of morality itself becomes, therefore, the guarantee of their existence. Practise thus gives what speculation fails to give. So after all the theologian could take courage from the Kantian philosophy, and the pragmatism of to-day could find a basis in the searching criticism of the Königsberg professor. The significance of Kant for modern philosophy has thus been wide and varied. He has been at once an inspiration and a check to free speculation, and also a source of renewed progress in moral and religious inquiry. Yet, it must be admitted that his importance has waned considerably in recent years. His central idea that there are necessities of thought and practise which of themselves significantly determine the content of our knowledge and belief has come to lack its authoritative tone. This has been brought about not so much by direct refutation as by the steady advance in stability of scientific knowledge, which insists that we can be really compelled only by the exigencies of the things with which we deal. Kant in his early years was no mean scientist. Indeed he thought that his philosophy could give to science its only stable basis and its only correct interpretation. The result is in striking contrast with his conviction.
RECENT PROGRESS IN THE STUDY OF RADIOACTIVITY.
The dream of the alchemists had without doubt a strong philosophical foundation, and although the desire to accomplish transmutations of the elements has lost all power as an incentive to the study of natural phenomena, one can not help noticing the small amount of reverence modern physics has for the identity of the atom of a chemical element. The electronic theory of matter, well set forth by Sir Oliver Lodge in his Romanes lecture at Oxford, which was published in this magazine last August, holds that there is no more difference between the atoms of the different elements than between houses of different shapes and sizes, but built of the same kind of bricks, the little electrons being the bricks of which the atoms are built, although the structure of an atom is more like that of a planetary system than that of a house. Confidence in the stability of this structure in the case of ordinary atoms has not been shown to be misplaced, but in the case of the radioactive substances—elements they are by the usual tests—evidence of atomic disintegration continues to accumulate. Their radiations consist chiefly of projected particles, far smaller than the atoms of the radioactive elements, and, as Professor Rutherford and Mr. Soddy have shown, the radioactive matter passes successively through a series of unstable forms. The final product of this atomic disintegration must be stable and therefore not radioactive, and since the gas helium is found in all radioactive minerals it is suggested that helium is one of the stable residues left by the heavy and unstable radioactive atoms.
During the past summer Professor Ramsay, the discoverer of terrestrial helium, and Mr. Soddy followed up this suggestion with experiments and came to the conclusion that helium is continuously produced by radium. The experiments consisted in examining in a spark tube the spectrum of the radioactive gas, or emanation, given on dissolving in water fifty milligrams of nearly pure radium bromide that had been in the solid state for some time. This radioactive gas is not stable, but decays in a geometrical progression with the time, the rate being about half in four days. Of course the most careful precautions were taken to free the spark tube from foreign gases, especially hydrogen, oxygen and carbon dioxide. When first prepared the tube gave a new and hitherto unknown spectrum, probably that of the radioactive gas. After four days the lines of the helium spectrum began to appear, growing brighter for several days, while the new spectrum observed at
