MU. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
539 
110. An examination of this Table shows that the action is by no means confined to 
the rays usually called heat, i. e. to the extreme- and ultra-red of the spectrum. The 
strong action obtained when the light is filtered through greenish glass and alum, or 
through ammonio-sulphate of copper, shows that luminous rays produce a similar 
movement of repulsion. 
Unfavourable weather has prevented me from obtaining good quantitative results with 
the different rays of the solar spectrum ; but I have tried numerous qualitative experi- 
ments which leave so doubt on my mind that any ray, from the invisible ultra-red to 
the invisible ultra-violet, will produce repulsion in a vacuum. The following is an 
experiment tried with the electric light. The spectrum was formed with a complete 
quartz train, no glass whatever being in the path of the rays. The purity of the 
spectrum was evidenced by the fact of the lines being sharp when thallium, sodium, or 
lithium was put between the carbon poles. The spectrum was so arranged that any 
desired ray could be thrown on to a lampblacked pith surface, screens being interposed 
to cut off the action when desired. The torsion-balance was similar to the one used in 
the last-named series of experiments (104), but was not quite so sensitive. 
The extreme-red rays were first brought into position. On removing the screen the 
luminous index moved 9 divisions on the scale. The screen being replaced, the index 
returned to zero. A solution of iodine in disulphide of carbon was now interposed, and 
the screen again removed. The repulsion was almost as strong as before, showing that 
this liquid was transparent to the ultra-red rays. 
The iodine solution was then replaced by a clear plate of alum 5 millims. thick, and 
the screen removed ; a very slight movement only took place. The iodine solution was 
then put in front of the alum plate, so as to subject the extreme-red rays to a double 
process of sifting. No trace of action could be detected. 
Whilst this double screen was in front of the pith disk, the spectrum was gradually 
passed along, so as to bring the rays, one after the other, into position. No effect, 
however, was produced, showing that alum and iodine solution practically obliterate 
the whole of the spectrum. 
The alum plate and iodine-cell were now removed, and the green of the spectrum 
(the thallium line) was brought into position. The luminous index moved 6 divi- 
sions. The plate of alum cut off only a small amount of this action, but the iodine- 
cell brought the index to zero. This is a proof that the action in this case was not due 
to the heat-rays of the spectrum, for these are practically transmitted by iodine, and cut 
off by alum. 
The indigo-rays were next brought into position. The spot of light moved three 
divisions on the graduated scale. Alum cut off only a very little of the action ; but the 
iodine-cell was completely opaque to the rays, and brought the index to zero. 
Finally, the invisible ultra-violet rays of the spectrum were brought into position. 
The train being of quartz these were abundant. Care was taken to keep any of the 
luminous rays away from the pith disk. I think I succeeded in this ; but it was not 
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