Heroes of Science: Chemists

by M. M. Pattison Muir

These examples, while illustrating the connection that exists between the composition and the properties of bodies, also illustrate the need there is for giving a scientific chemical training to the man who is to devote his life to chemical manufactures. Pure and applied science are closely connected; he who would succeed well in the latter must have a competent and a practical knowledge of the former.

That composition—molecular composition—and properties are closely related is generally assumed, almost as an axiom, in chemical researches nowadays.

Lavoisier defined acids as substances containing oxygen; Davy regarded an acid as a compound the properties of which were conditioned by the nature and by the arrangement of all the elements which it contained; Liebig spoke of acids as substances containing "replaceable" hydrogen; the student of the chemistry of the carbon compounds now recognizes in an organic acid a compound containing hydrogen, but also carbon and oxygen, and he thinks that the atoms of hydrogen (or some of these atoms) in the molecule of such a compound are, in some way, closely related to atoms of oxygen and less closely to atoms of carbon, within that molecule,—in other words, the chemist now recognizes that, for carbon compounds at any rate, acids are acid not only because they contain hydrogen, but also because that hydrogen is related in a definite manner within the molecule to other elementary atoms; he recognizes that the acid or non-acid properties of a compound are conditioned, not only by the nature of the elements which together form that compound, but also by the arrangement of these elements. Davy's view of the nature of acids is thus confirmed and at the same time rendered more definite by the results of recent researches.

The physical student is content to go no further than the molecule; the properties of bodies which he studies are regarded, for the most part, as depending on the size, the nature, and perhaps the grouping together of molecules. But the chemist seeks to go deeper than this. The molecule is too large a piece of matter for him; the properties which he studies are conceived by him to be principally conditioned by the nature, the number, and the arrangement of the parts of the molecule—of the atoms which together build up the molecule.

In these elementary atoms he has, for the present, found the materials of which the heavens and the earth are made; but facts are being slowly gained which render it probable that these atoms are themselves structures—that they are built up of yet smaller parts, of yet simpler kinds of matter. To gather evidence for or against this supposition, the chemist has been obliged to go from the earth to the heavens, he has been obliged to form a new science, the science of spectroscopic analysis.

This subject has been considered in "The Astronomers," belonging to this series of books; but the point of view from which the matter is there regarded is astronomical rather than chemical. I should like briefly to recall to the reader the fundamental facts of this branch of science.

Fig. 4.

When a ray of light is allowed to pass through a glass prism and then fall on to a white surface, the image produced on this surface consists of a many-coloured band of light. The blue or violet part of this band is more bent away from the plane of the entering ray than the orange part, and the latter more than the red part of the band. This is roughly represented in Fig. 4, where r is the ray of light passing through the prism P, and emerging as a sevenfold band of coloured lights, of which the violet, V, is most, and the red band, R, is least bent away from the plane of the ray r. If the surface—say a white screen—on which the many-coloured band of light, or spectrum, falls, is punctured by a small hole, so as to admit the passage of the violet, or blue, or orange, or red light only, and if this violet, etc., light is then passed through a second prism, no further breaking up of that light takes place. This state of matters is represented in the part of the figure towards the right hand, where the red ray, R, is shown as passing through the screen, and falling on to a second prism, P': the red ray is slightly bent out of its direct course, but is not subdivided; it falls on the second screen as a ray of red light, R'. But if a quantity of the metal sodium is vaporized in a hot non-luminous flame, and if the yellow light thus produced is passed through a prism, a spectrum is obtained consisting of a single yellow line (on a dark background), situated on that part of the screen where the orange-yellow band occurred when the ray of sunlight was split up by the action of the prism. In Fig. 5 the yellow light from a flame containing sodium is represented by the line Y. The light emitted by the glowing sodium vapour is said to be monochromatic.