3o6 



NATURE 



[November 3, 192 1 



Qualities of Valency. 



In a letter under the above heading in Nature for 

 October 13, p. 210, Dr. R. M. Caven directs attention 

 to the difficulty felt by chemists in accepting Dr. 

 Langmuir's view that the sodium and chlorine in 

 sodium chloride are not united by a chemical bond, 

 being always ionised, although molecules of sodium 

 chloride actually exist in a state of vapour at 2000° C. 

 Perhaps the following considerations may help to re- 

 move the difficulty. 



In the first place it is necessary to distinguish be- 

 tween the two separate processes of intramolecular 

 ionisation and electrolytic dissociation. Intramolecular 

 ionisation is the expression introduced by Sir J. J. 

 Thomson to describe the transfer of an electron from 

 one atom to another within the molecule, an "ionic 

 molecule " being thereby produced. Electrolytic disso- 

 ciation is the breaking up of such an ionic molecule 

 into separate ions. 



Sodium chloride is therefore an intramolecularly 

 ionised compound, and the chemist's difficulty is the 

 question of how the ions are united in the molecule. 



Now the conception of the valency bond can be 

 retained if we accept the convention first suggested 

 by Sir Oliver Lodge (Nature, vol. 70, p. 176, 1904), 

 according to which the electron and positive charge 

 are united by a very large number of lines of force 

 when in combination. In the chloride ion we have 

 a kernel with seven positive charges surrounded by 

 eight electrons. The electrons will, therefore, be un- 

 saturated, and in a molecule of sodium chloride we 

 shall have a bundle of lines of force passing from the 

 electrons of the .chloride ion to the nucleus of the 

 sodium ion. This is a typical instance of a strong 

 electrolyte. 



In non-electrolytes two or more electrons are shared 

 between two atoms, giving Dr. Langmuir's covalency 

 bond. In this case, without specifying the particular 

 electrons involved, we may assume a double bond con- 

 sisting of two equal bundles of lines of force passing 

 in opposite directions, that is to say, in a molecule 

 AB, one bundle will pass from the electrons of A to 

 the nucleus of B, and the other one from the elec- 

 trons of B to the nucleus of A. All intermediate 

 stages of combination between these two extremes are 

 possible, as I have pointed out in a series of papers, 

 in which the subject is discussed in detail (Trans. 

 Chem. Soc., vol. iii, p. 253, 1917; vol. 115, p. 278, 

 1919; Phil. Mag., vol. 42, p. 448, 1921). 



Full consideration of the question, as shown in the 

 papers referred to, leads to the conclusion that a 

 simple and all-embracing theory of valency is not 

 possible, but that different theories of valency must 

 be devised for different types of compounds, such as 

 electrolytes, non-electrolytes, and molecular com- 

 pounds. In view of this difficulty I have used affinity 

 formulae only, and it would appear that the time is 

 approaching when the chemist will have to decide 

 whether the conception of valency can be retained for 

 general purposes, or whether it would not be better 

 to restrict its use to certain special branches of the 

 science, such as the chemistry of carbon compounds, in 

 which it has proved of supreme value. 



To the inorganic chemist the valency conception 

 has been of doubtful value. This is particularly 

 noticeable if we compare the rapid development of in- 

 organic chemistry since Werner introduced the co- 

 ordination theory less than thirty years ago with the 

 slow rate of progress in the previous thirty years 

 under, the valency theory. 



In anv case the restricted use of the valency bond 

 to the particular type of combination termed co- 

 valency by Dr. Langmuir will scarcely be accepted by 



NO. 2714, VOL. 108] 



chemists if the conception of valency is to be retained 

 for general purposes. 



The other facts mentioned by Dr. Caven all indicate 

 that there is no clear-cut distinction between electro- 

 valency and covalency, but that they represent ex- 

 treme types of combination with an indefinite number 

 of intermediate grades. S. H. C. Briggs. 



October 24. 



Relation of the Hydrogen-ion Concentration of the Soil 

 to Plant Distribution. 



With reference to Dr. W. R. G. Atkins's interesting 

 communication in Nature of September 15, may I be 

 allowed to submit the following comments? The im- 

 portance of the study of hydrogen-ion concentration in 

 physiology and biochemistry, soil science, and other 

 branches of research cannot be over-emphasised, and 

 is rapidly becoming appreciated by workers. There 

 appears, however, to be a tendency to apply methods 

 of measurement that have been standardised in one 

 branch of study to other departments — soils, for 

 example^ — with a minimum change of technique. 

 With the exception of Gillespie's pioneer, but by no 

 means exhaustive, work in America, the colorimetric 

 method of measuring the hydrogen-ion concentration 

 of soils has never been critically examined. After 

 considerable preliminary work with this method — an 

 account of which has been published elsewhere — the 

 present writer feels that the conditions under which 

 it can be applied in soil work so as to yield accurate 

 and reproducible results have not. yet been fully worked 

 out. As an example may be cited the fineness of 

 division of the soil sample which is often a factor 

 influencing the apparent ^H as determined colori- 

 metrically. Until much more work has been done from 

 this point of view the data being accumulated by 

 ecologists can scarcely have the strict quantitative 

 significance often attached to them, although when 

 regarded as provisional only they are undoubtedly of 

 great interest and no little importance. 



A further point is perhaps worthy of attention : it 

 mav possibly happen that in some soils the actual ^H 

 at the moment of measurement is of less importance 

 than the rate of change of the ^H under natural condi- 

 tions. The "buffer effects" imposed by the nature 

 of the soil on its reaction vary enormously in magni- 

 tude from soil to soil ; a dressing of basic slag may 

 alter considerably the ^H of a light sand while having 

 no appreciable effect on that of a heavy loam ; and 

 the pH of a light soil may vary regularlv or erratically 

 with fluctuations in local conditions. Such variations 

 mav be important in many cases, and would well repay 

 studv. The whole problem of the nature of soil re- 

 action is complicated, but some light would un- 

 doubtedlv be thrown upon it by measuring, not merely 

 the pH of soils, but the variations in pU with additions 

 of acids and alkalis, i.e. by plotting titration curves 

 the slopes of which can be correlated with "the magni- 

 tude of the buffer action of the soil. Very little work 

 along these lines has so far been done in this country 

 — or, indeed, in anv country — but that little ha< 

 afforded indications that such work would be fruitful 

 of result if attacked svstematically and with due 

 appreciation of all the difficulties. 



E. A. Fisher. 



The University, Leeds, September 28. 



Absorption of X-rays. 



At the suggestion of Prof. Richardson, we have for 

 some time been engaged in an investigation of the 

 connection between X-ray absorption coefficients and 

 critical frequencies. In this work we have met with 



