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The calculation is based on experiments made with the radiometer of Professor Crookes, an instrument which in itself is held to demonstrate the truth of the kinetic theory of gases. Such an attenuated gas as this is considered by Professor Crookes as constituting a fourth state of matter, which he terms ultra-gaseous.
In trying the preceding experiment with the electric radiometer the potential should not exceed a certain limit, as then the electrostatic attraction between the vanes and the glass of the bulb may be so great as to stop the rotation. A most curious feature of alternate currents of high frequencies and potentials is that they enable us to perform many experiments by the use of one wire only.
One of his ingenious expedients was to turn to use an ordinary radiometer of large bulb, and, having fitted this with tin-foil electrodes, he found that he was able to get strongly marked shadow pictures.
Phyl, who was as sensitive to impressions as a radiometer to light, noted the silence of the other and resented it as she hung up her old hat and cloak. She knew nothing of the true facts of the case, she looked on Pinckney as a being almost of her own age, and that he should dare to express disapproval of an act of hers not concerning him, even by silence, was an intolerable insult.
This is Wollaston's celebrated "Thimble Battery." It will slip readily into the pocket, yet he jestingly showed it to a visitor as "his entire laboratory." That is a model of the double-decked boat made by Sir William Petty, and there beyond is a specimen of almost, if not quite, the first radiometer devised by Sir William Crookes.
This application of the Röntgen principle will commend itself to many students who, being unable to provide themselves with the rare and expensive Crookes tubes, may buy a radiometer which will serve their purpose excellently in any laboratory supply store, the cost being only a few dollars, while the application of the tin foil electrodes is perfectly simple.
With such a company about us we are surely justified in feeling that the glory of England as a scientific centre has not dimmed in these first hundred and thirty years of the Royal Society's existence. And now, as we view the radiometer, the scene shifts by yet another century, and we come out of cloud-land and into our own proper age.
But what a splendid and useful building has been placed on this foundation by Clausius and Maxwell, and what a beautiful ornament we see on the top of it in the radiometer of Crookes, securely attached to it by the happy discovery of Tait and Dewar, that the length of the free path of the residual molecules of air in a good modern vacuum may amount to several inches!
Immense improvements have been effected, in the means of exhausting a given space of its gaseous contents; and experimentation on the phenomena which attend the electric discharge and the action of radiant heat, within the extremely rarefied media thus produced, has yielded a great number of remarkable results, some of which have been made familiar to the public by the Gieseler tubes and the radiometer.
The historian never stopped repeating to himself that he knew nothing about it; that he was a mere instrument of measure, a barometer, pedometer, radiometer; and that his whole share in the matter was restricted to the measurement of thought-motion as marked by the accepted thinkers. He took their facts for granted.
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