Popular Science Monthly/Volume 63/May 1903/A New Source of Heat: Radium



AT a meeting of the French Academy of Sciences held in March MM. Curie and Laborde announced a newly discovered property, of that extraordinary substance radium—its salts emit heat continuously and to a measurable extent. Headers of the Popular Science Monthly may remember that in the number for July, 1900, we sketched the history of the discovery of this new body by M. and Mme. Curie in 1898, and we gave some account of its marvellous physical and chemical properties so far as known at that date; its power of giving out light perpetually without any exciting cause, its emission of rays that penetrate solids like the X-ray, its faculty of acting on sensitized plates, and of causing air to conduct electricity. Now a fifth property must be added, that of the emission of heat.

During the few months that have elapsed since the publication of the above summary, physicists and chemists on both sides of the Atlantic have been actively experimenting with the interesting body, in no wise discouraged by its excessive rarity and by the great difficulty of obtaining it unmixed with the mineral substances by which it is always accompanied in nature. Tons of minerals have been submitted to laborious processes in the chemical laboratory to obtain a few grammes of the precious material; and at the end of the task the conscientious scientist can only claim that the product is such and such a salt containing a small, unknown percentage of radium.

To enumerate the peculiar activities of radium with any degree of completeness would occupy more pages of the magazine than could well be spared; for details we must refer to the purely technical journals, but some points arrest the attention of every one.

Becquerel, the French physicist whose name is attached to the rays emitted by uranium, observed the powerful physiological action of radium when in a comparatively pure state; a few grammes enclosed in a bottle carried in his waistcoat pocket burned holes into the flesh in six hours, producing superficial sores that took several weeks to heal. Some experimenters have remarked that their fingers are made sore by handling its salts. Aschkinass and Caspari have exposed cultures of Micrococcus prodigiosus to the influence of its rays and ascertained that they were fatal to the bacteria.

The character of the rays given out by radium has been the subject of special research; MM. Curie and Danne observed that solid bodies submitted to the rays issuing from radium in a confined space, became active themselves in an analogous manner. On removing the bodies from this influence the power thus excited passes off in accordance with a given law independent of the nature of the bodies. In this connection experiments were made with bodies of diverse constitution, such as aluminium, copper, lead, bismuth, platinum, silver, glass, alum, paraffine, celluloid and caoutchouc.

Professor Rutherford, of Montreal, has found that this induced activity is produced by an 'emanation' that behaves like a gas, but this gas has not been isolated, or tested chemically or physically. In this connection it is of interest to note that Dr. Giesel, of Germany, also mentions a peculiar, colorless gas, having radio-active properties obtained by the decomposition of radium bromide.

The nature and extraordinary energy of the rays emitted by this singular substance has attracted much attention; it has been shown that they are of different kinds, a part being identical with cathode rays and another part capable of being still further divided into very penetrating rays, and those easily absorbed. Their energy is estimated by Rutherford and McClung to be prodigious; they calculate that one gramme of radium would radiate in a year energy equivalent to 3000 gramme-calories, which is about one foot-pound per hour. The source of this energy is a mystery; the savants last named suggest that it is due to the breaking down of atoms into smaller particles which themselves constitute these radiations.

Since it is universally admitted that the radiations are material the problem arises, does radium lose weight in the course of time? This question has been answered differently by two authorities. Becquerel has calculated from experimental data that one square centimeter of radium-surface would lose 1.2 milligrammes of matter in one thousand million years. On the other hand, Heydweiller found that five grammes containing only a small percentage of pure radium lost about 0.02 of a milligramme per day, and he observed a total loss of one half milligramme in a time not stated. The excessively small quantities of material available for examination and its exceeding rarity (a very small sample is valued at twenty-five dollars) will account for such contradictory statements.

The discovery by Curie and Laborde that radium emits heat was the result of two experiments. By a thermo-electric method they ascertained that a specimen of barium chloride containing one sixth of its weight of radium chloride indicated a temperature 1.5° C. (2.7° Fah.) higher than a sample of pure barium chloride; the temperature was determined by comparing the heat emitted with that excited in a wire of known resistance by an electric current of known intensity. In the second experiment they employed a Bunsen calorimeter. The experimenters found that one gramme of active barium chloride emits about fourteen small calories per hour. The specimen contained only about one sixth its weight of radium chloride, but on testing 0.08 gramme of purer material they obtained identical results, from which it can be calculated that one gramme of radium would emit 100 small calories per hour, or one atom-gramme (225 grammes) would emit each hour 22,500 calories, an amount comparable with the heat disengaged by the combustion in oxygen of one atom-gramme of hydrogen.

The continuous emission of such a large quantity of heat can not be explained by any chemical action, and must be due to some modification of the atom itself; if so, such a change must be very slow. As a matter of fact, Demarçay observed no change in the spectrum of radium examined at intervals of five months.

An English writer, commenting on the figures given by M. Curie, says that a radium salt in a pure state would melt more than its own weight of ice every hour; and half a pound of radium salt would evolve in one hour an amount of heat equal to that produced by burning one third of a cubic foot of hydrogen gas. And the extraordinary part of this is that the evolution of heat goes on without combustion, without chemical change of any kind, without alteration of its molecular structure, and continuously, leaving the salt at the end of months of activity just as potent as in the beginning. Yet this state of things must have a cause, for it must not be imagined that perpetual motion has been at last attained.

Persons who are not practically familiar with the work carried on in the laboratories of physics and chemistry are in danger of drawing unwarrantable conclusions from the statements made by imaginative reporters in the daily press, and of concluding that radium will eventually replace gas for illuminating purposes as well as anthracite for heating. Such persons do not realize the great scarcity of the raw material yielding this substance, nor the exceedingly minute quantities used in the experiments which have furnished these astounding results. A tea spoon would probably hold all the pure radium as yet prepared, and its price would amount to thousands of dollars.

And what may be expected from future researches? Do the other rare bodies, polonium, actinium and thorium, that behave in many respects like radium, also share its most recently discovered power of emitting heat? Will not scientists be compelled to revise some of the theories of physics that they regard at present as cardinal? And what are the conditions in the earth beneath our feet, when inert matter manifests energy to such an amazing extent without a known cause? The future opened to students and to philosophers is fraught with mysteries, the solution of which will be eagerly awaited by the rest of the world.