Part 11 (1/2)
Radium compounds spontaneously give off energy in the form of heat. A quant.i.ty of radium chloride which contains 1 gram of radium continuously gives out, per hour, a quant.i.ty of heat sufficient to raise the temperature of 1 gram of water through 100 C., or 100 grams of water through 1 C. The heat given out by 1 gram of radium during twenty-four hours would raise the temperature of 2400 grams of water through 1 C.; in one year the temperature of 876,000 grams of water would be raised through 1 C.; and in 1800 years, which is approximately the half-life period of radium, the temperature of 1,576,800 _kilograms_ of water would be raised through 1 C. These results may be expressed by saying that if 1 gram (about 15 grains) of radium were kept until half of it had changed into inactive substances, and if the heat spontaneously produced during the changes which occurred were caused to act on water, that quant.i.ty of heat would raise the temperature of about 15 tons of water from its freezing- to its boiling-point.
Radium compounds send forth three kinds of rays, distinguished as _alpha_, _beta_, and _gamma_ rays. Experiments have made it extremely probable that the [alpha]-rays are streams of very minute particles, somewhat heavier than atoms of hydrogen, moving at the rate of about 18,000 miles per second; and that the [beta]-rays are streams of much more minute particles, the ma.s.s of each of which is about one one-thousandth of the ma.s.s of an atom of hydrogen, moving about ten times more rapidly than the [alpha]-particles, that is, moving at the rate of about 180,000 miles per second. The [gamma]-rays are probably pulsations of the ether, the medium supposed to fill s.p.a.ce. The emission of [alpha]-rays by radium is accompanied by the production of the inert elementary gas, helium; therefore, the [alpha]-rays are, or quickly change into, rapidly moving particles of helium. The particles which const.i.tute the [beta]-rays carry electric charges; these electrified particles, each approximately a thousand times lighter than an atom of hydrogen, moving nearly as rapidly as the pulsations of the ether which we call light, are named _electrons_. The rays from radium compounds discharge electrified bodies, ionise gases, that is, cause them to conduct electricity, act on photographic plates, and produce profound changes in living organisms.
The radium emanation is a gas about 111 times heavier than hydrogen; to this gas Sir William Ramsay has given the name _niton_. The gas has been condensed to a colourless liquid, and frozen to an opaque solid which glows like a minute arc-light. Radium emanation gives off [alpha]-particles, that is, very rapidly moving atoms of helium, and deposits exceedingly minute quant.i.ties of a solid, radio-active substance known as radium A. The change of the emanation into helium and radium A proceeds fairly rapidly: the half-life period of the emanation is a little less than four days. This change is attended by the liberation of much energy.
The only satisfactory mental picture which the facts allow us to form, at present, of the emission of [beta]-rays from radium compounds is that which represents these rays as streams of electrons, that is, particles, each about a thousand times lighter than an atom of hydrogen, each carrying an electric charge, and moving at the rate of about 180,000 miles per second, that is, nearly as rapidly as light.
When an electric discharge is pa.s.sed from a plate of metal, arranged as the kathode, to a metallic wire arranged as the anode, both sealed through the walls of a gla.s.s tube or bulb from which almost the whole of the air has been extracted, rays proceed from the kathode, in a direction at right angles thereto, and, striking the gla.s.s in the neighbourhood of the anode, produce a green phosph.o.r.escence. Facts have been gradually acc.u.mulated which force us to think of these _kathode rays_ as streams of very rapidly moving electrons, that is, as streams of extraordinarily minute electrically charged particles identical with the particles which form the [beta]-rays emitted by compounds of radium.
The phenomena of radio-activity, and also the phenomena of the kathode rays, have obliged us to refine our machinery of minute particles by including therein particles at least a thousand times lighter than atoms of hydrogen. The term _electron_ was suggested, a good many years ago, by Dr Johnstone Stoney, for the unit charge of electricity which is carried by an atom of hydrogen when hydrogen atoms move in a liquid or gas under the directing influence of the electric current.
Some chemists speak of the electrons, which are the [beta]-rays from radium, and the kathode rays produced in almost vacuous tubes, as non-material particles of electricity. Non-material means devoid of ma.s.s. The method by which approximate determinations have been made of the charges on electrons consists in measuring the ratio between the charges and the ma.s.ses of these particles. If the results of the determinations are accepted, electrons are not devoid of ma.s.s.
Electrons must be thought of as material particles differing from other minute material particles in the extraordinary smallness of their ma.s.ses, in the ident.i.ty of their properties, including their ma.s.s, in their always carrying electric charges, and in the vast velocity of their motion. We must think of an electron either as a unit charge of electricity one property of which is its minute ma.s.s, or as a material particle having an extremely small ma.s.s and carrying a unit charge of electricity: the two mental pictures are almost, if not quite, identical.
Electrons are produced by sending an electric discharge through a gla.s.s bulb containing a minute quant.i.ty of air or other gas, using metallic plates or wires as kathode and anode. Experiments have shown that the electrons are identical in all their properties, whatever metal is used to form the kathode and anode, and of whatever gas there is a minute quant.i.ty in the bulb. The conclusion must be drawn that identical electrons are const.i.tuents of, or are produced from, very different kinds of chemical elements. As the facts about kathode rays, and the facts of radio-activity are (at present) inexplicable except on the supposition that these phenomena are exhibited by particles of extraordinary minuteness, and as the smallest particles with which chemists are concerned in their everyday work are the atoms of the elements, we seem obliged to think of many kinds of atoms as structures, not as h.o.m.ogeneous bodies. We seem obliged to think of atoms as very minute material particles, which either normally are, or under definite conditions may be, a.s.sociated with electrically charged particles very much lighter than themselves, all of which are identical, whatever be the atoms with which they are a.s.sociated or from which they are produced.
In their study of different kinds of matter, chemists have found it very helpful to place in one cla.s.s those substances which they have not been able to separate into unlike parts. They have distinguished this cla.s.s of substances from other substances, and have named them _elements_. The expression _chemical elements_ is merely a summary of certain observed facts. For many centuries chemists have worked with a conceptual machinery based on the notion that matter has a grained structure. For more than a hundred years they have been accustomed to think of atoms as the ultimate particles with which they have had to deal. Working with this order-producing instrument, they have regarded the properties of elements as properties of the atoms, or of groups of a few of the atoms, of these substances. That they might think clearly and suggestively about the properties of elements, and connect these with other chemical facts, they have translated the language of sense-perceptions into the language of thought, and, for _properties of those substances which have not been decomposed_, have used the more fertile expression _atomic properties_. When a chemist thinks of an atom, he thinks of the minutest particle of one of the substances which have the cla.s.s-mark _have-not-been-decomposed_, and the cla.s.s-name _element_. The chemist does not call these substances elements because he has been forced to regard the minute particles of them as undivided, much less because he thinks of these particles as indivisible; his mental picture of their structure as an atomic structure formed itself from the fact that they had not been decomposed. The formation of the cla.s.s _element_ followed necessarily from observed facts, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. The conception of the elementary atom as a particle which had not been decomposed followed from many observed facts besides those concerning elements, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. Investigations proved radio-activity to be a property of the very minute particles of certain substances, and each radio-active substance to have characteristic properties, among which were certain of those that belong to elements, and to some extent are characteristic of elements. Evidently, the simplest way for a chemist to think about radio-activity was to think of it as an atomic property; hence, as atomic properties had always been regarded, in the last a.n.a.lysis, as properties of elements, it was natural to place the radio-active substances in the cla.s.s _elements_, provided that one forgot for the time that these substances have not the cla.s.s-mark _have-not-been-decomposed_.
As the facts of radio-activity led to the conclusion that some of the minute particles of radio-active substances are constantly disintegrating, and as these substances had been labelled _elements_, it seemed probable, or at least possible, that the other bodies which chemists have long called elements are not true elements, but are merely more stable collocations of particles than the substances which are cla.s.sed as compounds. As compounds can be changed into certain other compounds, although not into any other compounds, a way seemed to be opening which might lead to the transformation of some elements into some other elements.
The probability that one element might be changed into another was increased by the demonstration of the connexions between uranium and radium. The metal uranium has been cla.s.sed with the elements since it was isolated in 1840. In 1896, Becquerel found that compounds of uranium, and also the metal itself, are radio-active. In the light of what is now known about radio-activity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves, and change into some substance, or substances, different from uranium; in other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating.
This supposition is confirmed by the fact, experimentally proved, that uranium emits [alpha]-rays, that is, atoms of helium, and produces a substance known as uranium X. Uranium X is itself radio-active; it emits [beta]-rays, that is, it gives off electrons.
Inasmuch as all minerals which contain compounds of uranium contain compounds of radium also, it is probable that radium is one of the disintegration-products of uranium. The rate of decay of radium may be roughly expressed by saying that, if a quant.i.ty of radium were kept for ten thousand years, only about one per cent. of the original quant.i.ty would then remain unchanged. Even if it were a.s.sumed that at a remote time the earth's crust contained considerable quant.i.ties of radium compounds, it is certain that they would have completely disappeared long ago, had not compounds of radium been reproduced from other materials. Again, the most likely hypothesis is that compounds of radium are being produced from compounds of uranium.
Uranium is a substance which, after being rightly cla.s.sed with the elements for more than half a century, because it had not been separated into unlike parts, must now be cla.s.sed with the radium-like substances which disintegrate spontaneously, although it differs from other radio-active substances in that its rate of change is almost infinitively slower than that of any of them, except thorium.[12]
Thorium, a very rare metal, is the second of the seventy-five or eighty elements known when radio-activity was discovered, which has been found to undergo spontaneous disintegration with the emission of rays. The rate of change of thorium is considerably slower than that of uranium.[13] None of the other substances placed in the cla.s.s of elements is radio-active.
[12] The life-period of uranium is probably about eight thousand million years.
[13] The life-period of thorium is possibly about forty thousand million years.
On p. 192 I said, that when the radio-active substances had been labelled _elements_, the facts of radio-activity led some chemists to the conclusion that the other bodies which had for long been called by this cla.s.s-name, or at any rate some of these bodies, are perhaps not true elements, but are merely more stable collocations of particles than the substances called compounds. It seems to me that this reasoning rests on an unscientific use of the term _element_; it rests on giving to that cla.s.s-name the meaning, _substances a.s.serted to be undecomposable_. A line of demarcation is drawn between _elements_, meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts, and _true elements_, meaning thereby groups of identical undecomposable particles. If one names the radio-active substances _elements_, one is placing in this cla.s.s substances which are specially characterised by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the cla.s.s. To do this may be ingenious; it is certainly not scientific.