84 



KNOWLEDGE 



[May 2, 1892. 



entirely insignificant. On account of the very small 

 masses of the attendant hodies, tidal friction, as Professor 

 G. H. Darwin has shown, has not been of much importance 

 in the solar system, and consequently the orbits of the 

 planets and satellites have in general undergone but little 

 change, compared to the great expansion and elongation 

 experienced by the orbits of double-stars. 



The mass of our system is nearly all in the Sun, and 

 hence the mass-distribution is essentially single ; the 

 Binary Stars are systems of donhh' sunx, and the mass- 

 distribution is therefore essentially double. If we look 

 upon the Sun and his attendant bodies as one of the stellar 

 systems, we cannot fail to recognise its very exceptional 

 character. For the system is very complex, and the 

 satellites (except the Moon) are very small compared to 

 the planets, which are in turn equally insignificant com- 

 pared to the Sun. The orbits are, moreover, in general, 

 nearly circular, as tidal friction has been too unimportant 

 to enlarge materially the (small) eccentricities with which 

 they were originally endowed. By far the greater number 

 of stellar systems appear to be composed of only two stars 

 — two vast suns — but a few systems are made up of three 

 and more rarely four such bodies. AS'e could not discover, 

 with our present optical means, planets such as .Jupiter 

 even at the short distance of a Centauri, and hence we 

 canuot affirm that surJi bodies do not revolve elsewhere in 

 the sky, especially around many of the " single" stars. It 

 is doubtful, however, whether there are, in general, many 

 small bodies of a planetary character in the double-star 

 systems, for the simple reason that they could probably 

 not long be preserved where the attraction is so com- 

 plicated (by two centres of nearly equal importance, 

 varying greatly in distance owing to the high eccentricities 

 of the orbits) . Viewed as a stellar system therefore, our 

 system is quite unique, and its development has apparently 

 been radically diiJ'erent from that prevailing among the 

 double-stars, which would seem to be the normal form of 

 celestial evolution. 



THE 



CLASSIFICATION OF THE CHEMICAL 

 ELEMENTS. 



By Yaughan Cornish, B.Sc, F.C.S. 



THE researches of Stas appeared to show that the 

 connection between the atomic weights, which 

 Prout thought he had discovered, was either 

 unreal or, at all events, not demonstrable. Four 

 years after the publication of Stas's second series of 

 researches, the Eussian chemist, Mendelejeff, made known 

 his system of classifying Elements on the basis of atomic 

 weights — a system which has stood the test of experiment, 

 and has pointed the way to many new paths of fruitful 

 research in chemistry. 



Chemists had been for some time familiar with the fact 

 that among the Elements are certain "natural families," 

 the members of which bear a general similarity to one 

 another, and show a regular gradation of properties 

 following the increase in atomic weight as we proceed from 

 the lowest to the highest member. Thus Chlorine, Bromine, 

 and Iodine have certain properties iu common, known to 

 every tyro in chemistry, which mark them as members of 

 a group or family. Among the members of this family 

 the properties vary in a co]itinu(in.'i manner with the atomic 

 weight. Thus Chlorine (at. wt. about 8.5.',) is a gas, 

 Bromine (at. wt. about 80) is a liquid, and Iodine 

 (at. wt. about 126i) is a solid, and so in similar 

 gradation with their other characters. Lithium (7), 

 Sodium (23), and Potassium (89) are another natural 



family, the well-known Alkali metals. Here, again, there 

 is a continuous gradation of properties with rise of atomic 

 weight, Lithia, the oxide of Lithium, being a weaker base 

 than Soda, and Potash being the strongest base of the 

 three oxides. 



The members of a natural family are termed humohyjous 

 elements. The connection between the properties and the 

 atomic weight among groups of homologous elements was, 

 as we have said, known before the publication of Men- 

 delejeff's first paper, which appeared in 1869. Mendelejeff, 

 however, by comparing together the members of different 

 groups or families, was led to the discovery of a new and 

 peculiar relation between the weights of the atoms and 

 their properties, a relation which is the basis of the present 

 system of classification of the Elements, known as the 

 Periodic Sijstciii. 



Writing the Elements* after Hydrogen (1) in order of 

 atomic weight we have — 



Li. Be. B. C. N. 0. F. A'a. Mg. Al. Si. P. S. CI. K. Ca. 



7 9 11 12 14 10 19 28 24 27 28 81 32 35i 39 40 



and so on. There is found to be a regular gradation of pro- 

 perties with increase of atomic weight from Lithium to 

 Fluorine, which are as opposite in their characters as any 

 two elements with which we are acquainted. Increase of 

 atomic weight from 7 to 19 has contmuously diminished 

 the electro-positive or metallic character possessed by 

 Lithium till we reach Fluorine, a non-metal and the most 

 strongly electro-negative element known. But after 

 atomic weight 19 the gradation of properties does not 

 continue ; on the contrary, there is a sudden " reversion 

 to type," the next element. Sodium (Na. 23), being 

 strongly metallic in character. It is, as we have already 

 mentioned, a member of the same natural family as 

 Lithium, and in that family stands next to Lithunu in 

 order of atomic weight. As we proceed from Sodium, in 

 order of increasing atomic weight, we find once more a 

 gradation from the most strongly marked metallic pro- 

 perties to the most decided non-metallic character. 

 Magnesium (Mg. 24) is a metal as Sodium is, but its oxide 

 is less strongly basic than Soda. Alumina (the oxide of 

 Aluminium, Al. 27) is weakly basic or weakly acidic 

 according to circumstances. Silicon, a non-metallic body, 

 forms a weakly acidic oxide, and Sulphur (S. 32) is not 

 metallic in its physical properties, and forms an oxide 

 which is strongly acidic or acid-forming. The next 

 element. Chlorine (CI. 85^), is the first homologue of 

 Fluorine, and is a typical non-metallic Element. The 

 Element next following, Potassium (K. 39), is, however, a 

 metal, and forms a strongly basic o.xide. It is the third 

 member of the group of alkali metals ; so that in passing 

 from Chlorine to Potassium we have the second instance 

 of " reversion to type." 



After Potassium, the metallic character again begins to 

 decrease ; the next Element, Calcium (Ca. 40) forming an 

 oxide. Lime, which is a weaker base than Potash. Thus, 

 after Potassium as after Sodium, the variation of properties 

 goes on continuously with increase of atomic weight from 

 one element to another for another period. 



We will not follow these periodir variations further, 

 partly on account of limitations of space, partly because 

 the relations become more intricate and more difficult to 

 follow as we proceed to the higher atomic weights. We 



* Approximate numbers are, for conTenience, given for the atomic 

 weights. The names of the Elements sTmtJolized above are : 

 Lithium (Li.), Beryllium (Be.), Boron (B.). Carbon (C). Nitro- 

 gen (N.), Oxygen ((>.), I'luorine (F.), Sodium (Na.), Magne- 

 sium (Mg.), Aluminium (Al.), Silicon (Si.), Phosphorus (P.), 

 Sulphur (S. ), Chlorine (Cl._), Potassium (K.), and Calcium (Ca.). 



