ME. W. CEOOKES ON EEPULSION EES TILTING- EEOM EADIATION. 
363 
radiating action of the surface for heat comes into play, and the heat, which has just 
been engendered and absorbed, is quickly radiated back again. It would appear as if this 
radiation of heat from the surface of a body caused the latter to retreat backwards, and 
so produced the motion. This would account for the apparent viscosity of the vacuum ; 
for the heat radiating from the black surface of the pith would act in opposition to the 
torsion, and hold the latter force in check till it was itself all dissipated. The superiority 
of pith over metal is also accounted for. Pith is one of the worst conductors of heat, 
and thus allows all the heat to radiate from the same surface which absorbed it ; whilst 
metals, being the best conductors of heat, allow it to pass through and radiate almost 
as much from one surface as the other. 
The slight action of the blue rays is thus due to their short vibrations not being 
capable of transmutation into so much thermometric heat as are the longer rays; 
whilst the strong action of the red rays is owing to the degradation necessary to convert 
them into heat being but slight. 
This action is parallel to that of the production of phosphorescence. A ray of such 
high refrangibility as to be invisible falls on a suitable surface ; it is there absorbed, 
degraded in refrangibility, and radiated out again in the form of visible rays of longer 
wave-length. We have only to imagine our eyes to be unaffected by what we now call 
light, but capable of responding to an octave lower in the spectrum, and we should 
see the same thing when the blue ray falls on the blackened pith. In such a case an 
invisible beam would be thrown on a suitable surface of lampblack ; the latter would 
instantly respond, lowering the refrangibility and increasing the wave-length to the 
point of visibility ; the ray so generated would be absorbed and then radiated back 
again, the lampblacked surface glowing with light for some time after the original ray 
had ceased to fall on it. 
196. Making use of the property discovered by Dr. Tyndall of the almost perfect 
transparency for the invisible heat-rays of iodine dissolved in disulphide of carbon, and 
its opacity to the luminous rays, experiments were instituted with a view to obtain a 
numerical comparison of the mechanical action which was due to the invisible rays and 
that due to the visible rays. 
The apparatus was used as fitted up for spectrum observations (187), the prisms and 
lenses being removed. A ray of sunlight was reflected from the heliostat, then, reflected 
by means of a right-angled prism (placed in the usual position of the refracting prisms) 
into the apparatus. 
The action of the sunlight was far too powerful for the apparatus, and I accordingly 
passed it through a plate of alum, a thick piece of glass, and an empty glass cell. 
On opening the shutter the pith was powerfully repelled, the index ray moving 
300 divisions. The cell was filled with disulphide of carbon, and the experiment again 
tried. The index moved to the same point. 
The clear disulphide of carbon was removed, and it was replaced by a strong solution 
