ance is decreased sufficiently to enable the battery E to actuate the relay R. This closes a local circuit, in which the telectrograph receiver is included, and a mark appears on the paper. In this way a visible record is obtained, which greatly facilitates the process.

Wireless phototelegraphy may eventually prove of more utility than the closed-circuit methods, because it would bring America within reach of this country, and would enable communication to be made where telephone or telegraph lines did not exist. It is not limited to photographs-banking signatures, sketches, maps, plans, and writing could be transmitted. But I would point out most particularly that the work is as yet in the very earliest stages, and that in giving you some account of it to-night I may be bringing before your notice methods and systems on which a few years hence you will look back with a smile-as curious merely from a historical point of view.

MODERN IDEAS ON THE CONSTITUTION OF MATTER.1

By JEAN BECQUEREL,

Professor at the National Museum of Natural History, Paris.

For a number of years past physicists have been laying the foundations of a new theory of matter. A series of bold conceptions, based on unlooked-for facts, has worked a deep-seated transformation in the previously accepted ideas concerning the constitution of bodies.

Everyone knows that substances in general are divided into two groups, simple bodies or elements and complex bodies made up by the combination of these elements. For a long time these bodies have been considered as composed of atoms which have combined and formed molecules, the atom being the most minute quantity of matter characteristic of an element and capable of entering into chemical combinations, while the molecule of a body, simple or complex, is the smallest particle of this body which is capable of existing in a physical state. Let us consider an example: The molecule of water, the smallest quantity of water which can exist in a physical state, is the result of the combination of two atoms of hydrogen with one of oxygen. I shall repeat before you the classic experiment of decomposing water by an electric current; oxygen is set free at the positive pole and hydrogen at the negative pole, the two gases coming off in the proportion of two volumes of hydrogen to one volume of oxygen.

The molecule of a complex body is always made up of the atoms of at least two elements. The molecule of an element may be made up of only a single atom, as is the case with monatomic bodies such as helium, zinc, cadmium, or mercury, while in other cases the molecule of a simple body may be a group of several atoms of this body, for instance, hydrogen and oxygen are diatomic, while phosphorus and arsenic are tetratomic.

The discovery of Gay Lussac concerning the laws of the composition of gases led Avogadro and Ampere to declare that gases con

1 Lecture delivered at the Museum, Apr. 10, 1910. Translated by permission from the Revue Scientifique, Paris, 48, No. 14, Oct. 1, 1910,

tained in equal volumes the same number of molecules, and that the definite proportions in which they combined represented the invariable relation between the weights of the atoms which were in juxtaposition.

The theory is that in the interior of bodies the constituent molecules are perpetually animated by a movement which becomes correspondingly greater as the temperature becomes higher. If the swiftness of these thermic movements could be gradually reduced to zero, temperatures would be obtained which would approach more and more closely to the limit of temperature found at about -273° C. This temperature, the lowest conceivable, since it corresponds to a state of repose of the molecules, is called absolute zero.

The principles of mechanics which apply to this conception of molecules in movement takes account of all these laws to which gases and dissolved bodies are subjected. I can not enlarge here on the methods which have made it possible to count in a cubic centimeter of gas at ordinary temperature and pressure, thirty billion billions of molecules, and to evaluate the dimensions of one of these molecules. The diameter of a molecule of oxygen, for example, is a few ten-millionths of a millimeter. These figures give some idea, however, of the extreme divisibility of matter. In connection with this divisibility of matter it is interesting to recall that according to Berthelot the odor of one hundred-thousand-millionth of a gram of iodoform per cubic centimeter of air is perceptible to the sense of smell.

You are all aware that matter attracts matter, in accordance with the universal law of gravitation which rules the movements even of the stars. The invariability of the constant of gravitation has suggested the idea that the atoms of all bodies can be formed by the unequal condensation of a single principle and the relations discovered by chemists between the different elements lend themselves favorably to this hypothesis.

The idea of a single principle as the ultimate constituent of all things, dates in reality from the most ancient times. Twenty-five centuries ago, Thales propounded the existence of a primordial fluid to which he attributed a sort of soul and a power of attraction. Anaximander, Anaximines, and Herodotus spoke of a universal principle, and Pythagoras located above the air "ether, a celestial substance free from all perceptible matter." Five hundred years before our era Leucippus and Democritus had conceived of atoms indivisible and eternal which moved about in infinite space; Lucretius a little later expounded similar doctrines. Finally Descartes and Leibnitz developed for themselves an idea of matter which led them to similar conclusions.

About the end of the last century an English chemist, Prout, propounded the hypothesis that all elements could be made up by the progressive condensation of hydrogen, the lightest of all the bodies.

Several years ago, however, modern physicists took a still further step; they now attribute an atomic structure not only to matter, but to electricity as well, and consider matter as composed of electricity.

We shall see as a fact that electrified corpuscles have been isolated which themselves appear to be composed of electricity, entirely free from anything that can properly be termed matter, whose mass is of electro-magnetic origin and is nearly two thousand times as small as that of an atom of hydrogen.

These atoms of electricity are called electrons. They are present in all bodies; they are the atoms which are at the source of all phenomena of light, and again they are those atoms which allow the conduction of heat and of electricity. The electron appears to be in the nature of a universal constituent of matter, without being itself matter, in the ordinary sense of the word.

The first conception of an atom of electricity is a result of the phenomenon of electrolysis, of which you may see an example in the decomposition of acidified water by an electric battery. A solution which is a conductor of electricity and is decomposible by a current is termed an electrolyte. Every molecule of an electrolyte is separable into two atoms or atomic groups, called ions, which possess charges of equal quantity and opposite signs; thus, when sodium chloride is dissolved in water a certain number of molecules dissociate into negative chlorine ions and positive sodium ions. Under the influence of the molecular movements which go to make up heat and consequently from the shocks resulting there from there is a constant recombination of the ions and fresh decomposition of the molecules. In a very dilute aqueous solution, however, nearly all the sodium chloride is found to be in a state of dissociation. Now, if two electrodes connected to the poles of a battery are dipped into the solution, the negative ions (chlorine) are carried to the positive pole (anode) and the positive ions (sodium) to the negative pole (cathode).

The laws of electrolysis, established by Faraday and worked out completely by Edmond Becquerel have led to the conclusion that all the univalent ions, such as hydrogen, chlorine, sodium, and potassium, always carry the same charge (negative or positive), while the bivalent ions (copper and the like) carry a charge just double the preceding, and so on. The charge of the univalent ion is the smallest charge that has ever been observed, and when separated from its material support constitutes the electron or atom of elec tricity.

This elementary charge has been susceptible of measurement. It is easy to evaluate the quantity of electricity necessary to liberate a gram of matter-for example, a gram of hydrogen in the electrolysis of water-and thus to obtain the total charge of the hydrogen ions. These ions correspond to the molecules, and as the number of molecules in a gram of hydrogen is known, the charge carried by a single ion may be determined. This charge is very minute; it amounts to 4.10-10 in terms of the C. G. S. electrostatic unit.

The study of the radiations obtained in rarified gases has also been of assistance in making our knowledge of the atom of electricity more definite. When an electric discharge is produced in a gas by means of a static machine or an induction coil under ordinary pressure a disruptive spark is obtained. In a tube where the pressure is reduced the aspect of this spark is changed and when the pressure is not more than a few millionths of an atmosphere (Crookes's vacuum) a ray emanating from the cathode (negative pole) may be observed. Whatever may be the position of the anode or positive pole, this cathode ray is emitted perpendicularly to the surface of the cathode and is sent out in a straight line. The glass of the tube where the radiation strikes it takes on a beautiful green fluorescence. These cathode waves excite phosphorescent bodies and heat screens placed in their path.

1

These rays emanating from the cathode bear the name of cathode rays. They were discovered in 1869 by Hittorf, and have since been made the subject of study by a great many physicists, among whom are Crookes, J. J. Thomson, Jean Perrin, Marjorana, Lenard, Wien, Villard, and others. Sir William Crookes was the first to propound the hypothesis that they were due to a fourth state of matter, the radiant state, which took shape as a molecular bombardment, as it might be called. This truly remarkable idea met with much incredulity, as at that period (1880) the tendency of most men of science was to attribute all such phenomena to a vibratory movement and not to a flow of matter itself. Many physicists, therefore, considered the cathode rays to be due to undulatory movements analagous to light. This interpretation was soon to be abandoned, however. Later experiments confirmed in the most startling manner the ideas of Sir William Crookes, subject only to the qualification that the radiant state was due in the cathode rays not to a bombardment of particles of matter, but to a bombardment of electrified corpuscles which were much smaller than the molecules of known bodies and which were no other, as we shall see later, than negative electrons free from matter.

1 Experiment: A bouquet of phosphorescent material rendered luminous by cathode rays.