NA TURE 



[August 6, 1896 



vague atomic groupings of to-day be succeeded by definite 

 systems, in which each atom will have its orbit mapped 

 out with ever-increasing minuteness ; for as long as the 

 atomic theory endures, so long will it become more and 

 more of a mechanical theory ; and indeed it would be 

 absolutely inconsistent, when we are perpetually striving 

 to arrange the atoms of a molecule into groups, to give up 

 all attempt to determine the relative positions and motions 

 of the groups and of the atoms within them. It is as 

 true to-day as it was when Kekule published the state- 

 ment in his " Aims and Achievements of Scientific 

 Chemistry," that as the great present aim of physics is 

 the elaboration of a system of molecular mechanics, so 

 the great present aim of chemistry is the elaboration of a 

 system of atomic mechanics, in which every reaction will 

 be accounted for by the mass and motion of the reacting 

 atoms. This may be deplorable ; but those who think it 

 most so, most keenly realise that it is true. 



For instance, quite recently, in his plea for " Emanci- 

 pation from Scientific Materialism," ' Prof. Ostwald 

 wrote : — 



" We read and hear with countless repetition the 

 statement that the only intelligent explanation of the 

 physical world is to be found in a ' Mechanics of the 

 Atoms ' ; matter and motion appear as the final prin- 

 ciples to which natural phenomena in all their variety 

 must be referred." 



With regard to physics, a similar acknowledgment is 

 contained in the words of Duhem, uttered in 1894 : — 



" When the science of motion ceases to be the first in 

 logical order of the physical sciences, and becomes only 

 a special case of a much more general science, which 

 embraces in its formulre all the changes of bodies, the 

 temptation will be less to try to reduce to the study of 

 motion the study of all physical phenomena ; it will be 

 better understood that change of position in space is a 

 problem no simpler than change of temperature or of 

 any other physical property. Then we shall more easily 

 avoid the most dangerous reef of theoretical physics 

 — a mechanical explanation of the universe." {Jour, de 

 Mathanaiiyues, x. 207.) 



Such statements as these are valuable, in that they 

 remind us that even the most necessary of our present 

 theories is a temporary makeshift — a crutch which in- 

 dicates the weakness that it helps, and which we may 

 hope to be able to discard. 



This might be said, however, of most things that 

 are useful ; and it must be remembered that the same 

 theory is not the best for every one. For each man that 

 theory is the best which is the most stimulating, which 

 best spurs him on to useful work, which urges and 

 guides him forward into the unknown. Another theory 

 may have more facts in its favour, but if these facts do 

 not specially interest the worker in question, it will be of 

 less value to him than a theory, otherwise inferior, which 

 enables him vividly to realise, and aptly to utilise, those 

 facts which do interest him. 



Moreover, even if we admit that the atoinic theory 

 may be near the end of its existence, and that it may, 

 and should, shortly be superseded by a more widely 

 useful theory, it must yet be maintained that the way to 

 hasten this consummation is to push the theory with all 

 rapidity, and in every direction, to its extreme con- 

 sequences, in the full assurance that, so far as it is 

 incomplete, this will be the quickest way to demonstrate 

 its deficiencies. 



Now, among the consequences of the atomic theory, 

 the consideration of the space relations of the atoms 

 occupies the first place ; it is not an extreme, but an 

 immediate and a necessary consequence. For this 

 reason alone, if stereochemistry did not exist, it would 

 be necessary to invent it. But to find a raison dctrc, 



^ Science Progress^ February 1896. 



NO. 1397, VOL. 54] 



stereochemistry needs no such arguments. It has justi- 

 fied its existence by its achievements. 



The stereochemical explanation of the existence and 

 properties of the two different substances formed when 

 a carbon atom unites with four dissimilar groups of 

 atoms, has long been generally admitted. As to the 

 exact three-dimensional formuhv by which we should 

 represent these two substances, both of which correspond 

 to the ordinary formula CR'R-R''K\ differences of opinion 

 exist ; but it is certain that the formuhc must resemble 

 those given in the figures (1 and 2), in so far as these 

 represent three-dimensional arrangements, each unsym- 

 metrical, but such that the two together form a sym- 

 metrical whole ; in other words, each being the tnirrored 

 image of the other. And space-formula-, in these rc- 



Q 







O 



.0 o. 



o 



spects similar, must be admitted for the two compounds 

 formed by the union of a nitrogen atom with five different 

 groups. 



It is true that, beyond this, the services of stereo- 

 chemistry are questioned by some chemists. Yet it can- 

 not be denied that the tetrahedral grouping of the atoms 

 combined with carbon forms a connecting link between 

 whole groups of facts, in the most varied branches of 

 organic chemistry, which, without it, would have been 

 left in comparative isolalion. But without entering into 

 the necessarily complicated discussion of these develop- 

 ments, it may be shown, by the consideration of a single 

 instance, that the simple original conception of the three- 

 dimensional asymmetric grouping of dissimilar atoms 

 about the carbon-atom to which they are attached, 

 enables stereochemistry not merely to follow in the steps 

 of structural chemistry, and to explain many anomalies 

 which the latter leaves unaccounted for, but to push its 

 investigations in advance, and to declare the space- 

 relations prevailing in the molecules of substances as 

 yet never analysed, and even never isolated. 



The action on polarised light of a substance in solution 

 is a test for the asymmetric grouping of the atoms in its 

 molecules. Just as when we find a substance crystallising 

 in two forms, such as Figs. 3 and 4, having the relation 

 of the right and left hands, we know that these crystals 

 will have the power of rotating the plane of polarised 

 light to the right and to the left respectively ; so when 

 we find that a dissolved substance exerts a one-sided 

 action on the light, we know that it possesses a one-sided 

 molecule capable of existing in the right- and left-handed 

 forms (Figs, i and 2) ; which, it will be observed, bear the 

 same relation to each other as the crystal forms 3 and 4. 



Further, it is known that although the two members of 

 a pair of substances like those shown in Fig. i and 

 Fig. 2, through the identity of their atoms and the 

 equality of the distances dividing them, show no 

 difference in their behaviour towards any ordinary sub- 

 stance, yet they differ entirely in their behaviour towards 

 molecules which are themselves asymmetric. To go 

 back to Pasteur's simile, they resemble equal screws 

 with their threads turned in opposite directions. Both 

 will fit the same hole equally well if it is an ordinary 

 hole ; but if it is a hollow screw, then everything will 

 depend upon whether the thread of the hollow screw is 

 right- or left-handed. 



Conversely, if towards any substance the right- and 



