Sept. I, 1887] 



NATURE 



421 



•was first pointed out by Kekule in 1857 ; though we must not 

 forget that this great principle was foreshadowed so long ago as 

 1833 from a physical point of view by Faraday in his well- 

 known laws of electrolysis, and that it is to Helmholtz, in his 

 <;elebrated Faraday Lecture, that we owe the complete elucida- 

 tion of the subject ; for, whilst Faraday has shown that the 

 number of the atoms electrolytically deposited is in the inverse 

 ratio of their valencies, Helmholtz has explained this by the fact 

 that the quantity of electricity with which each atom is 

 associated is directly proportional to its valency. 



Amongst the tetrad class of elements, carbon, the distinctive 

 element of organic compounds, finds its place; and the remark- 

 able fict that the number of carbon compounds far exceeds that 

 of all the other elements put together receives its explanation. 

 For these carbon atoms not only jiossess four means of grasping 

 other atoms, but these four-handed carbon atoms have a strong 

 partiality for each other's company, and readily attach t)iem- 

 selves hand in hand to form open chains or closed rings, to 

 which the atoms of other elements join to grasp the unoccupied 

 carbon hand, and thus to yield a dancing company in which all 

 hands are locked together. Such a group, each individual occu- 

 pying a given position with reference to the others, constitutes 

 the organic molecule. When, in such a company, the individual 

 members change hands, a new combination is formed. And as 

 in such an assembly the eye can follow the changing positions 

 of the individual members, so the chemist can recognize in 

 his m.olecule the position of the several atoms, and expliin by 

 this the fiict that each arrangement constitutes a new chemical 

 compound possessing different properties, and account in this 

 way for the decompositions which each differently constituted 

 molecule is found to undergo. 



Chemists are, however, not content with representing the 

 arrangement of the atoms in one plane, as on a sheet of paper, 

 but attempt to express the position of the atoms in space. In 

 this way it is possible to explain certain observed differences in 

 isomeric bodies, which otherwise baffled our efforts. To Van 

 t'Hoff, in the first instance, and more recently to Wislicenus, 

 chemistry is indebted for work in this direction, whijh throws 

 light on hitherto obscure phenomena, and points the way to still 

 further and more important advances. 



It is this knowledge of the mode in which the atoms in the 

 molecule are arranged, this power of determining the nature of 

 this arrangement, which has given to organic chemistry the 

 impetus which has overcome so many experimental obstacles, 

 and given rise to such unlooked-for results. Organic chemistry 

 has now become synthetic. In 1837 we were able to build up 

 but very few and very simple organic compounds from their 

 •elements 4 indeed the views of chemists were much divided as to 

 the possibility of such a thing. Both Gmelin and Berzelius 

 argued that organic compounds, unlike inorganic bodies, cannot 

 be built up from their elements. Organic compounds were 

 generally believed to be special products of the so-called vital 

 force, and it was only intuitive minds like those of Liebig and 

 Wohler who foresaw what was coming, and wrote in 1837 

 strongly against this view, asserting that the artificial production 

 in our laboratories of all organic substances, so far as they do not 

 constitute a living organism, is not only probable but certain. 

 Indeed, they went a step farther, and predicted that sugar, 

 morphia, salicine, will all thus be prepared ; a prophecy which, 

 I need scarcely remind you, has been after fifty years fulfilled, for 

 at the present time we can prepare an artificial sweetening 

 principle, an artificial alkaloid, and salicine. 



In spite of these predictions, and in spite of Wbhler's memor- 

 able discovery in 1828 of the artificial production of urea, which 

 did in reality break down for ever the barrier of essential chemical 

 difference between the products of the inanimate and the animate 

 world, still, even up to a much later date, contrary opinions were 

 held, and the synthesis of urea was looked upon as the exception 

 which proves the rule. So it came to pass that for many years 

 the artificial production of any of the more complicated organic 

 substances was believed to be impossible. Now the belief in a 

 special vital force has disappeared like the ii^nis fatiius, and no 

 longer lures us in the wrong direction. We know now that the 

 same laws regulate the formation of chemical compounds in both 

 animate and inanimate nature, and the chemist only asks for a 

 knowledge of the constitution of any definite chemical compound 

 found in the organic world in order to be able to promise to prepare 

 it artificially. 



But the progress of synthetic organic chemistry, which has of 

 late been so rapid, was made in the early days of the half-century 



only by feeble steps and slow. Seventeen long years elapsed 

 between Wohler's discovery and the next real synthesis. This 

 was accomplished by Kolbe, who in 1845 prepared acetic acid 

 from its elements. But then a splendid harvest of results 

 gathered in by chemists of all nations quickly followed, a harvest 

 so rich and so varied that we are apt to be overpowered by its 

 wealth, and amidst so much that is alluring and striking we 

 may well find it difficult to choose the most appropriate ex- 

 amples for illustrating the power and the extent of modem 

 chemical synthesis. 



Next, as a contrast to our picture, let us for a moment glance 

 back again to the state of things fifty years ago, and then notice 

 the chief steps by which we have arrived at our present position. 

 In 1837 organic chemistry possessed no scientific basis, and 

 therefore no classification of a character worthy of the name. 

 Writing to Berzelius in that year, Wohler describes the con- 

 dition of organic chemistry as one enough to drive a man mad. 

 " It seems to me," says he, "like the tropical forest primaeval, 

 full of the strangest growths, an endless and pathless thicket in 

 which a man may well dread to wander." Still clearances had 

 already been made in this wilderness of facts. Berzelius in 1832 

 welcomed the results of Liebig and Wohler's reearch on 

 benzoic acid as the dawn of a new era ; and such it really was, 

 inasmuch as it introduced a novel and fruitful idea — namely, the 

 possibility of a group of atoms acting like an element by point- 

 ing out the existence of organic radicals. This theory was 

 strengthened and confirmed by Bunsen's classical researches 

 on the cacodyl compounds, in which he showed that a common 

 group of elements which acts exactly as a metal can exist 

 in the free state, and this was followed soon afterwards by 

 isolation of the so-called alcohol radicals by Frankland and 

 Kolbe. It is, however, to Schorlemmer that we owe our know- 

 ledge of the true constitution of these bodies, a matter which 

 proved to be of vital impo tance for the further development of 

 the science. 



Turning our glanc3 in another direction we find that Dumas 

 in 1834 by this law of substitution threw light upon a whole series 

 of singular and unexph-^ined pheno.nena by showing that an ex- 

 change can take place between the constituent atoms in a molecule. 

 Laurent indeed went farther, and assumed that a chlorine atom, 

 for example, took up the position vacated by an atom of hydrogen 

 and played the part of its displaced rival, so that the chemical 

 and physical properties of the substitution-product were thought 

 to remain substantially the same as those of the original body. 

 A singular story is connected with this discovery. At a soiree 

 in the Tuileries in the time of Charles X. the guests were almost 

 suffocated by acrid vapours which wera evidently emitted by 

 the burning wax candles, and the great chemist Dumas was 

 called in to examine into the cause of the annoyance. He found 

 that the wax of which the candles were made had been bleached 

 by chlorine, that a replacement of some of the hydrogen atoms 

 of the wax by chlorine had occurred, and that the suffocating 

 vapours consisted of hydrochloric acid given off during the com- 

 bustion. The wax was as while and as odourless as before, and 

 the fact of the substitution of chlorine for hydrogen could only 

 be recognized when the candles were destroyed by burning. 

 This incident induced Dumas to investigate more closely this 

 class of phenomena, and the results of this investigation are 

 embodied in his law of substitution. So far indeed did the 

 interest of the French school of chemists lead them that some 

 assumed that not only the hydrogen but also the carbon of 

 organic bodies could be replaced by substitution. Against this 

 idea Liebig protested, and in a satirical vein he informs the 

 chemical public, writin { from Paris under the nom de plume of 

 S. Windier, that he has succeeded in substituting not only the 

 hydrogen but the oxygen and carbin in cotton cloth by 

 chlorine, and he adds that the London shops are now selling 

 nightcaps and other articles of apparel made entirely of chlorine, 

 goods which meet with much favour, especially for hospital 

 use ! 



But the debt which chemistry, both inorganic and organic, 

 thus owes to Dumas' law of substitution is serious enough, for it 

 proved to be the germ of Williamson's classical researches on 

 etherification, as well as of those of Wurtz and Hofmann on the 

 compound ammonias, investigations which lie at the base of the 

 structure of modem chemistry. Its influence has been, how- 

 ever, still more far-reaching, inasmuch as upon it depends in 

 great measure the astounding progress made in the wide field 

 of organic synthesis. 



It may here be permitted to me to sketch in rough outline the 



