ON

ATTRACTION AND REPULSION RESULTING FROM RADIATION.*

By WILLIAM CROOKES, F.R.S., &c.

(1). IN a paper "On the Atomic Weight of Thallium," presented to the Royal Society June 18, 1872, after describing a balance with which I was enabled to perform weighings of apparatus, &c., in a vacuum, I noted a peculiarity in relation to the effect of heat in diminishing the apparent weight of bodies. I said, "That a hot body should appear to be lighter than a cold one has been considered as arising from the film of air or aqueous vapour condensed upon or adhering to the surface of the colder body, or from the upward currents of air caused by the expansion of the atmosphere in the vicinity of the heated body. But neither hypothesis can be held when the variation of the force of gravitation occurs in a vacuum as perfect as the mercurial gauge will register, and under other conditions which I am now supplying, and which I embodying in a paper to be submitted to the Royal Society during a subsequent session."+

purpose

With the vacuum-balance mentioned above I carried out many experiments, but was unable to obtain results which were at all concordant; and it was soon found necessary to investigate the phenomena with smaller and less complicated apparatus.

(2). Most chemical manuals warn beginners against the errors occasioned by weighing substances while hot; and, up to a moderately high degree of exhaustion, I was prepared to find a piece of glass apparatus, when hot, apparently lighter than the weights which should balance it were the whole system at the same temperature. But, instead of the interfering causes diminishing as the rarefaction proceeded, they seemed rather to increase, or at all events to become irregular in their action, sometimes appearing to oppose, and at others to supplement the force of gravity. In such a vacuum as a good air-pump would produce, the actions of the ascending current of air and of the adhering film, it might be presumed, should cease to exert an influence; and I could think of no other disturbing cause except the lengthening of the beam, owing to the heat radiated from the apparatus below it. An increase in the length of the beam should make a mass suspended at its extremity appear heavier; but, whilst I frequently noticed an action which might be due to this cause, I occasionally obtained results which were so anomalous as to convince

From the Philosophical Transactions of the Royal Society of London, vol. clxiv., part 2. + Phil. Trans., 1873, vol. clxiii., p. 287.

me that some cause which I had not hitherto recognised was at work (49), and to lead me to hope that perhaps I might succeed in tracing a connection between heat and the force of gravity.

(3). Many physicists have worked on the subject of repulsion by heat. I give here a brief resume of the state of knowledge on this subject up to the time of my commencing these experiments, premising, however, that much of this historical information was unknown to me until some of the experiments here recorded were finished and I commenced putting my notes together. The earliest mention I can find is by the Rev. A. Bennet, F.R.S., who in the year 1792 published a paper on "A New Suspension of the Magnetic Needle intended for the Discovery of Minute Quantities of Magnetic Attraction; also an Air-Vane of Great Sensibility; with New Experiments on the Magnetism of Iron Filings and Brass." Mr. Bennet used a spider's thread as a means of suspension. This he found by experiment to be absolutely free from torsion. I quote the following experiments from his paper :

"Experiment IV.-A bristle was suspended horizontally by a spider's thread, somewhat stronger than the last, and after turning the wheel till it produced 4800 revolutions, it shortened the thread from 3 inches to 1 inch; yet either end of the bristle would move towards any warm substance which was presented to it either with or against the direction of the twist.+

"Experiment V.-Several other light substances were suspended by fine spiders' threads and placed in a cylindrical glass about 2 inches in diameter, as the thinnest part of the wing of a dragon-fly, thistle-down, and the down of dandelion; of these the last appeared most sensible to the influence of heat; for when this down was fastened to one end of a fine gold wire, suspended horizontally on to one end of two bits of straw joined together in the form of the letter T inverted, it would turn towards any person who approached it at the distance of 3 feet, and would move so rapidly towards wires heated by my hand, as very much to resemble magnetic attraction.

Experiment VI.-A bottle filled with cold water was brought near the glass cylinder standing in a warm room, and soon after the down of dandelion appeared to be repelled by the bottle by turning away from it. The bottle was removed to the other side, and the dandelion again moved towards the opposite side.

Experiment VII.-A piece of paper was tied over the mouth of a glass jar, about 4 inches in diameter. Two holes were made in the paper opposite to each other, and *Phil. Trans., 1792, p. 81.

For a re-discovery of this fact, seventy-nine years after, see par. 14.