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

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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.

ENCE PUBLI

LIBRARY

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Art 7.-THE EARLY HISTORY OF TOBACCO.

1. Segunda parte del libro de las cosas que se traen en nuestras Indias Occidentales, que siruen al vso de medicina. Do se trata del Tabaco, y de la Sassafras, y de Carlo Sancto, etc. By Nicholas Monardes. Seville, 1571. 2. Instruction sur l'herbe Petun ditte en France l'Herbe de la Royne, ou Medicée. By I. G. P. [i.e. Jacques Gohorry, Parisien]. Paris, 1572.

3. Tabacologia; hoc est Tabaci, seu Nicotianae descriptio Medico-Cheirurgico-Pharmaceutica: vel ejus praeparatio et usus in omnibus fermè corporis humani incommodis. By Johannes Neander of Bremen. Leyden : Isaac Elzevir, 1626.

4. Geschichte des Tabaks und anderer ähnlicher Genussmittel. By Friedrich Tiedemann. Frankfurt a/M, 1854. 5. Bibliotheca Nicotiana. By William Bragge. First edition, 1874; second edition, 1880. (Both privately printed.)

TOBACCO first found its way into Europe rather as a medicament than as the solace and companion of fallen male nature. Both in its native continent, and even more in those western European countries which early adopted it, almost miraculous healing qualities were at one time attributed to the herb. The gradual displacement of tobacco as a drug and its acceptance as one of the amenities of civilised life will become apparent as our story unfolds itself.

There can be no doubt that the knowledge of tobacco reached the old world from America, and that the first acquaintance of Europeans with the herb is contemporary, almost to a day, with the discovery of the western continent. Columbus appears to have seen the plant a few hours after he set foot on American soil. This information has reached us in comparatively modern times through the publication of the manuscripts of Bartholoméo de Las Casas, the apostle of the Indies (1474-1566), who deserves remembrance for his tireless denunciation of the cruelty of his people towards the American natives. Las Casas possessed a number of the papers of Columbus, and among them was the explorer's original holograph journal, of which he prepared an

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