state of equilibrium that each product is being produced from the one preceeding it in the series, just as rapidly as it is being transformed into the next succeeding one, and is therefore neither increasing nor decreasing in amount.

Since radium is being constantly though slowly transformed, it must disappear from the earth in the course of a few thousand years unless it is being produced in some way. It is now believed that radium is a disintegration product of uranium, for uranium has the greater atomic weight, they are always found together, and it has been shown that the amount of radium in any radioactive mineral always bears a constant ratio to the amount of uranium which it contains, viz., about 1 to 2,630,000.

The transformation of uranium itself is so slow that it will require, according to Rutherford, a period of at least ten thousand million years for any large fraction of it to be transformed.

Boltwood has been able to separate a radioactive substance, which he has named ionium, which appears to be one of the intermediate steps in the change from uranium to radium.

785. Helium. In 1868 Lockyer proposed the name helium for an unknown substance existing in the sun and causing a line in the solar spectrum which could not be obtained from any known terrestrial substance. Nearly thirty years later Ramsay, an English chemist, identified it with a gas obtained from some radioactive minerals. Its occurrence in these minerals suggested to Rutherford and Soddy that it might be a product of the disintegration of radium. Later Ramsay and Soddy collected in a tube some of the emanation from radium, and though at first there was no indication of the presence of helium, after five days its complete spectrum was obtained. Helium is a gas having twice the density of hydrogen, and it seems probable that the alpha particles given off in the transformations of radium form the atoms of helium. This result is remarkable as the first case in which one stable element has been derived from another.

786. Final Product of Radium.-If the atomic weight of the alpha particle is 4 (the same as helium), then, since the atomic weight of uranium is 238.5, the loss of three alpha particles would bring it down to 226.5, which is almost the same as 225, the observed atomic weight of radium. Then, as five alpha

particles are given off from radium in the series of changes indicated above ($784), the atomic weight of the final product would be expected to be 206.5, which is in close agreement with 206.9, the atomic weight of lead. The suggestion that lead may be the final product of the radium changes is due to Boltwood and is supported by the fact that lead is always found associated with radioactive minerals which are rich in uranium.

Rutherford says: "It does not appear likely that we shall be able for many years to prove or disprove experimentally that lead is the final product of radium."

These discoveries that radium, helium, and possibly lead also may be products of the disintegration of uranium give new emphasis to a very old suggestion that the atoms of what we call the elements are a number of stable aggregates built up from some simple primordial atom. Whether the elements as we now know them are to be regarded as so many stages in the gradual disintegration of originally more complex atoms of high atomic weight or are various stable aggregates formed independently and perhaps simultaneously under physical conditions that we cannot now distinctly conceive, is a question that cannot now be answered, though the absence of any perceptible radioactivity in the case of most substances points toward the latter origin.

787. Internal Energy of Atoms.-That there exists an enormous amount of energy in the interior of atoms is evident from the heat which is given out by radium in its slow transformation. Thus it has been shown that one gram of radium gives out 100 gram-calories of heat per hour, but as it requires 2000 years for one-half of a gram to be transformed, it is calculated that the whole amount of heat emitted by one gram of radium in the process of transformation is about 10,000,000,000 gram-calories, or more than a million times as much heat as is given out in the combustion of a gram of coal. And it must not be forgotten that this enormous amount of energy can represent only a small fraction of the total internal energy of the atoms in a gram of radium, being merely that part which is given out when the atoms pass from one state of equilibrium to another. This energy is detected in radioactive elements only in consequence of their disintegration, but there is no reason to suppose

that the internal energy of these elements is of a different order of magnitude from that of the other elements.

There is no known process by which the store of energy locked up in the atoms of matter may be made available for the service of man, for it does not seem possible to disturb the equilibrium of the electrons in the atoms by any outside force. The radioactivity of radium is not changed either by cooling to the temperature of liquid air or heating it to 200° C.

788. The Electron Theory of Matter. It is now generally accepted that waves of light, having their source in the atoms of matter, are electromagnetic waves and must originate in electric oscillations of some sort, and there is evidence that electrons, or the negatively charged particles making up the cathode rays, are constituents of every kind of matter, for it has been discovered that the electrons in cathode rays have the same mass and charge and are apparently identical whatever may be the nature of the electrodes or of the residual gas in the vacuum tubes. Similar electrons are also given out from a glowing .carbon filament and from a zinc plate acted on by ultraviolet light and are emitted at high velocities from radioactive substances. These discoveries have led to the idea that electrons are fundamental units entering into the structure of all kinds of atoms.

According to J. J. Thomson's development of the electron theory, an atom consists of a group of electrons moving with great velocity in a sphere of uniform positive electrification by which they are held together and kept in equilibrium, the number and grouping of the electrons determining the properties of the resulting atom. Thus in the hydrogen atom there are 2000 electrons, while in the oxygen atom there are 32000.

The electrons are conceived as arranged in the atom in concentric rings, those in each ring rotating together with great velocity about the center of the atom. A single electron moving in such a way would radiate a large amount of energy. But when a large number of electrons all move together at equal intervals in a circle the radiation is extremely small and consequently the atom may lose energy so slowly that there is no change in its condition of equilibrium even in millions of years. The electron is estimated to have only one hundred-thousandth

the diameter of the atom, so that if an atom were magnified to be 100 feet in diameter, each electron would have a diameter of only about one hundredth of an inch. It is easy, therefore, to conceive that in an atom there may be many groups of electrons moving with great velocity and yet without interfering with one another.

According to this theory, when such an atom loses an electron it becomes electropositive and when it gains one it becomes negative. A negatively charged body has an excess of electrons, while in a positively charged body there is a deficiency of them, and an electric current is conceived as the streaming of free electrons through a conductor.

This theory has been shown by J. J. Thomson to afford a possible explanation of the periodic law of the elements as well as valency. It is supported also by the observed high velocities with which electrons escape from radioactive elements, for it can hardly be supposed that these enormous velocities can have been given to them wholly in the act of escape. And Lorentz of Holland has shown that the assumption that light waves originate in moving electrons in an atom not only explains the Zeeman effect (§968), but leads to a value of the ratio of the mass of an electron to its electric charge which agrees with that found by J. J. Thomson for the electrons in cathode rays.

Whether such a theory shall stand or fall depends on whether it will enable the physicist to coordinate his knowledge and form a clear mental picture of the interrelation of the various isolated facts known about atoms. A satisfactory theory besides giving a basis for the explanation of radioactivity and the chemical relations of the elements must lend itself to an explanation of the peculiar series of lines in the spectra of the elements as well as of the other peculiarities of atomic radiation revealed by the spectroscope.

For an account of this theory the student is referred to "Electricity and Matter," by J. J. Thomson.

J. J. Thomson.

The Modern Theory of Physical Phenomena. Righi.
The Discharge of Electricity through Gases.
Electricity and Matter. J. J. Thomson.
Radioactive Transformations. Rutherford.

LIGHT

SHADOWS AND PHOTOMETRY.

789. Light. In a perfectly dark room we cannot see any objects, we have no sensation of sight,-showing that vision requires something more than simply the eye and the object to be seen. That additional something is called light. We may define it as the agent which excites the sensation of sight.

When a candle is lighted in the room we see it and also the other objects near. The candle flame is said to be self-luminous and a source of light.

Conditions of Vision.-When the candle is so screened that its light falls only on the eye of the observer but not on other objects in the room, then only the candle itself is seen. It thus appears that illuminating the eye does not give it power to see other objects from which light is excluded. Light must fall upon the objects themselves if we are to see them.

And even when an object is illuminated, if a screen is interposed across the straight line from the object to the eye, the former is hidden and we see the screen but not the object behind it. This leads to the inference that in order that a body may be seen light must pass from it to the eye, and usually this takes place along straight lines.

790. Transparent and Opaque Bodies.-Bodies differ greatly in their capacity for transmitting light. Those that transmit it freely are said to be transparent, while those that intercept it are called opaque. Opaque bodies are of two kinds: those that turn back the light at the surface and those into which light penetrates and is absorbed and transformed into heat. The opacity of metals is largely of the first kind, while that of most other substances is due to absorption.

Substances like paper or milk-glass, or milky or muddy waters, which transmit light but through which we cannot see objects, are said to be translucent. They are not homogeneous bodies, but light in passing through them is scattered in all