434 Prof. H. A. Bumstead on the Heating Effects 
possibility appeared to be excluded (as well as any sensible 
difference in the emissivity of the surfaces) by a control 
experiment in which the strips were heated by exposure to 
the light of an incandescent lamp, instead of Rontgen rays. 
The deflexions of the radiometer were almost exactly equal 
in this case, and the whole behaviour was such as to indicate 
that the conditions specified above were fulfilled ; the 
question was discussed in the previous paper and the ex- 
periments with light were taken as excluding the possibility 
of the result being due to the more rapid loss of heat by the 
zinc. My recent experiments, however, force me to the 
conclusion that the zinc did lose heat more rapidly (for a 
given temperature) mainly over the supports, and that, in 
the control experiments, this was accidentally compensated 
by a greater absorption of the incident light by the zinc, 
possibly owing to a thinner layer of wax between it and 
its covering of aluminium-foil. The experiments with light 
were made at the very end of my stay in Cambridge and 
could not be repeated on account of lack of time : if they 
had been repeated under slightly varied conditions, the error 
would doubtless have been discovered. The agreement 
between the two metals, however, w^as so good, and the 
improbability of two unrelated large errors w T hich exactly 
compensated each other was so great, that I was led to put 
more confidence in this result than it deserved. 
When the experiments were again taken up, however, 
I did consider the possibility that the emissivity of a surface 
and its absorption of light might vary together — thus 
destroying the force of the control experiment. It was not 
likely that the emissive power of a surface for low tempera- 
ture radiations would be proportional to its absorption of 
the high temperature radiation from an incandescent fila- 
ment, but I nevertheless made some experiments to test the 
matter. For this purpose two lead strips w T ere used, one 
of which was left w T ith its original dull surface and the other 
covered with aluminium -foil. Rontgen rays and light were 
both used : the rays gave deflexions agreeing to about 
5 per cent. ; with light, on the other hand, the dull lead strip 
gave four times the deflexion of the other. It thus appeared 
(as was expected) that the rate at which heat was lost varied 
very little with the state of the surface, while the absorption 
of light varied greatly. Thus there appeared to be no reason 
for distrusting the control experiment. 
A series of experiments was next made, by means of an 
electroscope, on the amount of secondary radiation from lead 
.and zinc, in order to test the possibility that a considerable 
