NA TURE 



409 



THURSDAY, FEBRUARY 28, 1895. 



WHAT DOES THE CHEMIST MEAN 

 BY " VALENCY"? 

 Handbuck der Stereochemie. Von Dr. C. A. Eischoff, 

 unter Mitwirkung von Dr. P. Walden. II. Band 

 (Schluss). (Frankfurt: H. Bechhold, 1894.) 



THE author of the review of the first part of this book 

 (Nature, vol. xII.k. p. 409) has given an account 

 of it with which the present writer entirely agrees. Dr. 

 Bischoffs book embodies a tolerably complete epitome 

 of all that is known of a subject which at the present 

 time has probably a larger number of cultivators than 

 any other department of chemistry, and as a register of 

 facts and references to original sources of information it 

 is indispensable to the worker engaged upon stereo- 

 chemical problems. On the other hand, the book is not 

 only eminently unreadable, but the arrangement is some- 

 times far from clear. 



Of this second and concluding volume about one third 

 is occupied with an enumeration, continued from vol. i., 

 of the various known cases of isomerism, inorganic as 

 well as organic, which may be possibly referred to the 

 geometrical hypothesis. The second third deals with 

 chemical changes, such as ring formation and intramole- 

 cular changes regarded from the stereochemical point 

 of view. The last part consists of additions to preced- 

 ing pages, so as to bring the matter up to the most recent 

 date possible. Examination of a volume such as this 

 compels the reflection that spite of the great array of 

 established fact, confusion still prevails among stereo- 

 chemical writers, chiefly for want of some common agree- 

 ment as to fundamental ideas. 



"Atomicity," as it used to be called, meant originally 

 the power exhibited by the several elements to combine 

 in the proportions of one atom to I, 2, 3, 4 or n atoms of 

 some other element. Hence arose the idea of diflTerent 

 atomic values in the process of exchanging one element 

 for another. Later, from these notions about "quanti- 

 valence" it was assumed (without sufficient evidence) 

 that when an atom, say, of carbon combines with lour 

 other atoms, the force of attraction or union for each of 

 these four atoms is the same. Attempts were then made 

 to estimate by thermal methods the mechanical value of 

 each unit of " atomicity" or " quanti valence." 



During all this time much discussion has occurred as 

 to what becomes of units of "valency," to use the more 

 modern term, which are not exercised as in unsaturated 

 compounds generally, and Frankland's idea that even 

 the " bonds " belonging to one and the same atom may 

 saturate each other in pairs has been commonly used. 



Since the observations of Van t'Hoff and Le Bel on 

 the relations of chemical constitution to optical activity, 

 and the revival of the hypothesis of spacial arrangements, 

 and especially in the generally adopted theory of tetra- 

 gonal carbon, the idea of direction which is implied in 

 all sp.ice arrangements has been universally adopted. As 

 to whether the direction in which valency can act is 

 fixed or variable, opinion seems a little divided, and yet 

 it is obvious that double linkage between two atoms, or 

 the formation of any closed chain of tetragonal carbon 



NO. 1322, VOL. 51] 



atoms (whether conceived according to Van t'Hoff or 

 Wunderlich, or otherwise), would be impossible on the 

 assumption that the valency of carbon atoms can act only 

 in certain directions which are quite fixed. C oncerning 

 this question the conclusion seems inevitable that we 

 must suppose something analogous to a magnetic field 

 between combining atoms. 



But what, after all, is valency? 



The fact is that while one atom of chlorine combines 

 habitually with one atom of hydrogen, an atom of oxygen 

 combines with two atoms of hydrogen, an atom of nitro- 

 gen with three, and so forth, and it is hardly assumption 

 to say that so long as the hypothesis of atoms is used to 

 express chemical combination,. rtiw^ geometrical arrange- 

 ment of combined atoms in space is an inevitable part 

 of the hypothesis. This was long ago pointed out by 

 VVollaston. The valency of an element is then the 

 numerical expression of its habit of combination. It is 

 well known to be variable, and to depend to a great 

 extent upon physical conditions. It may be that one 

 atom can only act upon another in certain directions, 

 but this does not render necessary the assumption that 

 each atom is always doing something in these several 

 directions, whether other atoms are present or not. This, 

 however, is what is assumed by those who use the term 

 " bond " in any of the recognised senses, or who attempt 

 to determine the dynamical value of units of valency. 

 There is some evidence in favour of the hypothesis that 

 combination can only occur when atoms get within 

 a certain maximum distance from each other, and the 

 possibility that isomerism may result from changes in 

 the distance between the atoms united within a molecule, 

 is deserving of greater consideration than it has hitherto 

 received. If we think, for example, of the exchange of an 

 atom of chlorine for an atom of hydrogen in, say, marsh 

 gas, it is difficult to believe that the chlorine atom 

 occupies exactly the same place relatively to the carbon 

 as the hydrogen atom for which it is substituted. 

 Reasons may possibly be found for this in the difference 

 of the force of "affinity" between carbon and chlorine 

 on the one hand, and between carbon and hydrogen on 

 the otner, or it may result from the " residual valency " of 

 chlorine acting upon the neighbouring hydrogen atoms, 

 or from the difference of "mass" of the chlorine and 

 hydrogen. But throughout all the various hypotheses 

 used generally by chemists, runs the idea of direction 

 variable within narrow limits, already referred to. Now, 

 however much the direction along which a force acts 

 may be supposed to vary according to circumstances, it 

 is inconceivable on any mechanical principle that one 

 force can act in several directions at once unless there is 

 reason to suppose that it is divided in some way into 

 component forces. Hence the " centric " formula for 

 benzene, which has lately come into vogue, represents a 

 wholly new idea which is incompatible with previously 

 received ideas about valency and with the hypothesis of 

 the tetragonal carbon atom. According to this formula 

 an atom of carbon is capable of acting in eight different 

 directions — that is, it acts directly, in the conventional 

 sense, upon one atom of hydrogen and upon two atoms 

 of carbon, at the same time that it " influences " these two 

 atoms of carbon and three others. This idea is wholly un- 

 necessary to the explanation of the stability of the benzene 



T 



