August 28, 1884] 



NA TURE 



423 



of such I would refer to the finely crystalline phospho-molybdates, 

 containing several hundred atoms in the molecule, lately prepared 

 by Wolcott Gibbs. 



Arising out of Kekule's theory of the tetrad nature of the 

 carbon atom, came the questions which have caused much debate 

 among chemists : (1) Are the four combining units of the carbon 

 atom of equal value or not ? and (2) Is the assumption of a dyad 

 carbon atom in the so-called non-saturated compounds justifiable 

 or not ? The answer to the first of these, a favourite view of 

 Kolbe's, is given in the now well-ascertained laws of isomerism ; 

 and from the year 1862, when Schorlemmer proved the identity 

 of the hydrides of the alcohol radicals with the so-called radicals 

 themselves, this question may be said to have been set at rest ; for 

 Lossen himself admits that the existence of his singular isomeric 

 hydroxylamine derivatives can be explained otherwise than by the 

 assumption of a difference between each of the combining units 

 of nitrogen, and the differences supposed by Schreiner to hold 

 good between the methyl-ethyl carbonic ethers have been shown 

 to have no existence in fact. With respect to the second point 

 the reply is no less definite, and is recorded in the fact, amongst 

 others, that ethylene chlorhydrin yields on oxidation chloracetic 

 acid, a reaction which cannot be explained on the hypothesis of 

 the existence in ethylene of a dyad carbon atom. 



Passing from this subject, we arrive, by a process of natural 

 selection, at more complicated cases of chemical orientation — 

 that is, given certain compounds which possess the same com- 

 position and molecular formula; but varying properties, to find 

 the difference in molecular structure by which such variation of 

 properties is determined. Problems of this nature can now be 

 satisfactorily solved, the number of possible isomers foretold, and 

 this prediction confirmed by experiment. The general method 

 adopted in such an experimental inquiry into the molecular 

 arrangement or chemical constitution of a given compound is 

 either to build up the structure from less complicated ones of 

 known constitution, or to resolve it into such component parts. 

 Thus, for example, if we wish to discriminate between several 

 isomeric alcohols, distinguishing the ordinary or primary class 

 from the secondary or tertiary class, the existence of which was 

 predicted by Kolbe in 1S62, and of which the first member was 

 prepared by Friedel in 1864, we have to study their products of 

 oxidation. If one yields an acid having the same number of 

 carbon atoms as the alcohol, it belongs to the first class and 

 possesses a definite molecular structure ; if it splits up into two 

 distinct carbon compounds, it is a secondary alcohol ; and if three 

 carbon compounds result from its oxidation, it must be classed 

 in the third category, and to it belongs a definite molecular 

 structure, different from that of the other two. 



In a similar way orientation in the much more complicated 

 aromatic hydrocarbons can be effected. This class of bodies 

 forms the nucleus of an enormous number of carbon compounds 

 which, both from a theoretical and a practical point of view, are 

 of the highest interest. For these bodies exhibit characters and 

 possess a constitution totally different from those of the ;o-called 

 fatty substances, the carbon atoms being linked together more 

 intimately than is the case in the latter-named group of bodies. 

 Amongst them are found all the artificial colouring matters, 

 and some of the most valuable pharmaceutical and therapeutical 

 agents. 



The discovery of the aniline colours by Perkin, their elabora- 

 tion by Hofmann, the synthesis of alizarin by Graebe and 

 Liebermann, being the first vegetable colouring matter which has 

 been artificially obtained, the artificial production of indigo by 

 Baeyer, and lastly the preparation, by Fischer, of kairin, a 

 febrifuge as potent as quinine, are some of the well-known recent 

 triumphs of modern synthetical chemistry. And these triumphs, 

 let us remember, have not been obtained by any such " random 

 haphazarding " as yielded results in Priestley's time. In the 

 virgin soil of a century ago, the ground only required to be 

 scratched and the seed thrown in to yield a fruitful crop ; now 

 the surface soil has long been exhausted, and the successful 

 cultivator can only obtain results by a deep and thorough pre- 

 paration, and by a systematic and scientific treatment of his 

 material. 



In no department of our science has the progress made been 

 more important than in that concerned with the accurate deter- 

 mination of the numerical, physical, and chemical constants upon 

 the exactitude of which every quantitative chemical operation 

 depends. For the foundation of an accurate knowledge of the 

 first of these constants, viz., the atomic weights of the elements, 

 science is indebted to the indefatigable labours of Berzelius. But 



"humanum est errare," and even Berzelius's accurate hand and 

 delicate conscientiousness did not preserve him from mistakes, 

 since corrected by other workers. In such determinations it it 

 difficult, if not impossible, always to ascertain the limits of error 

 attaching to the number. The errors may be due in the firss 

 place to manipulative faults, in the second to inaccuracy of the 

 methods, or lastly to mistaken views as to the composition of the 

 material operated upon ; and hence the uniformity of any series 

 of similar determinations gives no guarantee of their truth, the 

 only safe guide being the agreement of determinations made by 

 altogether different methods. The work commenced by Berzelius 

 has been worthily continued by many chemists. Stas and 

 Marignac, bringing work of an almost astronomical accuracy 

 into our science, have ascertained the atomic weights of silver 

 and iodine to within one hundred-thousandth of their value, 

 whilst the numbers for chlorine, bromine, potassium, sodium, 

 nitrogen, sulphur, and oxygen may now be considered correct to 

 within a unit in the fourth figure. Few of the elements, how- 

 ever, boast numbers approaching this degree of accuracy, and 

 many may even still be erroneous from half to a whole unit of 

 hydrogen. And, as Lothar Meyer says, until the greater number 

 of the atomic weights are determined to within one or two tenths 

 of the unit, we cannot expect to be able to ascertain the laws 

 which certainly govern these numbers, or to recognise the rela- 

 tions which undoubtedly exist between them and the general 

 chemical and physical properties of the elements. Amongst the 

 most interesting recent additions to our knowledge made in this 

 department we may note the classical experiments, in 18S0, of 

 J. W. Mallet on aluminium, and in the same year of J. P. Cooke 

 on antimony, and those, in the present year, of Thorpe on 

 titanium. 



Since the date of Berzelius's death to the present day, no dis- 

 covery in our science has been so far-reaching, or led to such 

 unforeseen and remarkable conclusions, as the foundation of 

 Spectrum Analysis by Bunsen and Kirchhoff in i860. 



Independently altogether of the knowledge which has been 

 gained concerning the distribution of the elementary bodies in 

 terrestrial matter, and of the discovery of half a dozen new 

 elements by its means, and putting aside for a moment the 

 revelation of a chemistry not bounded by this world, but limit- 

 less as the heavens, we find that over and above all these results 

 spectrum analysis offers the means, not otherwise open to us, of 

 obtaining knowledge concerning the atomic and molecular 

 condition of matter. 



Let me recall some of the more nmarkable conclusions to 

 which the researches of Lockyer, Schuster, Liveing and Dewar, 

 Wullner, and others in this direction have led. In the first place 

 it is well to bear in mind that a difference of a very marked kind, 

 first distinctly pointed out by Alex. Mitscherlich, is to be observed 

 between the spectrum of an element and that of its compounds, 

 the latter only being seen in cases in wdiich the compound is not 

 dissociated at temperatures necessary to give rise to a glowing 

 gas. Secondly, that these compound spectra — as, for instance, 

 those cf the halogen compounds of the alkaline-earth metals — 

 exhibit a certain family likeness, and show signs of systematic 

 variation in the position of the lines, corresponding to changes 

 in the molecular weight of the vibrating system. Still this 

 important subject of the relation of the spectra of different ele- 

 ments is far from being placed on a satisfactory basis, and in 

 spite of the researches of Lecoq de Boisbaudran, Ditte, Troost 

 and Hautefeuille, Ciamician, and others, it cannot be said that 

 as yet definite proof has been given in support of the theory that 

 a causal connection is to be found between the emission spectra 

 of the several elements belonging to allied groups and their 

 atomic weights or other chemical or physical properties. In 

 certain of the single elements, however, the connection between 

 the spectra and the molecular constitution can be traced. In 

 the case of sulphur, for example, three dittinct spectra are known. 

 The first of these, a continuous one, is exhibited at temperatures 

 below 500°, when, as we know from Dumas' experiments, the 

 density of the vapour is three times the normal, showing that at 

 this temperature the molecule consists of six atoms. The second 

 spectrum is seen when the temperature is raised to above 1000°, 

 when, as Deville and Troost have shown, the vapour reaches its 

 normal density, and the molecule of sulphur, as with most other 

 gases, contains two atoms, and this is a band spectrum, or one 

 characterised by channelled spaces. Together with this band 

 spectrum, and especially round the negative pole, a spectrum 

 of bright lines is observed. This latter is doubtless due to the 

 vibrations of the single atoms of the dissociated molecule, the 



