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must be in some of the electronic systems within the mole- cules; obviously, when the oxygen in no way has its attention en- gaged by hydrogen, its influence must in some way be felt more in the neighbouring carbon system; these, however, are dynamic changes, not evident in a model. The limitation is one to which our structural formulae have always been subject.

Whatever be the distribution of affinity in the carbon atom, in compounds it appears to be greatly modified, so that structural models, like structural formulae, must be interpreted with cau- tion. To take a simple case, the change from benzene to hexa- methylene seems to involve merely the addition of six more hydrogen atoms at the periphery, not the direct neutralization of the two sets of three affinities at the free carbon surfaces.

This pulling down of the affinities into two planes seems to be a general rule. Mr. Barlow finds, for example, that the model of tartaric acid constructed on this principle is in absolute accord with the crystallographic peculiarities of the acid.

Fig. 2 shows front and back views of the arrangement of the units in glucose, CeH^Gv The carbon units may be distin- guished without difficulty as the grey spheres, the cross denoting the position of the pyramidal apices; the white spheres are the hydrogen units; the twin dark spheres the oxygen atoms. Each layer consists of a succession of rows of four spheres. The free carbon area is of considerable extent at both surfaces: it will be obvious that, if some degree of residual affinity were exercised over these areas, successive layers of molecules might well be attracted into position, if once a single layer were deposited, as in crystallization.

FRONT

BACK

FIG. 2.

Front and Back Views of the Arrangement of Units in Glucose. (From Journal of the Society of Arts, Sept. 12 1919.)

Chemical Change: Determinant and Catalyst. Although the subject of chemical change has been much discussed of late years, the chief advance has been in the attention paid to catalysts, which have acquired popularity owing to their use in a number of industrial processes. Although it is recognized that often some determining agent is required to condition an interaction, and the feeling is widespread that this is more generally true than has been supposed, the primary conditions of chemical change are seldom set forth; seldom is the practice departed from of using simple equations in which the two agents and the resultants alone appear, the need of a third substance being left out of ac- count and unindicated.

The determining process is spoken of with increasing frequency as catalysis, the supposed agent being termed the catalyst. The conception was introduced by the great Berzelius in 1835, and was applied by him to such diverse changes as the hydrolysis of starch by acids and also by diastase; the oxidation of various sub- stances in presence of platinum, especially when used in the finely divided state (platinum black) ; and the formation of ether from alcohol by means of sulphuric acid. Berzelius drew no distinc- tion between interactions in solution and those in which solids such as platinum were involved. It is clear that he took an elec- tro-chemical view of the process his opinion being that the office of the catalyst is to awaken slumbering affinities through its presence and to determine a greater electro-chemical neutraliza- tion ; probably no one has been nearer to our modern conception. Whether he had been in any way influenced by Faraday's electro- chemical researches of 1833-4 is not clear.

Now that it is so generally admitted that Faraday's dictum is to be accepted, that ordinary chemical affinity is a consequence of the electrical attractions of the particles of different kinds of matter and that the forces termed chemical affinity and electri- city are one and the same, it may be asserted, as a necessary co- rollary, that the conditions which determine chemical action are those which determine electrolytic action.

Speaking generally, it may be affirmed that two substances cannot interact ; a third must be present to determine the neces- sary slope of potential and flux of current. Thus zinc, whether highly purified or amalgamated, is all but unattackedbyanacid, and it is logical to assert that, if pure, it would be unattacked; when coupled with an electro-negative inert conductor, it is dissolved, indeed, very rapidly, if the resistance in circuit be small. Or to condition attack, a depolarizer may be used, as in the case of copper, for example, which readily dissolves in dilute sulphuric acid in the presence of oxygen.

The rule appears to be that the three necessary factors must be conjoined in a conducting circuit ; one of them must be an elec- trolyte; one of the remaining two and the electrolyte must be substances that can interact ; the third may or may not take part in the chemical interchange if it takes part, by acting as a depo- larizer, it adds to the efficiency of the change, raising the electro- motive force. Brereton Baker, in the course of his refined studies on the influence of water as a determinant of chemical change, has given abundant proof of the accuracy of the above- given definition, particularly by showing that a mixture of hydro- gen and oxygen cannot be fired, even in presence of water; and that interaction takes place only when an impurity is present, which impurity, together with the water, forms an electrolyte. A trace of acid suffices.

Years before this result was obtained, it was possible to pre- dict that water alone would not condition the interaction of hy- drogen and oxygen because it was not an electrolyte. It is true that this contention is not generally admitted it is held that water per se has a slight conductivity; but this conclusion in- volves the unjustified assumption that Kohlrausch dealt with pure water and that the minimal conductivity which he observed was an intrinsic property of water. The course followed by Kohl- rausch in purifying water, however, involving as it did nothing more than distillation within closed glass tubes, was by no means a refined one from our modern point of view; to assume that he had reached finality is absurd: it is impossible to obtain a vessel without surface impurity which is not open to attack; access of atmospheric impurity cannot be entirely prevented ; further, some form of electrode must be used ; pure water, therefore, will ever remain an ideal, and whilst it is logical to extend the curve repre- senting the loss of conducting power, as the impurity is reduced, to zero origin, on no theoretical grounds are we called on to be- lieve that it should come to rest short of this point.

Especially is this the case, in view of the conclusion of the dis- sociationist school, that in aqueous solutions the dissolved sub- stance is alone resolved into its ions: muriatic acid, for example, is assumed to be a mixture of undissociated molecules of water with the separated ions, H and Cl. Unfortunately, the modern chemist too often lacks feeling for his material, and, without sympathy, understanding is impossible. It is impossible to put two so closely related and similar compounds as water (H 2 O) and hydrogen chloride (HC1) on the different planes they neces- sarily occupy, if the one be regarded as all but entirely stable and the other as entirely unstable.

That a profound chemical change takes place on bringing to- gether the two compounds, hydrogen chloride and water, is be- yond question. To regard the water as inert is impossible if it were, the gas would not be so attracted as it is. Equations such as the following are not merely rational but necessary expressions in illustration of the changes that may occur:

Cl H

The part played by water in activating hydrogen chloride may be compared with that of magnesium in Grignard's well-known