502 



NA TURE 



[Sept. 20, 1883 



that the iron molecule was dissociated by heat, and that it 

 different constituents, on account of their different volatility, or 

 some other cause, had floated away from one another. This 

 seems to me the easiest explanation of the phenomenon ; and, 

 as dissociation by heat is a very common occurrence, there is no 

 a frioti improbability about it. But we are not shut up to it, 

 for the different layers of atmosphere are certainly at different 

 temperatures, and most probably of different composition. If 

 they are of different temperatures, the variations of the spectrum 

 may only be an extreme case of what must be acknowledged by 

 every one more or less — that bodies emit, or cease to emit, 

 different rays as their temperature increases, and notably when 

 they pass from the liquid to the gaseous condition. And again, 

 if the composition of the two layers of atmosphere be different, 

 we have lately learnt how profoundly the admixture of a foreign 

 substance will sometimes modify a luminous spectrum. 



2. Peculiarities of Atomic Weights. — At the meeting of this 

 Association at Ipswich, in 1851, M. Dumas showed that in 

 several cases analogous elements form groups of three, the 

 middle one of which has an atomic weight intermediate between 

 those of the first and third, and that many of its physical and 

 chemical properties are intermediate also. During the discussion, 

 upon his paper, and subsequently, 1 attention was drawn to the 

 fact that tins is nol confined to groups of three, but that there exit 

 many series of analogous elements having atomic weights which 

 differ by certain increments, and that the>e increments are in 

 most cases multiples of 8. Thus we have lithium, 7 ; sodium, 

 23, i.e. 7+16; potassium, 39, i.e. 7 + (16 X 2); and the 

 more recently discovered rubidium, 85, i.e. 7 + (16 x 5) nearly; 

 and ccesium, 133, i.e. 7 + (16 x 8) nearly. This is closely 

 analogous to what we find in organic chemistry, where there are 

 series of analogous bodies playing the part of metals, such as 

 hydrogen, methyl, ethyl, &c, differing by an increment which 

 has the atomic weight 14, and which we know to be CH„. 

 Again, there are elements with atomic weights nearly the same 

 or nearly multiples of one another, instances of which are to be 

 found in the great platinum group and the great cerium group. 2 

 '1 his suggests the analogy of isomeric and polymeric bodies. 

 There is also this remarkable circumstance : the various members 

 of such a group as either of those just mentioned are found to- 

 gether at certain spots on the surface of the globe, and scarcely 

 anywhere else. The chemist maybe reminded of how in the 

 dry distillation of some organic body he has obtained a mixture 

 of polymerised hydrocarbons, and may perhaps be excused If he 

 speculates whether in the process of formation of the platinum or 

 the cerium group, however and whenever it took place, the 

 different elements had been made from one another and im- 

 perfectly polymerised. 



But this is not the largest generalisation in regard to the 

 peculiarities of these atouiic weights. Newlands showed that, 

 by arranging the numbers in their order, the octaves presented 

 remarkable similarities, and, on the same principle, Mendelecff 

 constructed his well-known table. I may remind you that in 

 this table the atomic weights are arranged in horizontal and 

 vertical series, those in the vertical series differing from one 

 another, as a rule, by the before-mentioned multiples of 8-— 

 namely 16, 16, 24, 24, 24, 24, 32, 32 — the elements being 

 generally analogous in their atomicity and in other chemical 

 characters. Attached to the elements are figures, representing 

 various physical properties, and these in the horizontal series 

 appear as periodic functions of the atomic weights. The table 

 is incomplete, especially in its lower portions, but, with all its 

 imperfections and irregularities, there can be no doubt that it 

 expresses a great truth of nature. Now, if we were to inter- 

 polate the compound bodies which act like elements — methyl, 

 15; ammonium, 18; cyanogen, 26 — into. Mendelecff's table, 

 they would be utterly out of place, and would upset the oider 

 both of chemical analogy and of the periodicity of the physical 

 properties. 



3. Specific Refraction. — The specific refraction has been de- 

 termined for a large majority of the elements, and is a very 

 fundamental property, which belongs to them apparently in all 

 their combinations, so long at lea^t as the atomicity 3 is un- 

 changed. If the figures representing this property be inserted 

 into Mendeleeff's table, we find that in the vertical columns the 



1 " Phil. Mag.," May, 1853. 



3 Another curious instance is the occurrence of nickel and cobalt in all 

 meteoric imns, wiih occasionally chromium or manganese, the atomic 

 weights and « Iher properties of which are very similar. 



3 1 his exception includes not merely such changes as that from a ferrous 

 to a ferric salt, hut the different ways in which the carbon is combined in 

 such bodies as ethene, benzene, and pyrene. 



figures almost invariably decrease as the atomic weights increase. 

 If, however, we look along the horizontal columns, or better still 

 if we plot the figures in the table by which Lothair Meyer has 

 shown graphically that themolecular volume is a periodic function 

 of the atomic weights, we shall see that they arrange themselves 

 in a series of curves similar to but not at all coincident with his. 

 The observations are not so complete or accurate as those of the 

 molecular volumes, but they seem sufficient to establish the fact, 

 while the points of the curves would appear to be, not the alka- 

 line metals, as in Meyer's diagram, but hydrogen, phosphorus 

 and sulphur, titanium and vanadium, selenium, antimony. Now, 

 if we were to insert the specific refractions of cyanogen, ammo- 

 nium, and methyl into this table, we should again show that it 

 was an intrusion of strangers not in harmony with the family of 

 elements. 



But there is another argument to be derived from the action of 

 light. The refraction equivalent of a compound body is the sum 

 of the refraction equivalents of its compounds ; and, if there is 

 anything known for certain in the whole subject, it is that the 

 refraction equivalent of an organic compound advances by the 

 same quantity (76) for every increment of CH 9 . If, therefore, 

 the increment between the different members of a group of 

 analogous elements, such as the alkaline metils, be of the same 

 character, we may expect to find that there is a regular increase 

 of the refraction equivalent for each addition of 16. But this is 

 utterly at variance with fact : thus, in the instance above quoted, 

 the refraction equivalent of lithium being 3'S, that of sodium is 

 4'8, of potassium 8'i, of rubidium i4'o, and of cresium about 

 137. Neither does the law obtain in those series in which the 

 increment is not a multiple of 8, as in the case of the halogens, 

 where the increment of atomic weight is 45, and the refraction 

 equivalents are chloiine 99, bromine 15 "3, and iodine 24'5. 



The refraction equivalents of isomeric bodies are generally 

 identical, and the refraction equivalents of polymeric bodies are 

 in proportion to their atomic weights. Among the groups of 

 analogous elements of the same, or nearly the same, atomic 

 w eight we do find certain analogies : thus cobalt and nickel are 

 respectively IO'S and 104, while iron and manganese are re- 

 spectively l2"o and 122. But, as far as observation has gone at 

 present, we have reason to conclude that, if metals stand to one 

 another in the ratio of 2 : 1 in atomic weight, their refraction 

 equivalents are much nearer together than that ; while, on the 

 other hand, the equivalent of sulphur, instead of being the double 

 of that of oxygen, is at least five times as great. 



The general tendency of these arguments is evidently to show 

 that the elementary radicals are essentially different from the 

 compound radical", though their chemical functions are similar. 



There remains still the hypothesis that there is a " primordial 

 element, " from which the others are derived by transmutation. 

 With the sages of Asia it was the " blue ether," with Thales 

 water, with Dr. Trout hydrogen. The earlier views have passed 

 awav, and the claims of hydrogen are being fought out on the 

 battle-field of atomic weights and their rigorous determination. 



There does not appear to be any argument which is fatal to the 

 idea that two or more of our supposed elements may differ from 

 one another rather in form than in substance, or even that the 

 whole seventy are only modifications of a prime element ; but 

 chemical analogies seem wanting. The closest analogy would 

 be if we could prepare two allotropic conditions of some body, 

 such as phosphorus or cyanogen, which should carry their allo- 

 tropism into all their respective compounds, no compound of the 

 one form being capable of change into a compound of the other. 

 Our present knowledge of allotropism, and of variations in 

 atomicity, affords little, if any, promise of this. 



The remarkable relations between the atomic weights of the 

 elements, and many peculiarities of their grouping, force upon 

 us the conviction that they are not separate bodies created with- 

 out reference to one another, but that they have been fashioned 

 or built up from one another, according to some general plan. 

 This plan we may hope gradually to understand better, but if we 

 are ever to transform one of these supposed elements into 

 another, or to split up one of them into two or three dissimilar 

 forms of matter, it will probably be by the application of some 

 method of analysis hitherto unknown. 



Nothing can be of greater premise than the discovery of new 

 methods of research ; hence I need make no apology to others 

 who have lately done excellent work in chemistry if I single out 

 the liakerian Lecture of this year, by Mr. Crookes, on " Radiant 

 Matter Spectroscopy." It relates to the prismatic analysis, not 

 of the light transmitted or absorbed in the ordinary way by a 

 solid or liquid, nor of that given out by incandescent ga=, but the 



