COMMUNICATION OE HEAT BETWEEN A SUEEACE AND A GAS. 
731 
dp x dv . 
p 2 v 
and by the foregoing 
dv 1 dr 
v 2 r 
dp x dr 
" P * 
Therefore if, as we have calculated, 
^=•0008, 
p 
- = •0032, 
T 
and taking r=520° F., 
<Zr=l*6640. 
If, therefore, the difference of temperature caused by the light were not greater than 
1 0, 7 F., it would appear from these measurements that the forces arising from the com- 
munication of heat would not be adequate to cause the effect produced. That is to say, 
l° - 7 is the lowest limit that the theory admits for the heat reaction to have caused the 
effects in this particular case. The theory points to the probability, however, that the 
difference was considerably greater than l° - 7. 
To put this to the test it was necessary to obtain some measure of the 
Actual Difference of Temperature on the Black and Bright sides of the Plates. 
So far as I am aware there is no recognized means of measuring this difference ; and 
although it is admitted that a black surface exposed to light will attain a higher degree 
of temperature than a white or bright surface, no comparative experiments have been 
made. 
While taking part with Dr. Schuster in his experiments, I held an ordinary thermo- 
meter containing some dark red fluid in the place which the mill had occupied exposed 
to the light. This came from a lime-light, and was condensed by an ordinary lantern. 
The thermometer rose to 130° F., and was still rising when the experiment had to be 
discontinued. 
This measure, great as it was, was not satisfactory, for it was not comparative, and a 
white-bulbed thermometer would obviously have risen to some extent. I therefore took 
two similar mercurial thermometers, blackened the bulb of one and whitened that of the 
other, and exposed them to similar intensities of light. Under all circumstances the 
black bulb was the most affected, for however long a time the exposure was continued ; 
the light of a candle which caused the light-mill to make 30 turns per minute made a 
difference of 2^° in the thermometers, Avhereas a feeble sun, which gave the mill about 
60 turns, caused a difference of 5°. These results showed a close agreement with the 
action of the light-mill ; but whereas the light acted instantaneously on the mill, the 
