BEPULSION EESULTING- FEOM EADIATION. 
503 
light was admitted through a small hole, and also the focus of a large lens was thrown 
upon the paper, with the intention of observing whether it would be moved by the 
impulse of light ; but though these experiments were often repeated, and once with the 
paper suspended in an exhausted receiver, yet I could not perceive any motion distin- 
guishable from the effects of heat. Perhaps sensible heat and light may not be caused 
by the influx or rectilinear projection of fine particles, but by the vibrations made in 
the universally diffused caloric or matter of heat or fluid of light. I think modern dis- 
coveries, especially those of electricity, favour the latter hypothesis.” 
4. In his £ Elementary Treatise on Heat’* Professor Balfour Stewart, F.R.S., cites 
this experiment of Mr. Bennet’s as one of the arguments against the emissive and in 
favour of the undulatory theory of light and heat. Bearing in mind the overwhelming 
proofs we now possess that the undulatory theory more nearly expresses the truth than 
does the emissive theory, it is not likely that the very different results I have succeeded 
in obtaining (56, 57, 58), by the employment of instruments of a delicacy unattainable 
eighty years ago, will have any weight in modifying the accepted theories of light and 
heat. 
5. The next mention of the dynamic action of heat is by Laplace, who, in his ‘ Me- 
canique Celeste ’ f , speaks of the “ repulsive force of heat ” as subsisting among the 
particles of a fluid, but observes that experiment shows it has no other effect on 
capillary attraction than what results from its diminishing the density of the fluid. 
6. In the year 1824 Libri J published some experiments on the movement of trans- 
lation experienced by a drop of liquid suspended to a metallic wire, one of the ends of 
which is heated. This he inferred was due to repulsion produced by the heat between 
the wire and the particles of the liquid. The Rev. Baden Powell, F.R.S., says § that 
trying to repeat Libri’s experiment he has never been able to succeed, except in pro- 
ducing a slight apparent motion in the drop, which seems explicable from the mere 
effect of evaporation on the side next the heat. 
7. In the ‘Annales de Chimie et de Physique’ for 1825 1| are two papers by Fresnel, 
in which he gives an account of an experiment on the repulsion exerted by heat. To 
the two extremities of a fine magnetic needle, suspended by a cocoon-fibre, he attached 
vertical disks of foil and mica, so as to test with the same apparatus an opaque and a 
transparent body. The fixed body which was to repel the torsion-balance was another 
disk of foil. The whole was covered with the receiver of an air-pump, and a vacuum, 
up to 1 or 2 millimetres, was obtained. The whole was then taken into sunshine, and 
turned so that the needle was kept slightly out of the magnetic meridian by pressure 
of the fixed disk against one of the movable disks. On concentrating the sun’s rays 
on either of these disks, they instantly separated, sometimes to 'the extent of a milli- 
metre. On withdrawing the lens, the torsion-balance only gradually returned to its 
* Oxford, at the Clarendon Press, 1866, pp. 161, 352. 
f Snppl. livr. x. p. 75, A.n. 1799-1805. i Mem. Accad. Torino, xxviii. 
§ Philosophical Transactions, 1834, p. 485. |j Yol. xxix. pp. 57, 107. 
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