298 



NATURE 



[November 3, 192 1 



stration of the practicability of mechanical flight 

 — and for the next stage, which is the commercial 

 and practical development of the idea, it is prob- 

 able that the world may look to others. The 

 world, indeed, would be supine if it does not 

 realise that a new possibility has come to it, and 

 that the great" universal highway overhead is now 

 soon to be opened." 



The Wright brothers are equally clear in their 

 acknowledgment of Langley's work:- — 



"The knowledge that the head of the most pro- 

 minent scientific institution of America believed in 

 the possibility of human flight was one of the in- 

 fluences which led us to undertake the preliminary 

 investigations that preceded our active work. He 

 recommended to us the books which enabled us to 

 form sane ideas at the outset. It was a helping 

 hand at a critical time, and we shall always be 

 grateful." 



One feels that in relation to such remarks by 

 the two great American pioneers of aviation 

 the matter under discussion in "The Langley 

 Machine and the Hammondsport Trials " is un- 

 important. The transactions appear to have been 

 rather sordid and to reflect discredit on those com- 

 mercial systems of the world which exalt " patent- 

 ability " at the expense of solid service which is 

 not patentable. 



The Impurity of Pure Substances. 



Die Theorie der Allotropie. By Prof. A. Smits. 

 Pp. xvi-f5oo. (Leipzig: Johann Ambrosius 

 Barth, 192 1.) 100 marks. 



AT one time the term allotropy or allo- 

 tropism was confined mainly to the 

 chemical elements, such as oxygen, carbon, phos- 

 phorus, etc. In one of these cases — that of 

 oxygen and ozone — the explanation of the allo- 

 tropie forms was found in the existence of dif- 

 ferent molecular species, whereas in the case of 

 diamond and graphite, for example, this could 

 not be definitely proved. Another more striking 

 case of the latter sort is presented by the rhombic 

 and monoclinic forms of sulphur. As our know- 

 ledge increased concerning the different crystalline 

 forms, in which both elements and compounds 

 could occur, the idea of crystalline polymorphism 

 was developed. In these cases it was supposed 

 that one and the same molecular species only was 

 involved, that the polymorphism was due to dif- 

 ferent crystalline arrangements of the same units, 

 and that the vapours or solutions derived from 

 the different polymorphic forms would, for the 

 same values of pressure, temperature, and con- 

 centration, be chemically and physically identical. 

 This explanation certainly gave a satisfactory 

 NO. 2714, VOL. 108] 



account of the main facts involved — that is to 

 say, the various substances appeared to behave 

 as one-component systems. The curious facts 

 relating to the behaviour of water suggested, 

 however, to chemists that this substance was in 

 reality complex, water consisting at any given 

 temperature and pressure of a mixture of mole- 

 cular species differing in the degree of association 

 of the simplest (Avogadrian) molecular type. 



This idea of an inner complexity, or inner equi- 

 librium of simple and associated molecules, was 

 gradually extended to other classes of "pure " sub- 

 stances — that is to say, substances the thermo- 

 dynamical behaviour of which was that of a one- 

 component system. In spite of this inner com- 

 plexity, it was realised that such substances would 

 behave as one-component systems so long as, 

 with variation of the "external" parameters 

 (pressure, temperature, etc.), the inner equilibria 

 were readjusted with a speed incomparably 

 greater than the rate of variation of the para- 

 meters referred to. Cases gradually became 

 known, however, where certain one-component 

 systems exhibited a pseudo-binary character. 

 Thus the researches of Alexander Smith, Aten, 

 Kruyt, etc., on sulphur showed that liquid 

 sulphur must be a mixture of different molecular 

 species, for if the inner equilibrium existing at 

 any given temperature were " frozen " (or partly 

 so) by the use of certain inhibiting agents (so- 

 called negative catalysts], liquid sulphur showed 

 a marked pseudo-binary behaviour. 



Such phenomena were easily understood owing 

 to the knowledge possessed by chemists of the 

 behaviour of nitrogen peroxide. Here there is a 

 well-known inner equilibrium (of a simple and an 

 associated form) existing not only in the pure 

 liquid state, but also in the states of -vapour and 

 solution. The researches of Prof. H. B. Baker 

 showed that the speed of readjustment of this inner 

 equilibrium is positively catalysed by very minute 

 traces of water, and that it may be "frozen " by 

 very prolonged and intensive dehydration. We 

 may, in fact, have at a given temperature and 

 pressure an infinite number of different liquid or 

 gaseous "nitrogen peroxides," the former one- 

 component system passing^ thus into an infinite 

 number of states corresponding to a two-com- 

 ponent system. 



About fourteen years ago Prof. Smits, on the 

 basis of certain experimental results which he 

 had obtained, took a very daring and interesting 

 step. It occurred to him that perhaps all known 

 cases of allotropy, phase-polymorphism, etc., 

 were due to the existence of different inner equi- 

 libria of two or more different molecular species, 



