ME. TV. CEOOKES ON EEPULSION EESITLTING FEOM EADIATION. 
369 
the dehydrating alum might be carried away and prevented from interfering with the 
results, as it otherwise would have done (130). The last two experiments, however, 
show the effect of aqueous vapour. 
205. To test the accuracy of the explanation that the opacity caused increased action, 
I coated two disks of pith, one with powdered rock-salt and the other with powdered 
alum, and tested them against lampblacked pith in a similar apparatus to the one 
described in par. 198. The deflections were — 
Black pith. 
Powdered alum. 
Powdered rock-salt. 
110 
38 
18 
or reduced, 100 
34-5 
16-3 
As will be seen on reference to par. 203, the ratio between the black disk and the 
plate of rock-salt was 100:3. Powdering the rock-salt has therefore increased the 
action 13*3 per cent. The much larger action of the powdered alum is probably due 
to the fact that crushing the crystals facilitates efflorescence in vacuo. 
206. The alum and rock-salt plates were removed, a fresh alum plate ground and 
polished, and this and the rock-salt were coated with lampblack. They were then put 
into the apparatus as before, the black side being away from the source of radiation, so 
that the rays would have to pass through the crystal plates before meeting with the 
lampblack. The deflections were taken as soon as the vacuum was good. The deflec- 
tions were — 
Blackened alum. Blackened rock-salt. 
26 19 
or reduced, 100 73 
The rock-salt was more slow in its movements under the influence of radiation than 
the alum was, but they both returned to zero equally well. 
207. Avery sensitive apparatus, similar to the torsion-apparatus described in par. 198, 
fig. 5, was now employed, and a clear polished disk of rock-salt and a thin disk of 
glass of the same size were placed therein. The apparatus was very well exhausted, 
and the deflections taken when the radiation from a candle was allowed to fall on either 
disk. The mean of several concordant observations was — 
Eock-salt. Glass. 
39 40 
I quote the following from the article in c Nature ’ already referred to : — k£ Prof. Dewar 
then proceeded to show that the heating of the disk was the efficient cause of the action. 
Two equal disks, one of rock-salt, the other of glass, were attached to the glass fibre. 
The rock-salt was inactive when the beam [from a candle] was thrown on it ; the glass 
disk was active. The reason is evidently that the rock-salt is not heated, being trans- 
parent to heat, whereas the glass is opaque, absorbs the heat and is heated.” It will 
be seen that I have failed to obtain this marked difference in action between rock-salt 
