changes in 

 the force of the voltaic battery. 



The following are the dimensions and measures of the 

 instrument I have used : Length of the platinum strip, 

 1.35 inch; length of the part actually ignited, 1.14 inch; 

 w 7 idth of ditto, -fa of an inch ; length of the index from 

 its centre of motion to the scale, 7.19 inches; distance 

 of the centre of motion of index from the insertion of 

 the platinum at the point Z>, .22 inch ; multiplying effect 

 of the index, 32.68 times; length of each division on the 

 ivory scale, .021 inch. From this it would appear by a 

 simple calculation, using the coefficient of dilatation of 

 platinum given by Dulong and Petit, that each of the 

 divisions here used is equal to 114.5 Fahrenheit degrees. 

 For the sake of perspicuity I have generally taken them 

 at 115. 



The Grove's battery I have employed has platinum 

 plates thr.ee inches long and three quarters wide; the 

 zinc cylinders are two inches and a half in diameter, 

 three high, and one third thick. As used in these ex- 

 periments it could maintain a current nearly uniform for 

 an hour. I commonly employed four pairs. 



By the aid of resisting wires of different lengths, or 

 the rheostat, the force of the current in the platinum 

 could be varied, and therefore its temperature. The first 

 attempt was, of course, to discover the degree at which 

 the metal began to emit light. 



The platinum and the voltaic battery were placed in 

 a dark room, the temperature of which was 60 Fahr.; 

 and after I had remained therein a sufficient length of 

 time to enable my eyes to become sensible to feeble 

 impressions of light, I caused the current to pass, gradu- 

 ally increasing its force until the platinum \vas visible. 

 In several repetitions of this experiment it was uniform- 



