42: 



NA TURE 



[August 28, 1884 



been appointed to the Chair of Practical Chemistry in the same 

 College, vacant by the death of poor Fownes. At the same time, 

 Hofmann, in whom Liebig found a spirit as enthusiastic in 

 the cause of scientific progress as his own, bringing to England 

 a good share of the Giessen fire, founded the most successful 

 school of chemistry which this country has yet seen. 



At the Edinburgh meeting of this Association in 1S50, 

 Williamson read a paper on " Results of a Research on /Etherifi- 

 cation," which included not only a satisfactory solution of an 

 interesting' and hitherto unexplained problem, but was destined 

 to exert a most important influence on the development of our 

 theoretical views. For he proved, contrary to the tben prevail- 

 ing ideas, that ether contains twice as much carbon as alcohol, 

 and that it is not formed from the latter by a mere separation of 

 the elements of water, but by an exchange of hydrogen for ethyl, 

 and this fact, being in accordance with A vogadro's law of molecular 

 volumes, could only be represented by regarding the molecule 

 of water as containing two atoms of hydrogen to one of oxygen, 

 one of the former being replaced by one of ethyl to form alcohol, 

 and the two of hydrogen by two of ethyl to form ether. Then 

 Williamson introduced the type of water (subsequently adopted 

 by Gerhardt) into organic chemistry, and extended our views of 

 the analogies between alcohols and acids, by pointing out that 

 these latter are also referable to the water-type, predicting that 

 bodies bearing the same relations to the ordinary acids as the 

 ethers do to the alcohols must exist, a prediction shortly after- 

 wards (1852) verified by Gerhardt's discovery of the anhydrides. 

 Other results followed in rapid succession, all tending to knit to- 

 gether the framework of modern theoretical chemistry. Of these 

 the most important was the adoption of condensed types, of 

 compounds constructed on the type of two and three molecules 

 of water, with which the names of Williamson and Odling are 

 connected, culminating in the researches of Brodie on the higher 

 alcohols, of Berthelot on glycerine, and of Wurtz on the dibasic 

 alcohols or glycols ; whilst, in another direction, the researches 

 of Hofmann on the compound amines and amides opened out an 

 entirely new field, shoving that either a part or the whole of the 

 hydrogen in ammonia can be replaced by other elements or 

 elementary groups without the type losing its characteristic 

 properties. 



Again, in 1852, we note the first germs of a theory which was 

 destined to play an all-important part in the progress of the 

 science, viz., the doctrine of valency or atomicity, and to 

 Frankland it is that we owe this new departure. Singularly 

 enough, whilst considering the symmetry of construction visible 

 amongst the inorganic compounds of nitrogen, phosphorus, 

 arsenic, and antimony, and whilst putting forward the fact that 

 the combining power of the attracting element is always satisfied 

 by the same number of atoms, he does not point out the 

 characteristic tetrad nature of carbon ; and it was not until 1858 

 that Couper initiated, and Kekule, in the same year, thoroughly 

 established, the doctrine of the linking of the tetrad carbon atoms, 

 a doctrine to which, more than to any other, is clue the extra- 

 ordinary progress which organic chemistry has made during the 

 last twenty years, a progress so vast that it is already found im- 

 possible for one individual, even though he devote his whole time 

 and energies to the task, to master all the details, or make himself 

 at home with the increasing mass of new facts which the busy 

 workers in this field are daily bringing forth. 



The subject of the valency of the elements is one which, since 

 the year above referred to, has given chemists much food for 

 discussion, as well as opportunity for experimental work. But 

 whether we range ourselves with Kekule, who supports the un- 

 alterable character of the valency of each element, or with Frank- 

 land, who insists on its variability, it is now clearto most chemists 

 that the hard and fast lines upon which this theory was sup- 

 posed to stand cannot be held to be secure. For if the pro- 

 gress of investigation has shown that it is impossible in many 

 instances to affix one valency to an element which forms a large 

 number of different compounds, it is also equally impossible to 

 look on the opposite view as tending towards progress, inasmuch 

 as to ascribe to an element as many valencies as it possesses com- 

 pounds with some other element, is only expressing by circuitous 

 methods what the old Daltonian law of combination in multiple 

 proportion states in simple terms. Still we may note certain 

 generally-accepted conclusions : in the first place, that of the 

 existence of non-saturated compounds both inorganic'and organic, 

 as carbon-monoxide on the one hand, and malic and citraconic 

 acids on the other. Secondly, that the valency of an element is 

 not only dependent upon the nature of the element with which it 



combines, but that this valency is a periodic function of the atomic 

 weight of the other component. Thus the elements of the 

 chlorine group are always monads when combined with positive 

 elements or radicals, but triad, pentad, and heptad with negative 

 ones. Again, the elements of the sulphur group are dyads in the 

 first case, but tetrad and hexad in the second. The periodicity 

 of this property of the atoms, increasing and again diminishing, 

 is clearly seen in such a series as 



AgClj, CdCL, InCI :! , SnCl 4 , SbH 3 , TeH.,, IH, 

 as well as in the series of oxides. The difficulties which beset 

 this subject may be judged of by the mention of a case or two : — 

 Is vanadium a tetrad because its highest chloride contains four 

 atoms of chlorine ? What are we to say is the valency of lead 

 when one atom unites with four methyls to form a volatile pro- 

 duct, and yet the vapour-density of the chloride shows that the 

 molecule contains one of metal to two of chlorine ? Or, how can 

 our method be said to determine the valency of tungsten when 

 the hexachloride decomposes in the state of vapour, and the penta- 

 cl loride is the highest volatile stable compound ? I low again are 

 we to define the point at w hich a body is volatile without decom- 

 position ? — thus sulphur tetrachloride, one of the most unstable of 

 compounds, can be vaporised without decomposition at all tem- 

 peratures below — 22°, whilst water, one of the most stable of 

 known compounds, is dissociated into its elements at the 

 temperature of melting platinum. 



But. however many doubts may have been raised in special 

 instances against a thorough application of the law of valency, it 

 cannot be denied that the general relations of the elements 

 which this question of valency has been the means of bringing to 

 light are of the highe-t importance, and point to the existence of 

 laws of Nature of the » ide-t significance ; I allude to the periodic 

 law of the elements first foreshadowed by Newlands, but fully 

 developed by Mendeleeff and Lothar Meyer. Guided by the 

 principle that the chemical properties of the elements are a 

 periodic function of their atomic weights, or that matter becomes 

 endowed with analogous properties when the atomic weight of an 

 element is increased by the same or nearly the same number, we 

 find ourselves for the first time in possession of a key which en' 

 ables us to arrange the hitherto disjecta membra of our chemica 

 household in something like order, and thus gives us means of 

 indicating- the family resemblances by which these elements are 

 characterised. 



And here we may congratulate ourselves on the fact that, by 

 the recent experiments of Brauner, and of Nilson and Pettersen 

 respectively, tellurium and beryllium, two of the hitherto out- 

 standing members, have been induced to join the ranks, so that 

 at the present time osmium is the only important defaulter 

 amongst the sixty-four elements, and few persons will doubt that 

 a little careful attention to this case will remove the stigma which 

 yet attaches to its name. But this periodic law makes it possible 

 for us to do more ; for as the astronomer, by the perturbations of 

 known planets, can predict the existence of hitherto unknown 

 ones, so the chemist, though, of course, with much less satis- 

 factory means, has been able to predict with precision the pro- 

 perties, physical and chemical, of certain missing links amongst 

 the elements, such as ekaluminium and ekaboron, then unborn, 

 but which shortly afterwards became well known to us in the 

 flesh as gallium and scandium. We must, however, take care 

 that success in a few cases does not blind us to the fact that the 

 law of Nature which expresses the relation between the properties 

 of the elements and the value of the atomic weights is as yet un- 

 known ; that many of the groupings are not due to any well- 

 ascertained analogy of properties of the elements, and that it is 

 only because the values of their atomic weights exhibit certain 

 regularities that such a grouping is rendered possible. So, to 

 quote Lothar Meyer, we shall do well in this, as indeed in all 

 similar cases in science, to remember the danger pointed out in 

 Bacon's aphorism, that "The mind delights in springing up to 

 the most general axioms, that it may find rest, but after a short 

 stay here it disdains experience," and to bear in mind that it is 

 only the lawful union of hypothesis with experiment which will 

 prove a fruitful one in the establishment of a systematic inorganic 

 chemistry which need not fear comparison with the order which 

 reigns in the organic branch of our science. And here it is well 

 to be reminded that complexity of constitution is not the sole 

 prerogative of the carbon compounds, and that before this sys- 

 temaiisation of inorganic chemistry can be effected we shall have 

 to come to terms with many compounds concerning whose 

 constitution we are at present wholly in ignorance. As instances 



