Intelligence and Miscellaneous Articles. 147 



certain length, which becomes heated to a temperature which can 

 be estimated in different ways. The deflection of the compass is 

 observed when it has become stationary. The wire is then replaced 

 by a thinner one, of which the length is varied until the compass 

 gives the same deflection as before ; the rest of the circuit remains 

 unchanged. The resistance of the stout and long and that of the 

 short and thin wire are evidently equal, and the quantities of heat 

 evolved in their interiors are the same. 



Operating thus with two platinum wires taken from the same 

 sample, one of them 0*62 millim. in diameter and 538*7 millims. in 

 length, and the other of 0*31 millim. diameter and 77*7 millims. 

 length, the same deflection was obtained. The ratio of the two ra- 

 diation-surfaces is therefore 



2x538-7 _ 13 . 9 

 77-7 

 The radiation per unit of surface of the thin wire was therefore 14 

 times as much as that of the thick wire (neglecting the heat taken 

 away by contact of the air). 



On the other hand, the thick wire was raised to a temperature 

 much below red heat (it did not char wood), while the thin wire 

 became white-hot. If we admit that the difference of temperature 

 was only 600° (a figure certainly too low), the heat emitted per 

 unit of surface by the two wires should have been, according to the 

 law of Dulong and Petit, in the ratio of 1 to 100, and not 1 to 14 

 as was given by the experiment*. 



I made some more trials by a very different method. A globule 

 of platinum, as large as possible, placed in a magnesia cupel, is 

 fused by the flame of a little blowpipe with illuminating-gas and 

 oxygen well mixed. The heat evolved by the gas in burning is 

 about 5600 calories per cubic metre; therefore, if the pipe con- 

 sumes 1 litre per minute, neglecting the losses (which are necessa- 

 rily considerable), the maximum of heat that can be communicated 

 to the platinum is 5*6 calories. Then, if we estimate the radiating 

 free surface of the metallic globule, which is raised to the tempera- 

 ture of fusion of platinum at least, we can calculate the quantity of 



* M. Ed. Becquerel also arrived at similar results, although to my 

 knowledge he has not discussed them from the point of view occupied by 

 us. He operated upon a platinum wire placed in vacuo and passed through 

 by an electric current of varying intensity ; and he calculated the heat 

 evolved internally by multiplying the square of the intensity of the cur- 

 rent by the resistance of the wire, measured at each experiment. Passing 

 thus from a temperature much below red heat to one near the melting- 

 point of platinum, those quantities of heat did not change in the ratio of 

 1 to 13.— La Lumiere, t. i. p. 92. 



We may also cite Professor Tyndall's experiments, who, on measuring 

 with a thermoelectric pile the intensity of a single kind of obscure radia- 

 tions, found that the energy of the radiation of a platinum wire changes 

 in the ratio of 6 to 122 in passing from a temperature below red heat to 

 that of an intensely white heat. According to the law of Dulong and 

 Petit, the ratio ought to have been as 6 to 600, supposing that the differ- 

 ence between these two temperatures amounted to only 600°. — La CJialeur, 

 trad, franc, p. 414. 



