448 Prof. G. Magnus on the Change in the Radiation 



Knoblauch himself mentions that, by scratching, the parts under 

 the strokes proper are condensed, but the upturned edges are 

 loosened. Melloni maintains, on the contrary, that scratching 

 does not loosen the part, but only lays bare the less dense interior. 



Under these circumstances it seemed desirable to examine the 

 phenomenon a little more minutely ; and with this view the author 

 experimented with platinum plates instead of copper and other 

 easily oxidizable metals, so that other alterations also on the sur- 

 face, such as are produced on silver by small quantities of sul- 

 phuretted hydrogen, were not to be feared. 



A platinum plate which had been made as hard as possible by 

 rolling, after having been strongly heated, radiated as much heat 

 as before. Here the hardness cannot have affected the radiation. 



Another platinum plate had been passed under very great pres- 

 sure between two rollers, one of which was finely grooved; so 

 that after this treatment one of the sides exhibited small eleva- 

 tions, while the other remained smooth. The first radiated 

 slightly more than the other; but after the plate had been 

 strongly heated, even this difference was no longer perceptible. 

 Hence it follows that, when the surface is otherwise the same, 

 inequalities, and even regularly alternating elevations and de- 

 pressions, may exist without any increase in the radiation. 



When, on the contrary, a plane platinum plate, which had 

 been heated by a glassblower's lamp and was quite soft, was 

 roughened by means of fine emery-paper, the radiation was 

 doubled. 



In order to be able to make the comparison, the radiating plate 

 was heated by a small brass apparatus which was kept at 100° C. 

 by means of steam. It consisted of a horizontal cylinder of 50 

 millims. internal diameter and of the same length; the plate 

 under investigation formed one of the ends. In order readily 

 to exchange this plate for another, the cylinder was provided with 

 a broad welt, against which the plate was pressed by means of 

 a brass ring and three screws. In order to make the whole 

 apparatus quite tight, rings of stout paper were used as washers, 

 and rendered it perfectly steamtight. 



To be certain that in the treatment of the plate no foreign sub- 

 stance had been left upon it (traces, for instance, of the glue of the 

 emery-paper, although it was used quite dry), the plates before 

 being fastened in the apparatus were heated for a time in con- 

 centrated nitric acid, then washed with distilled water until all 

 traces of acid were removed, and then dried without being touched 

 by a cloth or any other object. 



It is difficult to conceive that by the slight treatment with 

 emery-paper the density of the surface was so far increased that 

 the radiation was doubled. 



