THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



AUGUST 1865. 



XII. On the different Properties of the Heat radiated by Rough 

 and by Bright Surfaces. By Professor G. Magnus*. 



[With a Plate.] 



SINCE Sir John Leslie's researches upon heat, it has been 

 known that metals radiate more heat when their surface 

 is rough than when it is bright ; but this remarkable pheno- 

 menon has not, upon the whole, been much studied, and many- 

 questions connected with it remain still unanswered. Melloni 

 and his successors have examined the heat given out by various 

 bodies at different temperatures; but the differences in the 

 radiation of one and the same body at a constant temperature, 

 but with different surfaces, have hitherto been very little inves- 

 tigated. The first question to be answered in relation to these 

 differences, was whether the points which a rough surface pre- 

 sents are the cause of the increased radiation. Mellonif and 

 Knoblauch J have shown that this is not the case, but that in- 

 crease of radiation depends rather upon the diminution of den- 

 sity which the surface undergoes when it is roughened in a cer- 

 tain manner; and I have myself §, as well as others before me, 

 pointed out that the increased radiation is to be attributed to 

 the state of greater subdivision of the roughened surface, as well 

 as to its diminished density. Beyond this, scarcely anything is 

 known about the differences of radiation. 



Does the more abundant radiation which takes place from 



* From Poggendorff's Annalen, vol. cxxiv. p. 476 (1865, No. 3). 



t La Thermochrose, p. 90, note. 



X Poggendorff's Annalen, vol. lxx. p. 340. [Taylor's Scientific Me- 

 moirs, vji. v. p. 214.] 



§ Monatsber. d. Berl. Akad. 1864, p. 593. [Phil. Mag. Ser. 4. vol. 

 xxix. p. 58.] 



Phil. Mag. S. 4. Vol. 30. No. 201. Aug. 1865. G 



