4i8 



NATURE 



{Sept. I, 1 88 1 



formuls: the syaibol of the atom which I assume to act as con- 

 necting' element. If we coni-ider the atomic vahies usually found 

 in the?e elements, together with those represented by the above 

 \\A, we ^ee that their atomic values vary according to the 

 nvnnbers given in a line with them respectively in the following 

 table. It has yet to be proved that the atom of platinum is 

 tetravalent in any knovvu compound, for there is no sufficient 

 evidence to show that platinic chloride has a molecular weight 

 corresponding to the formula PtClj, instead of one corresponding 

 to PtoClg, each atom of platinum being partly combined with 

 the other, partly with chlorine. 

 Atomic Symbols. Atomic Values. 



C • 2,4 



S 2,4 



Pt 4(?).8 



Si 4,8 



Sn 4.8 



Cu 2,6 



Hg 2,4,6 



Mg 2,4,6 



Ag i>5 



B 3.5 



.1 '.7 



N ■ 3.5 



P 3.5.7 



As 3.7.9 



Sb 3.5.7.9 



Bi 3.7 



Au 3(?).5 



Not only are there elements of which an atom is found in 

 combination with a greater number of basylous and chlorous 

 monads together than of either kind alone, but there are also 

 elements which are not known to form chemical compounds 

 with hydrogen or potassium alone, and yet which combine with 

 either of them when also combined with chlorine, fluorine, &c. 

 This is illustrated by the following compounds, viz., HAuClj, 

 H.,PtCl6, NaBF^, K.,3iFg, K^FeFj, KX'uCl^. It is also well 

 known that there are many cases of elements of which an atom 

 cannot combine with as many monads of one kind as of another. 

 For inst.ince an atom of niirogen or of antimony is only known 

 to be trivalent in combination with hj'drogen ; but each of them 

 occurs in the form of a pentavalent com] ound with chlorine. 

 Antimony forms either no compound with five atoms of bromine, 

 or a compound more unstable than the higher chloride. 



Many more such instances might easily n iw be given, and a 

 vast number w ill doubtless be found when the investigations of 

 chemists are directed to the search for them. I have only given 

 these few by way of illustration of the leading conditions of 

 change of atomic values. 



In the course of their investigations of the precise interchanges 

 of atoms which take place between molecules, chemists were 

 frequently led to observe evidei.ces of the order in which the 

 constituent elements are combined ; and with the more wide and 

 accurate knowledge of reactions which is now in their posses- 

 sion, they have teen enaliled to follow up so far the study of the 

 respective state of combination of each atom in a molecule as to 

 arrive at simple and consistent explanations of facts which had 

 previously eluded the grasp of science. 



Our knowledge of the order of combination of atoms in a 

 molecule and of the differences between direct and indirect com- 

 binations of particukar atoms may be said to have originated 

 chiefly in the study of the compounds of nitrogen. Thus it was 

 found that the hydrogen in aiiunonia differs in many of its 

 chemical functions from hydrogen in hydrocarbons. A base 

 (called methylia) was discovered having a molecular composiion 

 corresponding to the empirical formula (CNHj), and this base 

 was found to contain two atoms of hydrogen like those of am- 

 monia, and three atoms like those in hydrocarbons. Its consti- 

 tution was accordingly represented by a formula describing it as 

 an ammonia, in which one atom of hydrogen is replaced by the 

 monad methyle, or, to be more explicit, as containing two atoms 

 of hydrogen directly combined with nitrogen, and three atoms 

 of hydrogen indirectly combined with that same atom of nitrogen 

 through the intervening atom of carbon. Writing in juxtapo>iticn 

 to one another the symbols of those atoms which are directly 

 combined, we can express the facts by the following formula, 

 viz. H.,NCH3. 



Those marvellous varieties of matter called isomeric com- 

 pounds found their natural explanation in differences of the 

 respective arrangements of like atoms. Thus two bases were 



discovered having the same empirical molecular formula C„NH,. 

 One of them is made by different reactions from the other, and 

 in its decompositions differs from the other. All these chemical 

 differences between them are found to be due to the fact that 

 one of them (called ethylia) contains two atoms of hydrogen 

 directly combined with the nitrogen, and the monovalent hydro- 

 carbon ethyle in place of the third atom of hydrogen ; whilst 

 the other (called diniethylia) contains only one atom of hydrogen 

 combined directly with niirogen, the carbon of the two atoms of 

 meihyle completing the saturation of the trivalent nitrogen, as 

 expressed by the foimula HN{CH3)2. 



It was subsequently proved that an atom of oxygen may com- 

 bine with two like or unlike monads, such monads being in- 

 diredly combined with one another through the intervening atom 

 of oxygen. Thus five of the atoms of hydrogen in common 

 alcohol were proved to be in direct combination with the car- 

 bon, whilst the other one is indirectly combined with it through 

 the oxygen, as expressed by the formula HO(CoH5). 



Another compound (called methyl-o.xide) was proved to have 

 the same empirical composition, but very different proporties 

 and reactions, its constitution being explained by the formula 

 H3COCH3. 



Again, two compounds of distinct reactions and properties 

 were found to have the same empirical molecular composition, 

 C.,NH3, and it va as clearly proved that in one of them the two 

 atoms of carbon are directly combined thus, NCCH3, whilst in 

 the other they are indirectly combined through the atom of 

 nitrogen CNCH3. 



An immense amoiuit of admirable work has been done of late 

 years (especially in Germany) in working out the evidences of 

 the atomic order of complex organic bodies, and in thereby 

 obtaining a command of their reactions. 



Evidences of the same kind have been obtained of the atomic 

 arrangement of some few of the simplest inorganic bodies, and 

 it is to be hoped that ere long chemists will recognise the impor- 

 tance of examining the corstitution of salts with the aid of the 

 principles established in organic chemistry. 



The foundation is already laid by our knowledge of the con- 

 stitution of such compounds .as 



IIOH, HOK, HONHj, IIONO., 

 HOPbOH, HOPbONOo, HOCOK, HOSOH 

 O Oj 



and there is a strong probability regarding the atomic constitution 

 of many other water compounds, e.g. 



SO4H2 -1- H.,0 = OS(OIl2l4,S05Hj -1- H„0 = S(OH)s. 



Amongst the extensions of our means of examining the 

 physical properties of matter, and thereby di covering new 

 varieties of matter for chemical inver-tigation, spectrum analysis 

 has played an important part, and is no doubt destined to do far 

 more. It has already led chemists to the di-covery of several 

 previously unknown elements, and has led to the detection of 

 various known elements in distant masses of which we had 

 previously no chemical kno" ledge. 



Up to this point the growth of the atomic theory will be seen, 

 from the general outline w hich I have endeavoured to trace, to 

 have consisted mainly in the more and more full .and exact 

 identification of each elementary atom, and in the accumulation 

 of more and more varied and accurate evidences of its functions 

 in relation to other atoms. A step was made towards a know- 

 ledge of the general relations of atoms to ore another by their 

 preliminary classification according to their best-known values. 



But a far greater step has been more recently made, one which 

 is evidently destined to lead to most impc.rlant results. 



It was discovered that if we arrange the elements in the em- 

 pirical order of their respective atomic weights, beginning with 

 hydrogen and proceeding thence step by step to the heaviest 

 atom, we have before us a natural series with periodically 

 ecurrent changes in the chemical and physical functions of its 

 members. 



Of course the series is imperfect, and exhibits gaps and 

 irregularities ; but \\ hat view of natural order was complete in 

 its infancy ? 



Some of the gaps have already been filled up by the discovery 

 of elements possessing the anticipated properties. The gene- 

 ralisation affords a brilliant addition to the previous corrobora- 

 tions of the reality of the units of matter which chemists have 

 discovered. 



Chemists have as yet taken but little account of atomic motion ; 



