
between Optical and Electrical Qualities of Metals. 171 
surface 50 mms. diameter) or the “black body” could be 
introduced. The “black body ” was a copper cylinder B 
of 47 mms. internal diameter, furnished with conic ends, and 
coated with lampblack on the inside. Its aperture at 
the outer end was 22 mms. in diameter. Boiling aniline 
served as heating liquid, which was in continual contact with 
the metal sheets to be heated. The heating was done elec- 
trically by means of a spiral of constantan ribbon inside the 
copper case. The temperature was generally regulated to 
170°. ‘The heating-box was put in “front of a metal dia- 
phragm C, or C,, cooled by flowing water. Behind this dia- 
phragm stood a screen D, kept at the temperature of the room. 
When this screen was lifted, the radiation to be measured 
underwent successive reflexions trom three or four suitably 
set-up surfaces of fluorite F, F, F;, and by means of a large 
concave silver mirror G were finally brought to focus on a 
Rubens thermopile H. By this process the “ residual rays ”’ 
of fluorite, corresponding to a wave-length of about 26u, were 
separated from the total emission. 
A direct comparison between the radiations of the metal 
surfaces and that of the“ black body ” being impossible on 
account of the different sizes of the respective radiating sur- 
faces, we had recourse to the following method. A thin 
copper disk, covered on one side with a very thin glass 
plate (5 cms. diameter) was introduced into one of the openings 
of the heating case. We then determined the ratio between 
the radiation of the “black body” and that of the glass 
plate by aid of the smaller diaphragm C, (14 mms. diameter). 
The value we found was 1°29. In our further experiments, 
the radiation of the different metal surfaces was compared 
with that of the glass plate. In all these cases we could 
make use of the large diaphragm C, (diameter 32 mms.). The 
numbers thus obtained had only to be divided by 1:29 in 
order to give the emission-power of the different metals, 
compared “with that of the black body. These results are 
given in Table IV. 
Mercury was the only metal that required another treatment. 
We therefore changed our arrangement in the following 
manner. Instead of the copper cube, a cup filled with mercury, 
heated to 100°, was placed opposite and a little below the 
diaphragm C,. In front of the diaphragm and above the 
mercury surface, there was adjusted a polished plate of 
fluorite. By means of this fluorite mirror the heat-rays 
emanating from the mercury were horizontally reflected 
through the diaphragm. In order to compare this radiation 
with that of the black body, we covered the surface of the 
