September 5, 1884. 



SCIENCE. 



207 



chemists that the hard and fast lines upon which this 

 theory was supposed to stand cannot be held to be 

 secure. 



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 gen- 

 eral relatione of tbe elements which this question of 

 valency has been the means of bringing to light are 

 of the highest importance, and point to the existence 

 of laws of nature of the widest significance; as seen 

 in the periodic law of the elements first foreshadowed 

 by Newlands, but fully developed by Mendelejeff and 

 Lothar Meyer. But this periodic law makes it pos- 

 sible 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 reliable means, has 

 been able to predict with precision the properties, 

 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. 



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 com- 

 pounds, justifiable, or not ? The answer to the first 

 of these, a favorite 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 be set at rest. 



Passing from this subject, we arrive, by a process 

 of natural selection, at more complicated cases of 

 chemical orientation; that is, given certain com- 

 pounds which possess the same composition and 

 molecular formulae 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 num- 

 ber of possible isomers foretold, and this prediction 

 confirmed by experiment. 



The discovery of the aniline colors by Perkin, their 

 elaboration by Hof mann ; the synthesis of alizarin by 

 Graebe and Liebermann, being the first vegetable 

 coloring-matter which has been artificially obtained; 

 the artificial production of indigo by Baeyer; and, 

 lastly, the preparation by Fischer of kairine, a 

 febrifuge as potent as quinine, — are some of the 

 well-known recent triumphs of modern synthetical 

 chemistry. 



In no department of chemistry has the progress 

 made been more important than in that concerned 

 with the accurate determination of the numerical, 

 physical, and chemical constants, upon the exactitude 

 of which every quantitative chemical operation de- 

 pends. Amongst the most interesting recent ad- 

 ditions to our knowledge, made in this department, 

 we may note the classical experiments, in 1880, 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. 



In referring to the work in spectrum analysis, Pro- 

 fessor Koscoe recalled some of the more remarkable 

 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 which the compound is 

 not dissociated at temperatures necessary to give rise 

 to a glowing gas; second, that these compound 

 spectra (as, for instance, those of the halogen com- 

 pounds of the alkaline-earth metals) exhibit a certain 

 family likeness, and show signs of systematic varia- 

 tion in the position of the lines, corresponding to 

 changes in the molecular weight of the vibrating sys- 

 tem. Still, 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 spec- 

 tra of the several elements belonging to allied groups 

 and their atomic weights, or other chemical or physi- 

 cal 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 distinct spectra are 

 known. The first of these, a continuous one, is ex- 

 hibited at temperatures below 500°, when, as we 

 know from Dumas' experiments, the density of the 

 vapor 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 vapor reaches its normal density: 

 and the molecule of sulphur, as with most other 

 gases, contains two atoms ; and this is a band-spec- 

 trum, or one characterized 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 vibra- 

 tions of the single atoms of the dissociated molecule, 

 the existence of traces of a band-spectrum demonstrat- 

 ing the fact, that, in some parts of the discharge, the 

 tension of dissociation is insufficient to prevent the 

 reunion of the atoms to form the molecule. 



The most remarkable results obtained by Abney 

 and Festing show that the radical of an organic body 

 is always represented by certain well-marked absorp- 

 tion-bands; differing, however, in position, according 

 as it is linked with hydrogen, a halogen, or with car- 

 bon, oxygen, or nitrogen. Indeed, these experiment- 

 ers go so far as to say that it is highly probable, that. 

 by this delicate mode of analysis, the hypothetical 

 position of any hydrogen which is replaced may be 

 identified; thus pointing out a method of physical 

 orientation, of which, if confirmed by other ob- 

 servers, chemists will not be slow to avail them- 

 selves. 



One of the noteworthy features of chemical prog- 

 ress is the interest taken by physicists in fundamental 



