PHYSICS. B()5 



3. Conduction and Radiation. — Specific Heat. 



Stenger has measured the conductibility of tourmaline by the method 

 of Weber, employing for this purpose plates whose homogeneity had 

 been tlioroughly established. He was unable to discover the least 

 trace of unilateral conductivity and therefore supposes that the con- 

 trary results obtained by Thompson and Lodge arose from the non- 

 homogeneity of the tourmaline they used. In consequence the author 

 does not consider as established their theory of the pyroelectricity of 

 tourmaline, based upon such conductivity. {Wied. Ann., xxii, 522; 

 J. PMjs., November, 1885, II, iv, 522.) 



From the electromagnetic theory of light Maxwell deduced the prop- 

 osition that a beam of light or heat falling normally on a surface exerts 

 upon it a pressure equal to the energy which exists in unit of volume of 

 the ether in consequence of its light-motion. Boltzmaun, combining 

 with this a relation deduced by him from the second law of thermody- 

 namics, has obtained the law of Stefan, that the radiation from a heated 

 body is j^roportional to the fourth power of the absolute temperature. 

 {Wied. Ann., xxii, 291; J. Phys., November, 1885, II, iv, 526). 



Schneebeli has called attention to a paper by Svanberg, published in 

 1851, in which is a description of an apparatus based on the same princi- 

 ple as the bolometer of Langley. With this apparatus he has measured 

 the coefficient of absorption of glass, and finds for it a value which varies 

 inversely as the temperature, being 2*4 at 100°, 1-47 at 250°, and 0-42 at 

 1,000°. He has also verilied the law of Stefan between 400° and 1,000°, 

 and has shown that the ratio of the luminous to the total radiation of a 

 Swan lamp increases rapidly with the temperature; but that the ratio 

 of this radiation to the square of the current-strength remains constant, 

 as Joule's law requires. {Wied. Ann., xxii, 430; J. Phys., November, 

 1885, II, IV, 527.) 



Bottomley, on the contrary, has obtained experimental results entirely 

 disagreeing with Stefan's law. A current of known strength is passed 

 through a platinum wire, the temperature of which is deduced from its 

 increase in resistance. When the temperature has become constant, the 

 heat generated by the current must be equal to that radiated from the 

 surface of the wire plus that lost at the ends of the wire by conduction. 

 The wire was placed in a high vacuum, one twenty-millionth of an at- 

 mosphere, made in a glass tube 6™™ in internal diameter. The wire 

 itself was 0-4">™ diameter and about half a meter long, and was sealed 

 into the ends of the tube, the exhaustion being eflected through a lateral 

 tube. The temperature of the room at the time of the experiment was 

 15°. In four experiments, the absolute temperature of the wire being 

 298, 383, 798, and 823, the ratio of the energy radiated was found to be 

 1, G'l, 71.9, and 90-2; whereas by the law of Stefan the values should 

 have been 1, 16, 438-8, and 499-8. {Nature, November, 1885, xxxiii, 

 85.) 



