Jan. 26, 1888] 



NA JURE 



)C5 



by the introduction of a working hypothesis, an absolutely 

 artificial mode of expression such as is here adopted being 

 perhaps pardonable in the absence of any explanation which 

 may serve to guide us in extending o.ur inquiries as regards the 

 structure of such compounds, a knowledge of which is all-im- 

 portant to a rational conception of the nature of chemical change 

 generally. Moreover, I do not hesitate to affirm that, from the 

 chemical point of view, it is impossible to adopt the Helmholtz 

 explanation of valency, unless physicists are prepared to grant 

 the possibility of the " division " of the unit charge soir.ewhat 

 in the manner here suggested ; and it is in order to impress this 

 that I have ventured to give utterance to these speculations. 



To return to the consideration of the compounds previously 

 referred to, it may be supposed that the nitrogen of irirr.ethyl- 

 amine and the iodine of methyl iodide are possessed of residual 

 affinity, and hence the two molecules unite to form the molecular 

 compound tetramethylanimonium. iodide, which may be repre- 

 sented thus : — 



=N- 



cn. 



The phosphonium and sulphine iodides may be regarded as 

 similarly constituted. It is well known that the ammonium 

 haloid compounds and their analogues are also capable of form- 

 ing still more complex molecular aggregates with the halogens, 

 &c. : they are therefore to be regarded as possessed of residual 

 affinity ; and that polyad elements, e.g. nitrogen, phosphorus, 

 and sulphur, should still exhibit residual affinity in such com- 

 pounds is not surprising in the light of the hypothesis advocated 

 in this note ; but it is scarcely compatible with the assumption 

 that the halogen in the ammonium haloid compounds serves as 

 the bond of union. On the other hand, if it be assumed, as I 

 think it should be, that the formation of double metallic chlor- 

 ides, &c., is the outcome of the possession of residual affinity 

 by the halogen, the complete analogy which appears to exist 

 between the ammonium haloid compounds and those of the 

 alkali metals would seem logically to involve the inference that 

 the halogen of the ammonium compound doe> not serve as the 

 bond of union. I see but one mode of escape from this conflict 

 of evidence, and that is to call in question the time-honoured 

 assumption that the radical ammonium is the true analogue of 

 potassium and sodium, which, be it remarked, is of necessity 

 subject to doubt if the hypothesis that the ammonium salts are 

 molecular compounds be entertained ; and evidence which sup- 

 ports the conclusion that the per-haloid compound is formed by 

 the addition of the halogen to the nitrogen (phosphorus or sul- 

 phur) is afforded by the observation that not only haloid ammo- 

 nium and sulphine compounds, but also the sulphates, combine 

 with halogens (Dobbin and Masson, Chetn. Soc. Trans., 1885, 

 p. 56 ; 18S6, p. 846). 



It is now proved by abundant experimental evidence that, 

 whatever the order in which the radicles A, 13, C, D are 

 introduced in forming a tetralkylic ammonium compound 

 N(ABCD)X, one and the same end product always results. 

 This is commonly regarded as proof, not only that nitrogen is 

 pentad, but also that the five affinities of the nitrogen atom are 

 of equal value, and it would appear to favour the conclusion 

 that the ammoniu.n salts are in truth "atomic" compounds; 

 but I see no reason why isomeric change should not occur at the 

 moment of formation of a molecular compound— why the in- 

 tegrant molecules, in fact, should not interchange radicles. If 

 the statement be confirmed ^ that the compound formed from 

 dimethyl sulphide and ethyl iodide is different from that ob- 

 tained on combining methyle:hyl sulphide and ethyl iodide 

 (Krliger, /^«r«. pr. Cheiii., 1876, xiv. p. 193), it \yill follow, 

 not that sulphur is a tetrad, and that the four affinities are of 

 unequal value, but that there is little or no tendency for isomeric 

 change to occur in the formation of sulphines. The possible 

 occurrence of isomeric change in the formation of molecular 

 compounds, however, is a subject which certainly deserves 

 careful study at the present time. 



In the case of phosphorus, the existence of the highly stable 

 gaseous peniafluoride PFj, discovered by Thorpe, is undoubt- 

 edly regarded by many as final proof of the pentadicity of this 



I The number ol Liebig' s Annalen last issued contains^a valuable paper by 

 KlingeranJ Maassen disproving Kriiger's statement. 



element ; but the existence of compounds such as HjFj, HFFR, 

 &c. , which clearly belong to the class of molecular compounds, 

 is an indication of so marked a tendency on the part of fluorine 

 to combine with itself, that for this reason alone (as Naumann 

 and others have asserted) the pentafluorideisby no means neces- 

 sarily regarded as an atomic compound. And I would here add 

 that stability affords no criterion as between atomic and mole- 

 cular compounds, every degree of stability being met with even 

 among those of the former class. 



An argument in favour of the pentadicity of phosphorus 

 which apparently cannot be disposed of by any explanation 

 based on conventional considerations has, however, been ad- 

 vanced by La Coste and Michaelis {Berichte, 1885, p. 21 18), 

 who have shown that the compounds obtained from diphenyl- 

 chlorophosphine, PCKCeHg).^, and phenol is not identical with 

 the triphenyl-phosphine oxide, OP(C(jH5)3, obtained by oxidiz- 

 ing triphenyl-phosphine, as it should be if the latter were a 

 compound of the formula {CQHr^.2V . OCgH., ; this last corre- 

 sponding to the formula ClgP . OCl, which has been suggested 

 as that of phosphorus oxychloride, and which appears to derive 

 considerable support from Thorpe's observations on the specific 

 volume of the oxychloride (Chem. Soc. Trans., 1880, p. 388). 

 It is, however, conceivable that the oxygen and phosphorus are 

 united by residual affinities, thus ; — 



D^ 



-CI 

 -CI 

 -CI 



03 



a 



CgHg 

 . CrHs 



Michaelis and Polls {Berichte, 1887, p. 52) have argued in the 

 case of bismuth, which also is a member of the nitrogen group, 

 that the pentadicity of this element is proved by the existence 

 of the triphenyl dibromide, (CoH5)3BiBro. But the mere pro- 

 duction of such a compound proves nothing so long as its con- 

 stitution is undetermined ; it at most serves to strengthen the 

 conviction gained from the general study of the element, that 

 bismuth is a member of the nitrogen- phosphorus group. 



In other cases also it is possible that undue importance may have 

 been attached to the existence of alkylic compounds of particular 

 types : thus lead, judging from its general chemical behaviour, 

 would appear to be a dyad ; yet the existence of the tetrethi.- 

 Pb(C2Hg)4, is commonly held to be a proof that it can functi< 

 as a tetrad. But the properties of lead are such that I am 

 tempted to suggest that it is one of the metals in which the 

 "charges " have but a small degree of freedom ; and it is con- 

 ceivable that the tetrethide is actually a compound of dyad lead, 

 each charge serving to bind two ethyl groups, thus : — 



H5C2 

 H5C., 



^ 



■C.2HS 

 -C2H5 



The same may be true of tin, although in this case the fact tha 

 we are dealing with an element of the carbon-silicon family tend 

 to favour the conclusion that it may be a tetrad. 



Also too much importance must not be attached to the exist- 

 ence of stable volatile chlorine compounds : thus tellurium tetra- 

 chloride may well be a compound of dyad tellurium, thus : — 



Iron, and the other members of the family which boron heads, 

 in like manner, I feel convinced, are triads even in their ic com- 

 pounds : recent vapour-density determinations all support this 

 conclusion. 



It IS even conceivable that chlorine may form closed-chain com- 

 pounds, and that a tetrachloride may exist, such as is represented 

 by the formula : — 



-eir 



-et- 



Te. 



-Gir 



-€i- 



I think it is especially noteworthy that so many well charac- 

 terized and comparatively stable double chlorides exist formed by 

 the union of chlorides of which one at least is persevevy unstable ; 

 the tin-sulphur chloride, SnC^ . 2SCI4, and the remarkable 

 series of aurous compounds recently described by Lepetit (.Ann. 

 Chim. Phys., 1887, p. 11) may be cited as examples. 



