THE NEW CHEMISTRY.

IV.-CARBON AND THE SHAPES OF ATOMS.

WHEN John Dalton visited London, Oxford, and Bristol a century ago, to expound his new theory of atoms to a series of educated but non-chemical audiences, he took a step which his successors to-day usually endeavour to avoid under similar circumstances; that is, he carried with him, for the use of those who attended his lectures, a number of printed sheets on which his ideas were expressed graphically by means of a system of chemical symbols. Dalton's idea, which proved sound, was that the symbols which had helped him to see his own notions clearly and definitely would help equally any other educated man or woman who might attempt to follow those notions. With this fact to encourage me I have ventured here and there in this article to repeat the successful experiment of the founder of the atomic theory.

I do not think I need apologise for this, as I feel sure the few and comparatively simple formulæ in these pages will help every one who may read what I have written; but I have commenced my story with these few words of explanation in the hope that they may prevent any one to whom the subject may seem attractive from putting the article aside, after a superficial examination, with the idea that it is more difficult or more technical in its nature than the other members of the series of which it forms a part; for I feel sure that the exact contrary is the case.

In a single thimbleful of the 'marsh gas' which occurs in every specimen of the 'fire-damp' of coal mines there are about sixty million million million distinct particles or molecules. In each of these molecules of marsh gas there is one atom of carbon. What is the shape of this atom of carbon? It is my aim in this essay to show how far the chemists have got in the direction of winning an answer to this question, and, still more, to show how they have attacked the problem.

At first sight, perhaps, the question asked in the previous paragraph may not appear so very difficult, but when we remember the minute dimensions of the chemical atoms, and reflect

that they are far beyond the reach of the most powerful microscopes, so that atoms are not to be isolated and looked at one by one, like crystals, it seems at once not simple but almost fantastic and impossible. Nevertheless in a limited sense it has been solved; and the key which has unlocked, or partly unlocked, the chamber in which the secret has long been hidden is called by chemists 'isomerism.' This key has been forged by many hands. Its construction may perhaps be said to have been begun by Liebig in the laboratory of Gay-Lussac, in a cellar, I believe, in the city of Paris, in the year 1823. And for the better information of those who think the members of the younger generation are always idle when they seem to do nothing, I may mention that Liebig appears to have started on the quest which resulted in the discovery of isomerism when dawdling as a boy in the marketplace of Darmstadt, watching a cheap-jack make the explosive constituent of fire-crackers by adding brandy to a solution of quicksilver in strong nitric acid.

The element carbon, so familiar to us in the form of coal, soot, charcoal, and diamond, which predominates in almost every part of the tissues of all plants and animals, stands out among the eighty elements of the chemist on account of the astonishing number and variety of its compounds. No less than sixty thousand compounds of this single element have already been isolated and studied by the chemists; and, although it is probable that the chemistry of carbon is still in its infancy, the compounds of this element already bulk so large in the book of chemistry that this volume would shrivel, perhaps, to less than half its present dimensions if the record of carbon and its derivatives were removed from its pages. It will help, I think, to give an idea of the fecundity of this unique element if I mention, before we proceed, that it has been calculated that no less than eight hundred and two substances (called 'tridecanes ') may possibly exist, each of which would have distinct properties, and yet be indistinguishable from the rest by chemical analysis, since every one of them contains the same proportions of carbon and hydrogen, and corresponds to the same chemical formula. When I add that very few of these eight hundred substances find a place among the sixty thousand carbon compounds known to chemists, that nearly everything necessary for maintaining life, and many luxuries; nearly all we eat and, with the exception of water, nearly all we drink; everything we wear; every flower of the field; every perfume, nearly every dye, and a thousand things besides largely

consist of carbon compounds, all will agree that even the wonders of radium pale before the possibilities that lie hidden in a handful of soot or in a lump of common charcoal, and will understand why it is that during the last eighty or ninety years scores and hundreds of men have devoted their lives to the study of this single element and its transformations.

The idea conveyed to the chemist by the word 'isomerism the idea, that is, that two or more distinct compounds may be built up by uniting the same elements in exactly the same proportions came into chemistry somewhat haltingly.

It has, of course, been known from the days of Lavoisier and Smithson-Tennant that diamond, blacklead, and charcoal, if pure, though so different from one another in appearance and in many of their properties, all are elementary in their nature; and, what is more, all consist of the same element, viz. carbon. But the notion that similar phenomena might occur among compounds was not at once grasped even after an example of isomerism had been discovered among the compounds of carbon and hydrogen by Dalton and his friend Dr. Henry. This curious fact may be ascribed to the circumstance that those who accepted the atomic theory in its early days started with the idea that a given set of atoms, when combined, would be sure to produce one compound and no more, or, to put the same idea in another form, that the properties of a compound must depend solely on the nature and number of the atoms contained in its molecules.

It was not until after the year 1825 that wider and truer views began to assert themselves.

If you dissolve a bit of silver-say, a sixpenny piece-in a dessertspoonful of strong nitric acid, applying a gentle heat, add to the solution while still hot rather more than twice its weight of spirit of wine, continue to heat the mixture for a short while, and then allow it to cool, small brilliant, colourless crystals in thin plates will presently separate from the solution. These crystals, which can only be preserved safely in extremely small quantities or under water, consist of the silver salt of fulminic acid,' and are so unstable that when heated, rubbed in a mortar, or touched with oil of vitriol they explode with the utmost violence, giving off great volumes of gas and leaving a residue of silver.

About the year 1824, when working with Gay-Lussac, Liebig

The mercury salt of this acid is the explosive material used for charging percussion caps.

succeeded in analysing this dangerous salt, and, owing to the ideas which then prevailed among chemists, he was not a little surprised to find that it consisted of the same elements, united in the same proportions, as the silver salt of another acid, then known as cyanic acid, which was analysed, about the same time, by his fellowcountryman Wöhler. Now the substance analysed by Wöhler possessed no explosive qualities whatever. It was as unlike a fulminate as anything well could be. Hence Liebig's analysis created not a little excitement. How could two substances so different from each other consist of the very same elements united in the same proportions? The idea seemed absurd. It almost savoured of alchemy, which was much more out of fashion in those days than it is just now. But repeated analyses only confirmed Liebig's discovery; presently his greatest opponent, Berzelius, himself discovered another case of a similar kind, and by the year 1830 it was agreed, at the suggestion of Gay-Lussac, who from the first had supported his younger colleague, that the properties of chemical compounds do not depend solely on the nature and number of the atoms which build up their molecules, but also on the manner in which the atoms are arranged within the molecules. About a year later Liebig completed the working out of his beautiful method of organic analysis (in the course of which process he discovered how to make bicycle tyres-that is, tubes of indiarubber), and thus the new organic chemistry was started on its triumphant career. I have sometimes wondered if, by any flash of inspiration, Liebig ever in any degree foresaw the results that were to flow from these two researches the new ideas; the vast additions to knowledge, amounting, in effect, to the creation of a new branch of science; the historic industries about to be destroyed; and the new industries about to be created, to the great and special gain of his own fatherland ?

When Berzelius introduced the term 'isomerism' into chemistry the known cases of this phenomenon were still but few. Dalton and Henry had discovered two isomeric substances among the compounds of carbon and hydrogen. Liebig had demonstrated the identical composition of silver fulminate and silver cyanate. Wöhler had effected a revolution by changing ammonium cyanate into urea by the action of heat; Berzelius had discovered

Before this achievement it was commonly supposed that compounds like urea were produced under the influence of a special' vital force,' and that no such compound could be prepared from its elements in the laboratory.

the existence in grape juice of racemic acid, an isomeride of tartaric acid, and that was all. To-day cases of isomerism have been discovered by hundreds, and the whole subject is reduced to such order that the skilled organic chemist can predict the existence of undiscovered compounds, foretell their properties, and suggest their probable modes of formation with a degree of accuracy almost equal to that of Mendeléeff when he predicted the existence and foretold the properties of gallium and germanium by means of the famous 'periodic law.'

From the earliest days since the introduction of the atomic theory into their science chemists have used a conventional system of symbols to assist them to picture chemical changes and the constitutions of chemical compounds; and, as I have already said, there is no doubt that this 'chemical algebra' did much to bring Dalton's ideas into favour by making them more easy to understand than they would have been without such assistance. Dalton's symbols, which represented the atoms by circles bearing various devices to distinguish them one from another, have, however, been a good deal altered since their inventor passed from the stage, and they now take the form of letters of the alphabet arranged according to certain simple conventions. No doubt they seem a little alarming at first sight to a beginner, but they are not really difficult to follow, and a formula such as

which represents one of the constituents of the fusel oil of pot-still whiskey, and rejoices in the not very attractive name of methyl-ethylcarbin-carbinol, soon becomes quite a lively and suggestive affair to the student of chemistry. In this and all similar formulæ each letter represents an atom, of fixed weight, of an element; C standing for an atom of carbon, H for an atom of hydrogen, and O for an atom of oxygen. Numerals, like the figure 3 in the group CH,, indicate the number of atoms of the given element in that group. Thus CH, means that here we have one atom of carbon united with three atoms of hydrogen.

I should add that it is found desirable to treat certain groups of atoms, such as the group CH,, the group CH,HO, and, again, the group OH, as if they were indivisible, like atoms, since these groups can, in fact, be transferred unbroken from one molecule to another in chemical changes almost as readily as atoms them