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further ; and in the same ratio if the apparatus is filled with air above the atmoss 
pheric pressure. And this he found to be the case, which is very much what we 
should expect a priori. 
Shortly after Mr. Crookes’ first paper on the subject was published, or at 
least when these interesting phenomena became generally known, Professor 
Osborne Reynolds propounded an explanation, which, if true, would at onée 
deprive them of almost all their scientific interest. His explanation is this, It is, 
he says, impossible to produce a perfect vacuum ; there must be always some slight 
residuum of gas remaining ; and clinging to the surfaces, which no power can 
remove ; and he believes that the movements of the balance are due to evapora- 
tion and condensation of the remaining gas upon the surfaces. Evaporation from 
a surface is attended with a force tending to drive the surface back ; and condensa- 
tion, with a force tending to draw the surface forward. In opposition to this 
theory of Professor Reynolds, we have certain facts established by Mr. Crookes’ 
previous experiments, which are inconsistent with this theory of Professor 
Reynolds. Thus Mr. Crookes found that whether he started with the apparatus 
full of air, carbonic acid, water, iodine, hydrogen, ammonia, or whatever it might 
be, when he reached the point of highest rarefaction there was not found any 
difference in the results, which can be traced to the original gas. A hydrogen 
vacuum appears the same as a water or an iodine vacuum. Again it appears that 
the repulsion produced by the heat of a flame is not caused only by those rays 
usually called heat-rays, @.e., the extreme or ultra red of the spectrum. Experi- 
ments were tried with the solar spectrum, and that of the electric light, by means 
of a train of quartz prisms, which prove the action on the balance to be also 
exerted by the luminous and ultra violet rays. This single fact that light rays of 
themselves (all heat rays having been rigorously excluded) are sufficient to cause a 
powerful action upon the balance, seems quite enough to overthrow Professor 
Osborne Reynolds’s hypothesis, But Mr, Crookes undertook a further experiment 
with this special object ; he had an apparatus blown from a thick and strong green 
glass such as they use for steam-boiler gauges, very difficult to fuse; in it was a 
thin bar of aluminium supported by a long platinum wire; the apparatus was 
sealed by fusion to the Sprengel pump, and exhaustion was kept going on for two 
days, until an induction-spark refused to pass across the vacuum, During this 
time the bulb and its contents were several times raised to a dull red heat. At the 
end of two days exhaustion, the tube was found to behave in the same manner as 
with his previous apparatus, viz., the aluminium bar was repelled by heatiof low 
intensity, and attracted by cold. A similar experiment was next tried, only water 
was placed in the bulb before exhaustion. The water was then boiled away in 
vacuo, and the exhaustion continued, with frequent heating of the apparatus to 
dull redness for about 48 hours ; at the end of this time the bar of aluminium was 
found to behave the same as the one in the former experiment, being repelled by 
heat. It is impossible to conceive that in these experiments sufficient gag or 
vapour was present to produce the effects Professor Reynolds ascribes to it. After 
the repeated heating to redness at the highest obtainable exhaustion, it. is impos- 
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