Radiation pressure

“ It may help a moderate rotation about the axis to break up the star. Consequently larger masses will survive only rarely; for the most part stars will be kept down to the mass at which the new force first becomes a serious menace. The force of gravitation collects together nebulous and chaotic material; the force of radiation pressure chops it off into suitably sized lumps.
This force of radiation pressure is better known to many people under the name ‘pressure of light’. The term ‘radiation’ comprises all kinds of ether-waves including light, so that the meaning is the same. ”

“ In the case of radiation, as in the case of a vapour, the latent heat consists partly of internal energy of formation and partly of external work of expansion at constant pressure. Since in the case of full or undirected radiation the pressure is one-third of the energy per unit volume, the external work for any expansion is one-third of the internal energy added. ”

“ One can foresee the possibility of establishing mechanics upon the dynamics of small material centers charged and in motion.
α-Rays.—The α-rays of radium have very little power of penetration. A piece of aluminium foil a few hundredths of a millimeter in thickness absorbs them almost completely. They are also absorbed by the air, and cannot penetrate air at the atmospheric pressure to a greater distance than 10 cm. The α-rays form the most important part of the radiation of radium, provided we measure the radiation by the amount of ionization which it produces in the air. ”

“ For a given intensity of radiation, the saturation P.D. decreases rapidly with the lowering of the pressure of the gas. This is due to two causes operating in the same direction, viz. a decrease in the intensity of the ionization and an increase in the velocity of the ions. The ionization varies directly as the pressure, while the velocity varies inversely as the pressure. This will obviously have the effect of causing more rapid saturation, since the rate of recombination is slower and the time taken for the ions to travel between the electrodes is less. ”

“ On the other hand, if an absorbing surface is moving, so that it recedes from the incident radiation, then the latter steadily performs work; the surface is therefore only capable of absorbing – i.e. transforming into inner heat – less radiation by the equivalent of the latter. However, if the absorbing surface is approaching the incident radiation, then work from outside must be steadily performed against the pressure of radiation. ”

“ The same energy quantity also occurs in , and a quite similar consideration as earlier teaches, that the energy quantity
is absorbed there, because the work must also be performed against the incident radiation from outside, which is also transformed into work.
Thus the same quantity is absorbed by both bodies and , as it is emitted; upon both surfaces (in opposite direction) the same pressure
is exerted, so that no work is performed altogether. ”

“ It has often been suggested that the stars will be retarded by the back-pressure of their own radiation. The idea is that since the star is moving forward the emitted radiation is rather heaped up in front of it and thinned out behind. Since radiation exerts pressure the pressure will be stronger on the front surface than on the rear. ”

“ The radio-active energy which accumulates in the radium tends to become dissipated in two different ways:— (1) By radiation (rays both charged and uncharged with electricity) ; (2) by conduction, i.e., by gradual transmission to surrounding bodies in a medium of gases and liquids (production of an emanation and transformation into induced radio-activity) .
The loss of radio-active energy, both by radiation and by conduction, increases with the amount of energy accumulated in the radio-active body. ”

“ In these experiments it is the radio-active energy accumulated in the gas that maintains the activity of the walls. If the gas be withdrawn and a vacuum caused in the enclosure, we have found that dissipation of activity at once occurs in the rapid method; the intensity of radiation being reduced to one-half in twenty-eight minutes. ”

“ These facts are important because they determine the magnitude and the character of the danger when the radioactivity finally falls out of the cloud and is deposited on the surface of the earth. Those radioactive particles which disintegrate while still in the cloud need not worry us since this radiation can have no effect on living organisms that may be underneath. Provided that the cloud is more than a few hundred feet above the ground, the beta and gamma rays released in these disintegrations merely dissipate their energy in ionizing the air. ”

“ We see that in loose particles the effective surface acessible to radiation is very much enlarged. Moreover, the resistance offered to the particles is not due to the individual solid lumps, but to the resistance of surface layer. ”

“ The Slow Evolution of Induced Activity.—A solid body acquires a very feeble, persistent, induced activity when it remains a month or less in contact with the radiation from radium. A substance withdrawn from the influence of the emanation after having been subjected to it for a long time loses its activity rapidly at first, according to the laws given. But the activity of the radiation does not disappear completely. There remains a radiation several thousand times more feeble than it was at first. ”

“ The intensity of emitted radiation is moderately strong, and this is an advantage in many cases. It sometimes becomes necessary to have a greater intensity without the attendant trouble inseparable from too long waves. I have been able to secure this by making a radiator, where the oscillatory discharge takes place between two hollow hemispheres and an interposed platinum ball. The intensity of radiation is by this expedient very greatly increased. The parallel pencil of electric radiation, used in many of the experiments to be described below, is only about one cm. ”

“ Relative penetration of alpha, beta, and gamma radiation.
For comparison, a sheet of paper stops alpha particles, a block of wood stops beta particles, and a thick concrete wall stops gamma rays.
9
In one way or another, the key to the usefulness of radioisotopes lies in the energy of the radiation. When radiation is used for treatment, the energy absorbed by the body is used either to destroy tissue, particularly cancer, or to suppress some function of the body. Properly calculated and applied doses of radiation can be used to produce the desired effect with minimum side reactions. ”

“ We have seen that a salt of radium that has been freshly prepared possesses an activity which increases with the time, and becomes about five times as great as the initial activity. It seems that the emanation emitted by the radium can escape only with difficulty from the solid salt, and that it accumulates there and is transformed at once into induced radiation. An equilibrium is established when the spontaneous loss becomes sufficient to compensate the production. ”

“ About half is in the visible band, and the human eye is adapted to make most efficient use of this radiation. Generally, thermal radiation is not visible when the temperature of a substance is less than 1000°F. If we heat a substance, such as an iron rod, it appears dull red when it first becomes hot enough to emit visible radiation. Continued heating causes it to change in color to bright red, and then to orange, yellow, and white as its temperature increases and the maximum radiation intensity shifts toward the shorter wavelengths. ”

“ Radiation strikes the receiving object from all of the points—in greatest intensity from points perpendicular and closest to the object, and in lesser intensity from the other points. Because so many points are producing radiant energy, the decrease in intensity with distance from a flame source is less than that from a line source and much less than that from a point source. The number of points from which radiation can be received also increases rapidly with the area of the radiating surface. ”

“ The electromagnetic waves of radiant energy move only along straight paths. Hence, the intensity of radiation received depends on the angle of the incoming radiation and the distance from the source. Radiation perpendicular to the receiving surface is most intense. For a point source of radiation the intensity decreases inversely with the square of the distance from the source. ”

“ In a complex way, the amount of thermal radiation reflected depends on the characteristics of the surface of the substance, the radiation wavelength, and the angle at which the radiation strikes the surface. In general, however, smooth, light-colored surfaces reflect more radiation than do dark, rough surfaces. A highly polished, smooth surface such as a mirror or chrome on a car, reflects a large part of the thermal radiation, but a black-topped road surface reflects very little. ”

“ Actually, your skin is losing more thennal radiation than it is receiving, and this produces the cold sensation. Because all substances radiate energy, they are always tending to move toward a common equilibrium temperature.
Only absorbed radiation is converted to heat; the reflected and transmitted radiation is not. In opaque materials, such as wildland fuels, the conversion of radiant to thermal energy takes place in a very thin layer at the surface. Heating of deeper layers must usually be accomplished by conduction. ”

“ Not all thermal radiation is absorbed Only a perfect black body can absorb all the thermal radiation that reaches it—other substances absorb only part of the radiation. Clear, translucent, and very thin opaque materials can absorb some wavelengths while allowing others to pass through unchanged. Glass, for example, transmits most wavelengths in the visible light band, but absorbs those in the infrared range. Thus, a glass window in a house will admit the shortwave light radiation, but block longwave radiation from lower temperature materials inside the house. ”

“ If in the interior of any body there is, owing to any cause, a continuous and constant evolution of heat, the heat accumulates in the body and the temperature rises until the loss of heat by radiation and conduction is in equilibrium with the constant gain of heat.
In general, except under certain special circumstances, the activity is not propagated through solid bodies. When a solution is kept in a sealed tube, the loss by radiation alone takes place, and the radiating activity of the solution is of a higher degree. ”

“ In this investigation it is essential that the substance examined should be completely protected from all disturbances, such as mechanical vibration, &c., and only subjected to the action of radiation. The experimental difficulties are very great. If we take a strongly sensitive positive substance—say iron, the effect of the first flash of radiation (a diminution of resistance) is very great, and the subsequent relatively feeble reversal effects, unless carefully looked for, are liable to pass unnoticed. ”

“ When the particles become continuous, the radiation can only affect the extremely thin layer of molecules on the surface, the mass in the interior being shielded by the outer conducting sheet; the molecular changes produced on the surface layer do not affect to any appreciable extent the conductivity of the mass. ”

“ The rays from the active substances carry off energy in a very concentrated form, and this energy is dissipated by the absorption of the rays in matter. The rays might be expected to cause a disintegration of the atoms of inactive matter on which they fall and thus give rise to a kind of radio-activity. This effect has been looked for by several observers. Ramsay and W. T. Cooke [361] state that they have noticed such an action, using about a decigram of radium as a source of radiation. ”

“ Of course, gases do radiate heat and light under some circumstances, but the radiation seems to be produced either by chemical action, as in the flames coloured by metallic vapours, or by electric discharge, as in vacuum tubes, the arc or the electric spark. ”

“ This radiation of energy would fall off with the time, but the total emission of energy during the life of the emanation would correspond to 60,000 horse-power days.
250. Heating effects of uranium, thorium, and actinium. Since the heat emission of radium is a direct consequence of its bombardment by the α particles expelled from its mass, it is to be expected that all the radio-elements which emit α rays should also emit heat at a rate proportional to their α ray activity. ”

“ There is no doubt that the process which takes place in the emission of radiation from the radioactive elements is a transformation of the element, an explosion of the atoms accompanied by the emission either of double-charged helium atoms or of electrons, and the forming of the atoms of a new element. The energy of the rays is an internal atomic energy, freed by these transformations. ”

“ No certain evidence has yet been obtained that any body can be made radio-active by exposure to Röntgen rays or cathode rays. A metal exposed to the action of Röntgen rays gives rise to a secondary radiation which is very readily absorbed in a few centimetres of air. ”

“ That is what radium does to that kind of glass. You see, if radium exists in a gram of inactive matter only to the extent of one in ten-thousand million parts its presence can be readily detected by this radioscope, and everything that has been rendered radioactive is the same. Ordinarily the air between the gold leaves is insulating. Bringing something radioactive near them renders the air a good conductor and the leaves fall under the radiation. ”

“ Successive dots in the ascending portion of the curve represent effect of rotation through 360°. The descending curve represents recovery.
here only one case. We have seen in experiments with electric radiation that substances sometimes fall into a sluggish molecular condition, when the responses almost disappear. Strong stimulation (induction shocks, &c.) or annealing is found to restore the sensitiveness. The same peculiarity is observed in the strain-cell. Lead, for example, specially on cold days, is apt to fall into a sluggish condition, when it becomes almost irresponsive. ”