Sept. I, 1881] 



NATURE 



417 



atoms with one another. Thus in the vast majority of hydro- 

 carbons, fcuch as CjHg, CoHj, C^Hj, &c., the atoms of carbon 

 do not appear to be tetravalent, inasmuch as each of the mole- 

 cules contains less than four atoms of hydrogen to every one 

 atom of carbon. It was well known, however, that polyvalent 

 atoms can combine partly with one element, partly with another, 

 and also that like atoms can combine with one another. Why 

 then should not two tetravalent atoms like carbon combine 

 respectively with three atoms of a monad, and also combine 

 with one another? The compound must be a single molecule 

 with the properties known to belong to methyle C^H,;. Again, 

 if this molecule were deprived of two of its atoms of hydrogen, 

 each of the atoms of carbon must combine further with the other 

 atom of carbon forming HjCCH, ; and a further step in this 

 same direction would give us acetylene HCCH, in which each 

 atom of carbon is combined with the other to the extent of 

 three quarters of its value, and with one atom of hydrogen. An 

 extension of this reasoning led to the discovery of long chains 

 of atoms of carbon, each atom forming a link, and each of them 

 (short of the ends) being combined with two other atoms of 

 carbon, while its saturation is completed by hydrogen. 



Similar partial combinations of like atoms with oi.e another 

 were recognised in many other classes of compounds, and there 

 is strong reason to expect that the application of the principle 

 will be far more vvidely extended in proportion as our knowledge 

 of the silicates and other complex classes of compounds becomes 

 somewhat definite. 



This incorporation of the doctrine of equivalence into the 

 atomic theory by the division of the elements into cla.sses con- 

 sisting respectively of equivalent atoms, was probably one of the 

 most important general steps as )et made in the development of 

 the atomic theory. It was seen to correspond in so clear and 

 striking a manner with a vast number of well-known properties 

 and reactions of compounds as to deserve and acquire the con- 

 fident trust of chemists. But, as often happens in such cases, this 

 confidence in the result carried many of them too far. It led them 

 to assume that atomic values in all other chemical compounds 

 must be alw ays the same as in the compounds under considera- 

 tion. They saw that they had got hold of the truth, and they 

 thought it was the whole truth. For instance, one most distin- 

 guished chemist a.ssumed that each elementary atom has only 

 one value in its conipoui.ds ; that the atom of nitrogen has 

 always the value three, as in ammonia and its products of sub- 

 stitution, and that in sal ammoniac the atom of nitrogen is 

 chemically combined only with three atoms of hydrogen, w^hilst 

 the molecule of ammonia is in a state of molecular combination 

 with h)dric chloride. Another most distinguished chemist ad- 

 mitted that nitrogen and phosphorus have two atomic values, but 

 not more than tw'o. He held that the resptclive combining 

 powers aie always satisfied by the same iiumber of atoms, no 

 matter what the character of the uniting atoms may be. 



With respect to these views it may be noticed that the assump- 

 tion of combination between molecules as due to some other 

 force than that which binds together the constituents of each 

 molecule — in fact the as-uniption of m ilecular combii.ation as an 

 unknown something different from chemical combination, is 

 open to even more grave objections than those which led us to 

 abandon the dualistic system. 



To represent a molecule of sal ammoniac as a compound 

 containing two molecules, each one built up by the chemical 

 combination of the constituent atoms, and the two united 

 together by some other force called molecular, was hardly a step 

 in advance of the view which represented it as containing two 

 molecules united together by the same kind of force as that 

 which holds together the atoms in each of the constituent 

 molecules. 



The other form of the theory of atomicity as an inherent 

 property of each atom enabling it to combine v ith an equal 

 number of other atoms, whatever the character of lho>e other 

 atoms may be, seems difficult to reconcile with such facts as the 

 following : — An atom of nitrogen is not known to combine with 

 more than three atoms of hydrogen alone, or of substances like 

 hydrogen, but it forms stable com; ounds with five atoms (as in 

 the ammonia salts), when four of them are basylous and one of 

 them is chlorous. An atom of sulphur i? not known to combine 

 with more than two atoms of hydrogen alone, but it forms stable 

 compounds with four atoms, if three of them are like hydrogen, 

 while the fourth is chlorous. Instances like these are plentiful, 

 and they lead us to look to the chemical characters of the 

 atoms bound together in one molecule as a fundamental con- 



dition of the atomic value of the element which binds them 

 together. 



Theoretical limitations of natural forces are very difficult of 

 proof, and it is well to be slow and cautious in adopting any such 

 limitation. 



A careful consideration of the facts of the case has led me not 

 only to doubt the validity of the supposed limits of atomic value, 

 but to doubt whether we have grounds for assigning any limits 

 whatever to such v.ilues. 



Atomic values appear to me to be in their very nature variable 

 quantities, and I venture to think that chemistry mil be greatly 

 advanced by a full and careful study of the conditions of varia- 

 tion of atomic values. 



Two conditions of change of atomic value are particularly 

 worthy of notice : — 

 I. Temperature. 



II. The chemical character of the uniting atoms. 



Atomic values increase w ith fall of temperature, and diminish 

 with rise of temperature. An atom which is combined with as 

 many basylous monads as it can take up by themselves, will 

 take up chlorous monads, or both chlorous and basylous, and 

 reciprocally. 



In illustration of the diminution of atomic values with rise of 

 temperature, I may adduce the following well-known reactions : 

 Sal ammoniac containing nitrogen combiiied with five monads 

 breaks up at a high temperature into ammonia and hydric 

 chloride; and in like manner other ammonia salts decompose 

 by heat forming ammonia or an amide, with trlvalent nitrogen. 

 The highest chlorides of phosphorus and of antimony are de- 

 composed by heat into free chlorine and the lower chloride. 

 Potassic fiuosilicate is decomposed by heat into silicic and 

 potassic fluorides ; and carbonic acid breaks up at high tempera- 

 tures into a mixture of carbonic oxide and oxygen. 



Amongst illustrations of the greater atomic values which 

 elements assume by combining w ith both chlorous and basylous 

 atoms than with atoms of the one kind only, we may take the 

 follow ing cases : platinum is a metal of which the atom has 

 been supposed to be always tetravalent, because it has not been 

 found capable of combining with more than four atcms of 

 chlorine. The common solution formed by aqua legia contains 

 the compound HoPtCl,;, a perfectly definite and crystallis-ible 

 hydrogen salt. Chemists are constantly making and using the 

 potassium and ammonium salts, &c., corresponding to it, yet 

 they conceal from themselves the fact that the atom of platinum 

 is directly combined with ei^ht monads by calling the compounds 

 double salts. The atom of silicon in the silico-fluorides such as 

 HjSiF,., or K„SiF|;, is combined with twice as many monads as 

 it can take up of one kind ; so boron in the crystalline salt 

 NaBF^ has a higher atomic value than in its fluoride, owing to 

 the presence of the atom of sodium. 



In like manner the atom of gold in the well-known salt 

 NaAuClj, has a higher value than it can assume with chlorine 

 alone. 



^Sulphur, of which the atom does not combine with more than 

 2 atoms of hydrogen, forms with 3 atoms of methyle, or ethyle, 

 and I atom of iodine, or chlorine, &c., the well-known com- 

 pounds like JSMcs ; and iodine, which is considered a monad, 

 forms the crystalline and stable periodate 0J(0H)5 and the 

 various metallic derivatives, such as 



OJ ONa OJ (ONa)^ OJ (OAg)^ 



(OH), (OH)3 



The crystalline compound of the perchlorate w'ith water 

 (HCIO42H.P) has probably a similar constitution. Chemical 

 journals abound with descriptions of definite and well-charac- 

 terised compounds, which have, like the above, been put aside 

 by the atomicity theory, as mere molecular compounds. The 

 following formulae are taken almost at random, in illustration of 

 the generality of atomic values far beyond those acknowledged 

 by the theory of atomicity. 



JPClg KoFeFj ICSnCIe 



KAsFg 2(K2FeCl5) KHgJa 



KjAsF, 2(K3FeF6) Ki,Hgj44H20 



KSbFs KXUCI4 KHgCl, 



K.,SbCl5 KjCuF, K.,HgCl4 



K.SbF, KMgCls 



KjBiCIs K^MgBr, 



K^AgJ^ 



I have for convenience written in the middle of each of these 



