NATURAL PHILOSOPHY. 139 



" (9.) If it were possible to produce a spectrum of the heat 

 radiated at 150 C., the spectrum would, if rock salt were the radi- 

 ating body, exhibit only one luminous band. If sylvin were used 

 as a radiator, the spectrum would be much more extended, but 

 would still occupy but a small portion of the spectrum which the 

 heat reflected from lamp-black would form." 



In concluding this part of his memoir Magnus makes some re- 

 marks on transparency, which seem to us very suggestive. If we 

 assume that there is a constant interchange of heat even between 

 bodies having the same temperature, we may fairly assume also 

 that there is such an exchange in the case of light. We cannot 

 observe the light which bodies emit, at ordinary temperatures, 

 but they do not absorb light, since this absorption produces their 

 colors. If such an exchange of light takes place at ordinary tem- 

 peratures, it would follow that transparent bodies cither radiate 

 only such rays as are not contained in the light emitted by ignited 

 bodies, and then they absorb none of these rays, or they emit 

 only one or a few of the wave-lengths of the light which is visible 

 to us. Since, then, they absorb only these, and allow all others to 

 pass through, so that the intensity of the transmitted light is but 

 little less than that of the incident light, we may therefore infer 

 that the transparency of bodies depends upon the fact that they 

 radiate only a few of the wave-lengths, which are contained in 

 the light known to us. Sillimaii's Journal, from Poggendorff. 



USE OP THE ELECTRIC CURRENT IN CALORIMETRY. 



BY M. J. JAMIN. 



"Joule's Law gives the heat which is developed in conductors 

 when traversed by currents. A metal wire may be regarded as 

 a focus. It may have any possible form and be placed where we 

 please, in the midst of liquids or gases ; a quantity of heat will be 

 given off proportional to the time, to its resistance, and to the 

 square of the intensity of the current. It will heat these bodies 

 by a quantity that can be measured, and which is inversely pro- 

 portional to their mass and to their specific heat. Hence results 

 a new process to determine this specific heat. After numerous 

 trials I fixed upon he following arrangements : 



"I. Case of Solids and Liquids. In dealing with a solid or a 

 liquid, I use as a calorimeter an elongated cylindrical vessel of 

 thin copper, on which is coiled 8 metres : of German-silver 

 wire 0.2 millim. in diameter, and covered with silk. This spiral 

 commences at the bottom of the vessel, and ascends to one-third 

 of its height ; it is connected with the circuit by thick copper wires ; 

 its resistance is measured for all the temperatures of the experi- 

 ment. I envelop it with a thick silk ribbon to keep it in its 

 place, some swan's down to insulate it, and I enclose the whole in 

 an envelope of thin copper polished. When the calorimeter con- 

 tains a liquid, and a current is caused to pass through the spiral, 

 nearly all the heat will be transmitted to the side, then to the 

 liquid ; a scarcely appreciable portion will be transmitted to the 

 swan's down. 



c ' 



