occurs much less easily, since the dust particles act as nuclei on which condensation can begin, and so facilitate it. The condensation of water vapour without the presence of these minute foreign substances to start the process is much more difficult to obtain. Now, when the air is free from dust, if Röntgen rays. be allowed to pass through it, or a quantity of radio-active material be brought near, it is found that precipitation is once more made easy. The ions formed in the expanding air by the Röntgen rays or radio-active projections act as nuclei on which the condensation of water vapour can take place, just as do the particles of dust; and once more a cloud is made visible. By measuring the rate at which the cloud falls it is possible to estimate the size of the individual drops of water, since it is known that large drops of water, which are less affected by the resistance of the air, fall faster than small ones, which are relatively more retarded by the resistance of the gas around them. Hence a knowledge of the total mass of water precipitated by the expansion enabled Mr Wilson to calculate the number of drops of water that had been formed, and therefore to estimate the number of ionic nuclei required to form the cloud. It was in this way that in 1898 Sir J. J. Thomson first obtained a direct value for the electric charge carried by each gaseous ion. This method has now been extended. Mr Wilson confines the air in a shallow cylindrical glass box with a flat glass top. The bottom of the box is formed of a moveable piston-head sliding in the box. This piston compresses the air in the box when raised, and expands it when lowered; it is thus possible to obtain the necessary rarefication. In one series of experiments a minute quantity of a radium salt was placed on the top of a wire projecting into the cloud chamber. On the sudden expansion of the air, the tracks of a number of a particles were seen radiating from the tip of the wire. These tracks are lines of condensed water-vapour cloud deposited by the super-saturated air on the nuclei supplied by the ions, which have been produced by the impacts of the a ray particles on the molecules of the air. In order to secure sharp photographs of these clouds, a flashlight illumination is necessary. This is obtained by means of an electric spark discharged through mercury vapour. The trigger used to set in action the mechanism for the expansion of the air liberates also a metal ball, which, falling between the terminals of a set of Leyden jars, produces a spark at an adjustable interval after the act of expansion. The photographs of Fig. 2 are specimens of Mr Wilson's results. The first one shows the tracks of the shower of a rays projected from a speck of radium placed on the tip of a wire. Each of these streaks of light is the cloud track of a single atom of helium. The more blurred lines show where the projectile has passed before the expansion of the air-the ions formed as it passes have diffused appreciably before the state of super-saturation has arisen to condense clouds upon them. The sharper lines, on the other hand, show the effect of a previous expansion; the a rays pass through air already super-saturated, and cloud drops form at once before the ions have time to pass away to a distance. In the second photograph the cloud tracks are seen on a larger scale. Here and there, especially towards the ends of their paths, a sharp change in direction is seen. This represents the collision with a molecule of the gas, which, as the velocity of the a particle becomes less towards the end of its flight, results in a sudden bend in its path. Thus not only the track of an atom, but its collision with the molecule of a gas, has been made visible to the human eye. The number of ions produced by B rays is much less than those produced by a rays. But, by using oblique illumination, Mr Wilson has succeeded in photographing the cloud particles formed round single ions in the track of B rays. Their course appears as a faint dotted line, like a minute, barely visible, necklace of pearls. Not merely the track of an atom but that of an infra-atomic corpuscle-a particle much smaller than the smallest chemical atom-has thus been revealed. The brief historical account of the atomic theory given in the foregoing pages makes it clear that the interest of these recent researches is not confined to the immediate object with which they were undertaken. The atomic theory never has been and never can be an affair of science only. Atoms and molecules were purely hypo thetical concepts, convenient, perhaps necessary, to bring order into our scheme of physical and chemical science, but with no direct evidence for the validity of the idea of the individual atom. But now the individual atom in its direct effects has become a matter of sense perception. It is as real as the wind in the upper air which we infer from the scudding clouds, as real as the stars which we perceive in the depths of space by means of the light that reaches our eyes. Science, it is true, has nothing to say in its own language about the ultimate realities, if such there be, that lie behind our concepts of wind or star or atom. Science, in its own field, deals merely with sense perceptions and the mental concepts and schemes we form to reduce them to order. But the consistency of the parts of the vision of science, and its marvellous continued concordance with the world of sense perceptions, are valid metaphysical arguments in favour of the existence of some sort of reality underlying the conceptual scheme a reality which in some unknown way corresponds to the model of it which, by the help of experimental science, our minds construct. Such deep questions cannot be judged or answered by the methods of science. But science, though it cannot sit as judge, may claim to be the first and chief witness to be examined when the problem of Reality is called up for judgment in the Courts of Philosophy. Not the least part of its evidence will be the demonstration to our senses of the direct effect of the individual atom. W. C. D. WHETHAM. |