RADIATION

1579

RADIOMICROMETER

duced a number of works, all of which bore the stamp of very high, if not the highest, genius. His last production, Phèdre, which appeared in 1677, was the one in which Rachel's genius, in the 19th century, reached its highest manifestation. His chief writings are Esther, Athalie, Phèdre, Bajazet and Iphigénie. He died at Paris, April 21, 1699, and was buried according to his own request at Port Royal.

Ra'dia'tion, one of the fundamental processes by which energy is transmitted from one point to another. The word is sometimes used also to denote the quantity of energy transmitted as well as the process. If one consider the incandescent carbon filament of an ordinary electric lamp and hold his hand under either at the side of or underneath it, the sensation of heat is distinctly recognized. The same is true when the hand is placed some inches underneath a heated ball which is emitting no light.at all. The query arises as to how the heat gets from the ball down to the hand. It certainly is not by conduction, for air is one of the very poorest of conductors. Nor can it be by convection, for convection-currents heat only the regions above the lamp or ball. Now it has been proved by elaborate series of experiments, extending over the entire 19th century and performed by Herschel, Prévost, Carnot, Melloni, Balfour Stewart, Kirchhoff, Hertz and others, that this "something" which passes from the ball to the hand behaves exactly as light-waves behave in nearly all particulars. It travels with the same speed as light, can be polarized as light, can be reflected and refracted as light, etc. The unavoidable inference, therefore, is that that which we call heat in the ball and heat after it is received by the hand is not heat while traveling from the ball to the hand, but is a wave-motion in the ether just as light is, the principal difference being that the heat-waves are longer than the light-waves. This is a process known as radiation, and the energy thus transmitted, whether in the form of long heat-rays or luminous rays or so-called actinic rays or electrical vibrations, is all grouped under the head of radiant energy. The student should distinguish carefully between heat, which is the energy of motion resident in atoms and molecules of ponderable matter, and radiant energy as defined above. See Balfour Stewart's Treatise on Heat,

Radicalism is the name generally applied to the principles of those who wish for fundamental political and social reforms. Radicalism, therefore, is the logical antithesis to conservatism. In England radicalism is recognized as the political principle of an organized party. The first English radical leader was John Wilkes; but it was not until the days of Mill, Bentham and Ricardo that radicalism had a definite philosophy of its own, nor did it become a very important political principle until after the Reform Bill of 1832. When that bill had been passed, there came about a gradual division between the Radicals, who looked upon it as a mere beginning to a series of reforms, and the Whigs, who were satisfied with the results of the bill and wished for the time to go no further. Since the rise of a separate Radical section within the body of the Liberals or Whigs, radicalism has become closely connected with socialism. There is a very strong radical body in France, and recently a similar movement has developed in Germany.

Radiometer {rā-dí-ŏm'e-tĕr), an interesting device invented by Sir William Crookes in 1873, while determining the atomic weight of the then recently found element called thallium. In the form in which Crookes left it the essential features are a windmill of four light mica-vanes placed in a glass-bulb B from which the air has been very completely, but not too -completely, exhausted. The proper degree of exhaustion is that in which the mean free path of the molecules is a little greater than the perpendicular distance from one of the vanes to the wall of the glass-bulb. As shown in the figure, the vanes are blackened on one side; and when sunlight or even lamplight is allowed to fall on the instrument, the vanes showing the blackened sides are more heated than the others, and hence the molecules between the blackened vanes and the glass wall bombard the vanes away, giving rise to the rotation indicated by the arrow in the figure. Only recently the radiometer has been made an instrument of precision, principally through the labors of Prof. E. F. Nichols of Dartmouth College. For a description of this instrument see Astrophysical Journal, March, 1901, pp. 101—41.

Radiomicrometer ( rā'dï-0-mî-krom1'ê-tēr), an instrument of marvelous sensibility for detecting radiation. It consists essentially

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radiometer

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Radiation of heat downward from a hot iron ball