64 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
By this means, the distance between the pile and the source of heat being consider- 
able, no currents of heated air from the latter would be able to reach the former ; 
and as the tube A B was blackened in the inside, and passed through water, re- 
flection and secondary radiation would both be avoided. By means of a lid fitting 
on the tube at A, the aperture might be diminished at pleasure. The pile was con- 
nected with a very sensitive galvanometer. 
When glass at a high temperature was the source of heat, a very small aper- 
ture was sufficient, and thus the advantage was gained of having the whole field 
covered with glass, all at a high temperature, which could not have been the case 
had the aperture been large. 
Slips of glass about 3 inch broad were used, and were set vertically, just touching 
a gas flame from a Bunsen’s burner. When two slips—one behind the other— 
were used, the one just touched that portion of the flame next the pile, and the other 
that portion furthest from it. A cross section of the arrangement is shown above. 
A single slip of glass about ‘1 inch thick thus heated gave a deviation of 16°°5, 
while two slips, the one behind the other, gave 18°5. When the slips were -05 
inch thick these numbers were 29°-1 and 36°°3. 
18. From these experiments we may conclude, that at a high temperature, 
700° or 800° F., the radiation from two plates of glass, one behind the other, 
is sensibly greater than that from one—a result which does not hold for glass at 
212°. Or the fact may be stated thus :-— 
The radiation of a single plate of glass bears a smaller proportion to the total 
radiation at 700° than at 212°. 
19. It was next tried whether the capacity of a glass screen for passing heat 
from blackened copper at 700° was altered by its being heated. 
In order to ascertain this, blackened copper at 700° F. was placed behind a 
slip of glass, and the amount of heat from the copper which passed the glass was 
observed 
Firstly, When the glass was cold. 
Secondly, When it was heated to between 700° and 800° F. 
20. As in these experiments the considerably fluctuating temperature of the 
source of heat causes a somewhat large difference between successive observations, 
and renders necessary a great number in order to arrive at a correct result, it was 
thought desirable, instead of using momentary deviations, to employ permanent 
ones. This was done with complete success ; the application of the heated copper, 
or its removal, causing an unmistakeable alteration of the position of the needle. 
21. The experiment was then varied in the following manner :—The needle 
was kept permanently deviated by the heated glass, and the momentary swing 
due to the application or withdrawal of the heated copper was noticed, and was 
compared with that occasioned by the hot copper when the glass was cold and 
the needle at zero. 
