and on the Isomeric Modifications of Selenium. 515 



excesses of temperature of the system above the surrounding 

 medium:— Mean 



temperature. Cooling. .., ; , ) 



'"'^ From 71 to 72 V r^ . 175-5 5*7 



70 to 71 . . . 136-6 2-1 



69 to 70 . . . 120-0 1-0 , 



■^''■- ■' 68 to 69 . . . 113-0 0-7 



KOr. ;. ^- g7 to 68 . . . 109-2 0-5 



66 to 67 .. . 106-8 0-34- 



65 to 66 . . . 105-0 0-25 



64 to 65 . . . 103-5 0-17 



63 to 64 . . . 102-3 0-12 



62 to 63 . . . 101-3 0-07 



61 to 62 . . . 100-5 0-02. 



M*) 



10;97 



Thus the system would have lost by radiation, during the 

 ascending period of the thermometer, 10°-97. By these two 

 causes, the total elevation of the thermometer was too little by 

 27°-8 + 10°-97, or by 38°-77. 



Hence, if the losses by radiation had not existed, the ther- 

 mometer would have been raised to 214 + 38-77, that is to 253 

 degrees ; and if the selenium had not been enclosed in an en- 

 velope of brass, which divided with it the heat emitted, the 

 elevation of temperature experienced above 100 degrees would 

 have been oo, given by the formula 



15-42.2?=23-12 . 153, whence ^=229°. 

 According to this calculation, which is only approximative, the 

 temperature of dry selenium would have been raised from 98 to 

 329 degrees in consequence of the molecular transformation, if 

 all the heat disengaged had been effective. 



I sought to determine, in another manner, the quantity of 

 heat which selenium disengages during its molecular transform- 

 ation, and made the following experiment : — 



The small vessel of brass containing the selenium recently 

 melted was placed in the oven for specific heats, exactly as in 

 the preceding experiment. When the thermometer of the oven 

 began to rise above 100 degrees, in consequence of the molecular 

 transformation, the calorimeter for specific heats was arranged, 

 and at the moment at which the thermometer of the oven exhi- 

 bited the maximum, the small vessel was rapidly lowered into 



* As the cooling was not observed in the second period under 1 08 de- 

 grees, the last partial coolings were calculated by means of the formula 

 A^= A^l ; 6 representing the excess of the temperature above 100 degrees. 

 The constant was calculated according to the observation (69 to 70), that is, 

 1*00 was assumed to be equal to A 20, whence A=^V' ' ' 



