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MR C. T. R. WILSON'S PHOTOGRAPHS OF THE CLOUD-
TRACKS OF a PARTICLES.

FIG. 2.

THE SAME ENLARGED.

[graphic]

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

small current which leaks through the gas in the tube by the direct action of the voltage applied is allowed to run to earth through the high resistance of a layer of gas made slightly conductive by radio-active material. While the stopcock F is closed, the needle is steady. But when F is opened, and a particles can reach the gas in the tube, the needle shows a series of sudden swings, which follow each other at irregular intervals. By varying the position of the iron tube containing the radio-active film, the number of swings can be adjusted to the three or four per minute convenient for observation; each swing has then time to die away, and the needle is able to return to its position of equilibrium before it is again thrown off its balance by the arrival of another particle and the consequent fresh access of conductivity in the gas.

Now each of these swings of the needle means that one a particle has entered the detecting tube and produced its train of primary and secondary ions by which the electric current is ferried across space. And, as we know, each a particle is an atom of helium electrically charged. The effect of a single atom is thus demonstrated by means of the change it produces in the electrical conductivity—a change registered and made visible by the sudden swing of the electrometer. Moreover, by comparing the number of a particles determined by this method with the number of scintillations visible on a screen of zinc sulphide, values were obtained which made it clear that each scintillation was in truth the effect of a single a projectile and represented the splash on the phosphorescent target produced by the impact of the bullet. Hence it followed that, in watching the scintillations, we were also watching the effect of single atoms of helium.

During the past year or two Mr C. T. R. Wilson has developed a method, introduced by himself some time ago, which provides us with another means of following the track of individual a particles. When a volume of air is saturated with water vapour, a sudden expansion of the containing vessel, which causes a cooling of the air, is accompanied by a precipitation of moisture in the form of mist or cloud. If the air be freed from dust particles by filtering it through cotton wool, this precipitation

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