the heat which flows from the hot interior of the earth to the colder crust. Now, when the earth first solidified it only possessed a certain amount of capital in the form of heat, and if it is continually spending this capital and not gaining any fresh heat it is evident that the process can not have been going on for more than a certain number of years, otherwise the earth would be colder than it is. Lord Kelvin in this way estimated the age of the earth to be less than 100,000,000 years. Though the quantity of radium in the earth is an exceedingly small fraction of the mass of the earth, only amounting, according to the determinations of Professors Strutt and Joly, to about 5 grams in a cube whose side is 100 miles, yet the amount of heat given out by this small quantity of radium is so great that it is more than enough to replace the heat which flows from the inside to the outside of the earth. This, as Rutherford has pointed out, entirely vitiates the previous method of determining the age of the earth. The fact is that the radium gives out so much heat that we do not quite know what to do with it, for if there was as much radium throughout the interior of the earth as there is in its crust, the temperature of the earth would increase much more rapidly than it does as we descend below the earth's surface. Professor Strutt has shown that if radium behaves in the interior of the earth as it does at the surface, rocks similar to those in the earth's crust can not extend to a depth of more than 45 miles below the surface.

It is remarkable that Professor Milne from the study of earthquake phenomena had previously come to the conclusion that rocks similar to those at the earth's surface only descend a short distance below the surface; he estimates this distance at about 30 miles, and concludes that at a depth greater than this the earth is fairly homogeneous.

Though the discovery of radioactivity has taken away one method of calculating the age of the earth it has supplied another.

The gas helium is given out by radioactive bodies, and since, except in beryls, it is not found in minerals which do not contain radioactive elements, it is probable that all the helium in these minerals has come from these elements. In the case of a mineral containing uranium, the parent of radium in radioactive equilibrium, with radium and its products, helium will be produced at a definite rate. Helium, however, unlike the radioactive elements, is permanent and accumulates in the mineral; hence if we measure the amount of helium in a sample of rock and the amount produced by the sample in one year we can find the length of time the helium has been accumulating, and hence the age of the rock. This method, which is due to Professor Strutt, may lead to determinations not merely of the average age of the crust of the earth, but of the ages of particular rocks and the date at which the various strata were deposited; he has, for example, shown in this

way that a specimen of the mineral thorianite must be more than 240,000,000 years old.

The physiological and medical properties of the rays emitted by radium is a field of research in which enough has already been done to justify the hope that it may lead to considerable alleviation of human suffering. It seems quite definitely established that for some diseases, notably rodent ulcer, treatment with these rays has produced remarkable cures; it is imperative, lest we should be passing over a means of saving life and health, that the subject should be investigated in a much more systematic and extensive manner than there has yet been either time or material for. Radium is, however, so costly that few hospitals could afford to undertake pioneering work of this kind; fortunately, however, through the generosity of Sir Ernest Cassel and Lord Iveagh, a radium institute, under the patronage of His Majesty the King, has been founded in London for the study of the medical properties of radium, and for the treatment of patients suffering from diseases for which radium is beneficial.

The new discoveries made in physics in the last few years, and the ideas and potentialities suggested by them, have had an effect upon the workers in that subject akin to that produced in literature by the renaissance. Enthusiasm has been quickened, and there is a hopeful, youthful, perhaps exuberant, spirit abroad which leads men to make, with confidence, experiments which would have been thought fantastic twenty years ago. It has quite dispelled the pessimistic feeling, not uncommon at that time, that all the interesting things had been discovered, and all that was left was to alter a decimal or two in some physical constant. There was never any justification for this feeling, there never were any signs of an approach to finality in science. The sum of knowledge is at present, at any rate, a diverging not a converging series. As we conquer peak after peak we see in front of us regions full of interest and beauty, but we do not see our goal, we do not see the horizon; in the distance tower still higher peaks, which will yield to those who ascend them still wider prospects, and deepen the feeling, whose truth is emphasized by every advance in science, that "Great are the Works of the Lord."

PRODUCTION OF LOW TEMPERATURES, AND

REFRIGERATION.

By L. MARCHIS.

(Translated by permission from the Revue générale des Sciences, pures et appliquées, Paris, 20th year, No. 5, March 15, 1909.)

The first International Congress of Refrigerative Industries, held at Paris October 5-12, 1908, was remarkable for the number and importance of the papers submitted by the men of science and engineers who responded to the call of the organization committee. The congress was divided into six sections as follows: First section. Low temperatures and their general effects. Second section. Refrigerating media.

Third and fourth sections. Application of refrigeration to food and in various other industries.

Fifth section. Application of refrigeration in commerce and transportation.

Sixth section. Legislation.

In this article we do not intend to summarize the memoirs and communications presented, but rather to give a general view of the congress that brought together men of science, engineers, biologists, legislators, and men from the business world.

I. LIQUID AIR AND THE PROPERTIES OF BODIES AT LOW TEMPERATURES.

The first section considered principally the production of liquid air and the preparation, with this as a starting point, of oxygen and nitrogen in a commercial way.

It is well known that air, like all gases, is brought into a liquid condition by the combined effect of lowering its temperature and expanding it sufficiently. In carrying out this process, gaseous air cooled to a low temperature is expanded suddenly from a pressure Po to a lower pressure p1. Part of it goes over into a liquid state, and the other part, gaseous and very cold, is led into an economizer, where it cools down the air that is being compressed to the pressure po for the first time.

The essentially adiabatic expansion of air can be effected in two different ways.

(a) Air compressed to the pressure p, may be expanded without doing available exterior work. It passes from the compression tank to the liquefaction tank by the way of a narrow orifice. This is the manner of expansion adopted by Linde in his liquid-air machines. The lowering of temperature obtained under such conditions is only appreciable if the difference between the pressures p, and p, is considerable. In Linde's apparatus gaseous air cooled to about -100° C. is expanded from a pressure of 200 to 40 atmospheres; the liquefied part of the gas at about -140° C. passes into a regenerator where it cools the air compressed at 200 atmospheres; it is then led into a pump which brings it up to this latter pressure. A second auxiliary pump draws air from the atmosphere to take the place of that part which has been liquefied. In the industrial machines the gases compressed to 200 atmospheres, before passing into the economizer where the gas at -140° circulates, are cooled by liquid ammonia.

Under such conditions, in machines which produce 50 liters per hour the yield of liquid air is about half a liter per horsepower-hour.

(b) The second method of air expansion consists in utilizing the exterior work which the gas is capable of doing when it passes from pressure po to p,. This mode of expansion with utilizable exterior work is the basis of the processes of G. Claude for the production of liquid air. Air compressed to a maximum pressure of 30 or 40 atmospheres passes first to an economizer, where it is cooled down as in Linde's apparatus by unliquefied gas. It is then expanded in the cylinder of a motor whose energy can be utilized in the original compression of the air. In course of time a partial liquefaction of the air occurs in the cylinder of the auxiliary motor. The lubrication of this cylinder is accomplished by means of a petroleum distillate having a specific gravity of 0.675 (automobile gasoline), which, at the low temperature at which the motor operates, attains a sirupy consistency comparable to industrial lubricants.

Applied in this form the process of G. Claude gives only unsatisfactory results. The expansion of the air, occurring at temperatures of 175° to -180° by the gas expanding in the auxiliary motor, takes place under unfavorable conditions. The appearance of liquid air in this auxiliary cylinder is likely to produce a peculiar waterhammer effect and is accompanied by a large increase in friction, that

a For a description of Linde's machine, see E. Mathias: La préparation industrielle et les principales applications des gaz liquéfiés." Revue générale des Sciences, vol. 12, 1901.

Readers desirous of obtaining the details of Claude's processes will find an account of them in the following: G. Claude: Air liquide, Oxygène, Azote. Paris, H. Dunod and E. Pinat, 1909.