On p. 162 I referred to the work of the German chemist Richter, by which the equivalents of certain acids and bases were established. Those quantities of various acids which severally neutralized one and the same quantity of a given base, or those quantities of various bases which severally neutralized one and the same quantity of a given acid, were said to be equivalent. These were the quantities capable of performing a certain definite action.

In considering the development of Dumas's substitution theory, we found that Laurent retained this conception of equivalency when he spoke of an equivalent of hydrogen being replaced by an equivalent of chlorine (see p. 272). A certain weight of chlorine was able to take the place and play the part of a certain weight of hydrogen in a compound; these weights, of hydrogen and chlorine, were therefore equivalent.

This conception has been much used since Laurent's time, but it has for the most part been applied to the atoms of the elements.

Hydrogen being taken as the standard substance, the elements have been divided into groups, in accordance with the number of hydrogen atoms with which one atom of each element is found to combine. Thus certain elements combine with hydrogen only in the proportion of one atom with one atom; others combine in the proportion of one atom with two atoms of hydrogen; others in the proportion of one atom with three atoms of hydrogen, and so on.

The adjective monovalent, divalent, trivalent, etc., is prefixed to an element to denote that the atom of this element combines with one, or two, or three, etc., atoms of hydrogen to form a compound molecule.

Let us consider what is implied in this statement—"The nitrogen atom is trivalent." This statement, if amplified, would run thus: "One atom of nitrogen combines with three atoms of hydrogen to form a compound molecule." Now, this implies (1) that the atomic weight of nitrogen is known, and (2) that the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are also known.

But before the atomic weight of an element can be determined, it is necessary (as we found on p. 146) to obtain, analyze, and take the specific gravities of a series of gaseous compounds of that element. The smallest amount of the element (referred to hydrogen as unity) in the molecule of any one of these gases will then be the atomic weight of the element.

When it is said that "the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are known," the statement implies that the compound in question has been obtained in a pure state, has been analyzed carefully, has been gasefied, and that a known volume of the gas has been weighed. When therefore we say that "the nitrogen atom is trivalent," we sum up a large amount of knowledge which has been gained by laborious experiment.

This classification of the elements into groups of equivalent atoms—which we owe to Frankland, Williamson, Odling, and especially to KekulÉ—has been of much service especially in advancing the systematic study of the compounds of carbon. It helps to render more precise the conception which has so long been gaining ground of the molecule as a definite structure.

A monovalent element is regarded as one the atom of which acts on and is acted on by only one atom of hydrogen in a molecule; a divalent as one, the atom of which acts on and is acted on by two atoms of hydrogen—or other monovalent element—in a molecule; a trivalent element as one, the atom of which acts on and is acted on by three atoms of hydrogen—or other monovalent element—in a molecule; and so on.

The fact that there often exist several compounds of carbon, the molecules of which are composed of the same numbers of the same atoms, finds a partial explanation by the aid of this conception of the elementary atom as a little particle of matter capable of binding to itself a certain limited number of other atoms to form a compound molecule. For if the observed properties of a compound are associated with a certain definite arrangement of the elementary atoms within the molecules of that compound, it would seem that any alteration in this arrangement ought to be accompanied by an alteration in the properties of the compound; in other words, the existence of more than one compound of the same elements united in the same proportions becomes possible and probable.

I have said that such compounds exist: let me give a few examples.

The alchemists poured a stream of mercury on to molten sulphur, and obtained a black substance, which was changed by heat into a brilliantly red-coloured body. We now know that the black and the red compounds alike contain only mercury and sulphur, and contain these elements united in the same proportions.

Hydrogen, carbon, nitrogen and oxygen unite in certain proportions to produce a mobile, colourless, strongly acid liquid, which acts violently on the skin, causing blisters and producing great pain: if this liquid is allowed to stand for a little time in the air it becomes turbid, begins to boil, gets thicker, and at last explodes, throwing a white pasty substance about in all directions. This white solid is inodorous, is scarcely acid to the taste, and does not affect the skin; yet it contains the same elements, united in the same proportions, as were present in the strongly acid, limpid liquid from which it was produced.

Two substances are known each containing carbon and hydrogen united in the same proportions: one is a gas with strong and irritating odour, and exerting a most disagreeable action on the eyes; the other is a clear, limpid, pleasant-smelling liquid.

Phosphorus is a very poisonous substance: it readily takes fire in the air at ordinary temperatures, so that it must be kept under water; but a modification of phosphorus is known, containing no form of matter other than phosphorus, which is non-poisonous, does not take fire easily, and may be handled with safety.

Once more, there is a compound of nitrogen and oxygen which presents the appearance of a deep-red, almost black gas; there is also a compound of nitrogen and oxygen which is a clear, colourless gas; yet both contain the same elements united in the same proportions.

But a detailed consideration of isomerism, i.e. the existence of more than one compound built up of the same amounts of the same elements yet possessing different properties, would lead us too far from the main path of chemical advance which we wish to trace.

The chemist is to-day continually seeking to connect the properties of the bodies he studies with the molecular structures of these bodies; the former he can observe, a knowledge of the latter he must gain by reasoning on the results of operations and experiments. His guide—the guide of Lavoisier and his successors—is this: "Similarity of properties is associated with similarity of composition"—by "composition" he generally means molecular composition.

Many facts have been amassed of late years which illustrate the general statement that the properties of bodies are connected with the composition of those bodies. Thus a distinct connection has been traced between the tinctorial power and the molecular composition of certain dye-stuffs; in some cases it has even become possible to predict how a good dye-stuff may be made—to say that, inasmuch as this or that chemical reaction will probably give rise to the production of this or that compound, the atoms in the molecule of which we believe to have a certain arrangement relatively to one another, so this reaction or that will probably produce a dye possessed of strong tinctorial powers.

The compound to the presence of which madder chiefly owes its dyeing powers is called alizarine; to determine the nature of the molecular structure of this compound was, for many years, the object of the researches of chemists; at last, thanks especially to the painstaking zeal of two German chemists, it became fairly clear that alizarine and a compound of carbon and hydrogen, called anthracene, were closely related in structure. Anthracene was obtained from alizarine, and, after much labour, alizarine was prepared from anthracene. Anthracene is contained in large quantities in the thick pitch which remains when coal-tar is distilled; this pitch was formerly of little or no value, but as soon as the chemical manufacturer found that in this black objectionable mass there lay hidden enormous stores of alizarine, he no longer threw away his coal-tar pitch, but sold it to the alizarine manufacturer for a large sum. Thus it has come to pass that little or no madder is now cultivated; madder-dyeing is now done by means of alizarine made from coal-tar: large tracts of ground, formerly used for growing the madder plant, are thus set free for the growth of wheat and other cereals.

This discovery of a method for preparing alizarine artificially stimulated chemists to make researches into the chemical composition, and if possible to get to know something about the molecular structure of indigo. Those researches have very recently resulted in the knowledge of a series of reactions whereby this highly valuable and costly dye-stuff may be prepared from certain carbon compounds which, like anthracene, are found in coal-tar.

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.

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.

Lastly, if the experiment is arranged so that a ray of sunlight or of light from an electric lamp passes through a layer of comparatively cool sodium vapour before reaching the prism, a spectrum is produced corresponding to the solar spectrum except that a black line appears in the position where the yellow line, characteristic of sodium, was noticed in the second experiment.

Fig. 6 represents the result of this experiment: the ray of sunlight or electric light, r, passes through a quantity of sodium vapour, and is then decomposed by the prism; the spectrum produced is marked by the absence of light (or by a dark line) where the yellow line, Y, was before noticed.

These are the fundamental facts of spectroscopic analysis: sunlight is decomposable into a band of many colours, that is, into a spectrum; light emitted by a glowing vapour is characterized by the presence of coloured lines, each of which occupies a definite position with reference to the various parts of the solar spectrum; sunlight—or the electric light—when allowed to pass through a mass of vapour, furnishes a spectrum characterized by the absence of those bright lines, the presence of which marked the spectrum of the light obtained by strongly heating the vapour through which the sunlight has passed.

The spectrum obtained by decomposing the light emitted by glowing vapour of potassium is characterized by the presence of certain lines—call them A and B lines. We are asked what element (or elements) is present in a certain gas presented to us: we pass a beam of white light through this gas and then through a prism, and we obtain a continuous spectrum (i.e. a spectrum of many colours like the solar spectrum) with two dark lines in the same positions as those occupied by the lines A and B. We therefore conclude that the gas in question contains vapour of potassium.

The solar spectrum, when carefully examined, is found to be crossed by a very large number of fine black lines; the exact positions of many hundreds of these lines have been carefully determined, and, in most cases, they are found to correspond to the positions of various bright lines noticed in the spectra of the lights emitted by hot vapours of various elementary bodies.

Assume that the sun consists, broadly speaking, of an intensely hot and luminous central mass, formed to a large extent of the elementary substances which build up this earth, and that this central mass is surrounded by a cooler (but yet very hot) gaseous envelope of the same elements,—and we have a tolerably satisfactory explanation of the principal phenomena revealed by the spectroscopic study of the sun's light.

On this assumption the central mass of glowing iron, chromium, magnesium, nickel, cobalt, hydrogen, etc., is sending out light; a portion of the light emitted by the glowing iron is quenched as it passes through a cloud of cooler iron vapour outside the central mass, a portion of the light emitted by the glowing chromium is quenched as it passes through a cloud of cooler chromium vapour, and so on; the black lines in the spectrum are the records of these various quenchings of this and that light.

So far then the study of the solar spectrum appears to be tolerably simple, and this study generally confirms the proposition that the material of which the sun is composed is, broadly, identical with those forms of matter which we, on this earth, call the chemical elements.

But whatever be the composition of the sun, it is, I think, evident that in dealing with a ray of light coming therefrom, we are dealing with a very complex phenomenon.

According to the hypothesis which is now guiding us, the solar light which passes into our spectroscope has probably had its beginning in some central part of the sun, and has passed through very thick layers of hot metallic clouds, agitated perhaps by solar cyclones. Could we examine the light coming from some defined part of the sun, we should probably obtain valuable information. During a solar eclipse red prominences are seen projecting beyond the dark shadow of the moon, which covers the sun's disc. Analysis of the light emitted by these prominences has shown that they are phenomena essentially belonging to the sun itself, and that they consist of vast masses of intensely hot, glowing gaseous substances, among which hydrogen is present in large quantities. That these prominences are very hot, hotter than the average temperature of the ordinary solar atmosphere, is proved by the fact that the spectrum of the light coming from them is characterized by bright lines. By special arrangements which need not be discussed here, but which have been partly explained in "The Astronomers" (see pp. 334, 335 of that book), it has been shown that these prominences are in rapid motion: at one moment they shoot up to heights of many thousand miles, at another they recede towards the centre of the sun.

We thus arrive at a picture of the solar atmosphere as consisting of layers of very hot gases, which are continually changing their relative positions and forms; sometimes ejections of intensely hot, glowing gases occur,—we call these prominences; sometimes down-rushes of gaseous matter occur,—we call these spots. Among the substances which compose the gaseous layers we recognize hydrogen, iron, magnesium, sodium, nickel, chromium, etc., but we also find substances which can at present be distinguished only by means of the wave-lengths of the light which they emit; thus we have 1474 stuff, 5017 stuff, 5369 stuff, etc.

Let us now turn to another part of this subject. By a special arrangement of apparatus it is possible to observe the spectrum of the light emitted by a glowing vapour, parts of which are hotter than other parts, and to compare the lines in the spectrum of the light coming from the hottest parts with the lines in the spectrum of the light coming from the cooler parts of the vapour. If this is done for sodium vapour, certain lines are apparent in all the spectra, others only in the spectrum of the light coming from the hottest parts of the sodium vapour: the former lines are called "long lines," the latter "short lines." A rough representation of the long and short lines of sodium is given in Fig. 7.

Now, suppose that the lines in the spectrum of the light emitted by glowing manganese vapour have been carefully mapped, and classed as long and short lines: suppose that the same thing has been done for the iron lines: now let a little manganese be mixed with much iron, let the mixture be vaporized, and let the light which is emitted be decomposed by the prism of a spectroscope, it will be found that the long lines of manganese alone make their appearance; let a little more manganese be added to the mixture, and now some of the shorter lines due to manganese begin to appear in the spectrum. Hence it has been concluded by Lockyer that if the spectrum of the light emitted by the glowing vapour of any element—call it A—is free from the long lines of any other element—say element B—this second element is not present as an impurity in the specimen of element A which is being examined. Lockyer has applied this conclusion to "purify" various elementary spectra.

The spectrum of element A is carefully mapped, and the lines are divided into long and short lines, according as they are noticed in the spectrum of the light coming from all parts of the glowing vapour of A, or only in the spectrum of the light which comes from the hotter parts of that vapour. The spectra of elements B and C are similarly mapped and classified: then the three spectra are compared; the longest line in the spectrum of B is noted, if this line is found in the spectrum of A, it is marked with a negative sign—this means that so far as the evidence of this line goes B is present as an impurity in A; the next longest B line is searched for in the spectrum of A—if present it also is marked with a negative sign; a similar process of comparison and elimination is conducted with the spectra of A and C. In this way a "purified" spectrum of the light from A is obtained—a spectrum, that is, from which, according to Lockyer, all lines due to the presence of small quantities of B and C as impurities in A have been eliminated.

Fig. 8 is given in order to make this "purifying" process more clearly understood. But when this process has been completed there remain, in many cases, a few short lines common to two or more elementary spectra: such lines are called by Lockyer basic lines. He supposes that these lines are due to light emitted by forms of matter simpler than our elements; he thinks that at very high temperatures some of the elements are decomposed, and that the bases of these elements are produced and give out light, which light is analyzed by the spectroscope. Such short basic lines are marked in the spectra represented in Fig. 8 with a positive sign.

Now, if the assumption made by Lockyer be admitted, viz. that the short lines, or some of the short lines, which are coincident in the "purified" spectra of various elements, are really due to light emitted by forms of matter into which our so-called elements are decomposed at very high temperatures, it follows that such lines should become more prominent in the spectra of the light emitted by elements the higher the temperature to which these elements are raised. But we know (see p. 308) that the prominences around the sun's disc are hotter than the average temperature of the solar atmosphere; hence the spectrum of the light coming from these prominences ought to be specially rich in "basic" lines: this supposition is confirmed by experiment. Lockyer has also shown that it is the "basic," and not the long lines, which are especially affected in the spectra of light coming from those parts of the solar atmosphere which are subjected to the action of cyclones, i.e. which are at abnormally high temperatures. And finally, a very marked analogy has been established between the changes in the spectrum of the light emitted by a compound substance as the temperature is raised, and the substance is gradually decomposed into its elements, and the spectrum of the light emitted by a so-called elementary substance as the temperature of that substance is increased.

But it may be urged that Lockyer's method of "purifying" a spectrum is not satisfactory; that, although all the longer lines common to two spectra are eliminated, the coincident short lines which remain are due simply to very minute quantities of one element present as an impurity in the larger quantity of the other. Further, it has been shown that several of the so-called "basic" lines are resolved, by spectroscopes of great dispersive power, into groups of two or more lines, which lines are not coincident in different spectra.

And moreover it is possible to give a fairly satisfactory explanation of the phenomena of solar chemistry without the aid of the hypothesis that our elements are decomposed in the sun into simpler forms of matter. Nevertheless this hypothesis has a certain amount of experimental evidence in its favour; it may be a true hypothesis. I do not think we are justified at present either in accepting it as the best guide to further research, or in wholly rejecting it.

The researches to which this hypothesis has given rise have certainly thrown much light on the constitution of the sun and stars, and they have also been instrumental in forcing new views regarding the nature of the elements on the attention of chemists, and so of awakening them out of the slumber into which every class of men is so ready to fall.

The tale told by the rays of light which travel to this earth from the sun and stars has not yet been fully read, but the parts which the chemist has spelt out seem to say that, although the forms of matter of which the earth is made are also those which compose the sun and stars, yet in the sun and stars some of the earthly elements are decomposed, and some of the earthly atoms are split into simpler forms. The tale, I say, told by the rays of light seems to bear this interpretation, but it is written in a language strange to the children of this earth, who can read it as yet but slowly; for the name given to the new science was "Ge-Urania, because its production was of earth and heaven. And it could not taste of death, by reason of its adoption into immortal palaces; but it was to know weakness, and reliance, and the shadow of human imbecility; and it went with a lame gait; but in its going it exceeded all mortal children in grace and swiftness."

There are certain little particles so minute that at least sixty millions of them are required to compose the smallest portion of matter which can be seen by the help of a good microscope. Some of these particles are vibrating around the edge of an orb a million times larger than the earth, but at a distance of about ninety millions of miles away. The student of science is told to search around the edge of the orb till he finds these particles, and having found them, to measure the rates of their vibrations; and as an instrument with which to do this he is given—a glass prism! But he has accomplished the task; he has found the minute particles, and he has measured their vibration-periods.

Chemistry is no longer confined to this earth: the chemist claims the visible universe as his laboratory, and the sunbeams as his servants.

Davy decomposed soda and potash by using the powerful instrument given him by Volta; but the chemist to-day has thrown the element he is seeking to decompose into a crucible, which is a sun or a star, and awaits the result.

The alchemists were right. There is a philosopher's stone; but that stone is itself a compound of labour, perseverance, and genius, and the gold which it produces is the gold of true knowledge, which shall never grow dim or fade away.