and on the Elementary Colours of the Solar Spectrum. 263 



below. The electric circuit was established by placing the 

 mercury and the upper extremity of the platina strip in com- 

 munication with the poles of a Grove's battery, of which the 

 force, properly moderated by a Wheatstone's rheostat, could 

 be kept constant for about an hour. By means of this ar- 

 rangement, the strip of platina, more or less ignited by the 

 action of the voltaic current, preserved its rectilinear direction ; 

 tJie copper wire which was fastened to the weight merely dip- 

 ping more or less deeply into the mercury. To determine 

 the dilatations of the strip, there was attached to its free ex- 

 tremity a long and very slender horizontal lever, arranged in 

 such a manner that the point of attachment was very near one 

 of the extremities of the lever which worked upon a pivot, and 

 the other end traversed over a divided scale, taking a position 

 more or less oblique to the horizon as the weight and wire 

 descended by the dilatation of the strip, and indicating on the 

 graduated arc the quantities sought. 



The temperatures were then calculated on the hypothesis 

 that they were proportional to the dilatation of the platinum, 

 employing for this purpose the coefficient of Dulong and Petit. 

 It will be understood that these calculations are easily made 

 when we know the length of the strip and of the lever, and 

 also the position of their point of attachment. In the instru- 

 ment employed by Draper, each degree on the graduated arc 

 corresponded to an elevation of 1 1 5° F. above the temperature 

 of the air. 



Suppose, now, we commence by using a feeble current, the 

 force of which is gradually increased by means of the rheostat. 

 The heat correspondingly augments in the strip, and finally 

 makes it visibly red-hot in the dark room in which the expe- 

 riment is to be conducted. The degree that the index marks 

 is then to be observed. This experiment was repeated several 

 times, and with the aid of different persons (who ought to 

 remain for some time in the dark for their eyes to acquire a 

 due sensibility), and from the mean of all the observations the 

 temperature at which the platina began to be red-hot was cal- 

 culated. From the experiments of our author this tempera- 

 lure is 977° F. 



To determine whether all substances become red-hot at the 

 same degree of heat, Professor Draper took a gun-barrel, the 

 touch-hole of which was closed, and placed in it successively 

 platina, copper, gas carbon, lead, earthenware, and other sub- 

 stances ; then, making the barrel red-hot in the fire, he ob- 

 served attentively the moment when the incandescence com- 

 menced to manifest itself in the barrel, and the substance which 

 it contained. He could discover no difference of time between 

 the two phajnomena ; for the gun-barrel and the substance 



