4i8 



NATURE 



[Sept. 



bodies which fifty years ngo were undiscernible are now clearly 

 manifest, and it is to these relationships that I would for a 

 moment ask your attention. I have a'ready stated that Dalton 

 measured the relative weights of the ultimate particles by 

 assumin'j hydrogen as the unit, and that Prout believed that 

 on this basis the atomic weights of all the other elements would 

 be found to be multiples of the atomic weight of hydrogen, thus 

 indicating that an intimate constitutional relation exists between 

 hydrogen and all the other elements. 



Since the days of Dalton and Prout the truth or otherwise of 

 Prout's law has been keenly contested by the most eminent 

 chemists of all countries. The inquiry is a purely experimental 

 one, and only those wlio have a special knowledge of the diffi- 

 culties which surround such inquiries can form an idea of the 

 amount of labour and self-sicrifice borne by such men as Dumas, 

 Stas, and Marignac in carrying out delicite researches on the 

 atomic weights of the elements What is, then, the result of 

 these most laborious experiments? It is that, whilst the atomic 

 weights of the elements are not exactly either multiples of the 

 imit or of half the unit, many of the numbers expressing most 

 accurately the weight of the atom approximate so closely to a 

 multiple of that of hydrogen, that we are constrained to admit 

 that these aoproximations cannot be a mere matter of chance, 

 but that some reason must exist for them. What that reason is, 

 and why a close approximation and yet something short of 

 absolute identity exists, is as yet hidden behind the veil ; but 

 who is there that doubts that when this Association celebrates 

 its centenary, this veil will have been lifted, and this occult but 

 fundamental question of atomic philosophy shall have been 

 brought into the clear light of diy? 



But these are by no means all the relationships which modern 

 science has discovered with respect to the atoms of our chemical 

 elements. So long ago as 1829 Dol)ereiner pointed out that 

 certain groups of elements exist presenting in all their properties 

 strongly marked family characteristics, and this was afterwards 

 extended and insisted upon by Dumas. We find, for example, 

 in the well-known group of chlorine, bromine, and iodine, these 

 resemblances well developed, accompanied moreover by a pro- 

 portional graduation in their chemical and physical properties. 

 Thus, to take th= most important of all their characters, the 

 atomic weight of the middle term is the mean of the atomic 

 weights of the two extremes. Bat these groups of triads 

 appeared to be unconnected in any way with one another, nor 

 did they seem to bear any relation to the far larger number of 

 the elements not exhibiting these peculiarities. 



Things remained in this condition until 1863, when Newlands 

 threw fresh light upon the subject, showing a far-reaching series 

 of relationships. For the first time we thus obtained a glance 

 into the mode in which the elements ai-e connected together, but, 

 IdvC so many new discoveries, this did not meet with the 

 recognition which we now see it deserves. But whilst England 

 thus had the honour of first opening up this new path, it is to 

 Germany and to Russia that we must look for the consummation 

 of the idea. Germany, in the person of Lothar Meyer, keeps, 

 as it is wont to do, strictly within the licnits of known facts. 

 Russia, in the person of Mendelejeff, being of a somewhit more 

 imaginative nature, not only seizes the facts which are proved, 

 but ventures upon prophecy. These chemists, amongst whom 

 Carnelley must be named, agree in placing all the elementary 

 bodies in a certain regular sequence, thus bringing to light a 

 periodic recurrence of analogous chemical and physical proper- 

 ties, on account of which the arrangement is termed the periodic 

 system of the elements. 



In order to endeavour to render this somewhat complicated 

 matter clear to you, I may perhaps be allowed to employ a 

 simile. Let us, if you please, imagine a series of human families : 

 a French one, represented by Dumas ; an English one, by name 

 Newlands ; a German one, the family of Lothar Meyer ; and 

 lastly, a Russian one, that of Mendelejeff. Let us next imagine 

 the names of these chemists placed in a horizontal line in the 

 order I have mentioned. Then let us write under each' the 

 name of his father, and again, in the next lower line, that of his 

 grandfather, followed by that of his great-grandfather, and so 

 on. Let us next write against each of these names the number 

 of years which has elapsed since the birth of the individual. We 

 shall then find that these numbers regularly increase by a definite 

 amount, i.e. by the average age of a generation, which will be 

 approximately the same in all the four families. Comparing the 

 ages of the chemists themselves we shall observe certain differ- 

 ences, but these are small in comparison with the period which 



h^s elapsed since the birth of any of their ancestors. Now each 

 individual in this series of family trees represents a chemical 

 element ; and just as each family is distinguished by certain 

 idiosyncrasies, so each group of the elementary bodies thus 

 arranged .shows distinct signs of consanguinity. 



But more than this, it not unfrequently happens that the 

 history and peculiarities of some member of a family may have 

 been lost, even if the memory of a more remote and more 

 famous ancestor may be preserved, although it is clear that 

 such an individual must have had an existence. In such a 

 case Francis Galton would not hesitate frim the characteristics 

 of the other members to reproduce the physical and even the 

 mental peculiarities of the missing member ; and shoild genea- 

 logical research bring to light the true personal appearance and 

 mental qualities of the man, these would be found to coincide 

 with Galton's estimate. 



Such predictions and such verifications have been made in 

 the case of no less than three of our chemical elements. Thus, 

 Mendelejeff pointed out that if, in the future, certain lacunae in 

 his table were to be filled, they must b^ filled by elements 

 possessing chemical and phy-ical properties which he accurately 

 specified. Since that time these gaps have actually been 

 stopped by the discovery of gallium by Lecoq de Boisbaudron, 

 of scandium by Nilson, and of germanium by Winkle^-, and 

 their properties, both physical and chemical, as determined by 

 their discoverers, agree absolutely with those predicted by the 

 Russian chemist. Nay, more than this, wenot unfreq lently have 

 had to deal with chemical foandlings, elements whose parentage 

 is quite unknown to us. A careful examination of the person- 

 ality of such waifs has enabled us to restore them to the family 

 from which they have been separated by an unkind fate, and to 

 give them that position in chemical society to which they are 

 entitled. 



These remarkable re ults, though they by no means furnish a 

 proof of the supposition already referred to, viz. that the 

 elements are derived from a common source, clearly point in 

 this direction, and lend some degree of colour to the speculations 

 of those whose scientific imagination, warying of dry facts, 

 revels in picturing to itself an elemental Bathybius, and in apply- 

 ing to the inanimate, laws of evolution similar to those which 

 rule the animate world. Nor is there wanting other evidence 

 regarding this inquiry, for here heat, the great analyzer, is 

 brought into court. The main portion of the evidence consists 

 in the fact that distinct chemical individuals capable of existence 

 at low temperatures are incapable of existence at high ones, but 

 split up into new materials possessing a less complicated struc- 

 ture than the original. And here it may be well to emphasize 

 the distinction which the chemist draws between the atom and 

 the molecule, the latter being a more or less complicated aggre- 

 gation of atoms, and especially to point out the fundamental 

 diiference between the question of separating the atoms in the 

 molecule and that of splitting up the a^om itself. The decom- 

 positions above referred to are, in fact, not confined to compound 

 Ijodies, for Victor Meyer has proved in the case of iodine that 

 the molecule at high temperatures is broken to atoms, and J.J. 

 Thomson has added to our knowledge by showing that this 

 breaking up of the molecule may be effected not only by heat 

 vibrations, but likewise by the electrical discharge at a com- 

 paratively low te nperature. 



How far, now, has this process of simplification been carried? 

 Have the atoms of our present elements been made to yield ? 

 To this a negative answer must undoubtedly be given, for even 

 the highest of terrestrial temperatures, that of the electric spark, 

 has failed to .shake any one of these atoms in two. That this is 

 the case has been shown by the results with which spectrum 

 analysis, that new and fascinating branch of science, has enriched 

 our knowledge, for that spectrum analysis does give us most 

 valuable aid in determining the varying molecular conditions of 

 matter is admitted by all. Let us see how this bears on the 

 question of the decomposition of the elements, and let us suppose 

 for a moment that certain of our present ele:nents, instead of 

 being distinct substances, were made up of common ingredients, 

 and that these compound elenents, if I may te allowed to use 

 so incongruous a term, are split up at the temperature of the 

 electric spark into less complicated molecules. Then the spec- 

 troscopic examination of such a body must indicate the existence 

 of these common ingredients by the appearance in the spark- 

 spectra of these elements of identical bright lines. Coincidences 

 of this kind have indeed been observed, but on careful examina- 

 tion these have been shown to be due either to the presence of 



