October 20, 19-ii] 



NATURE 



259 



source of loss and confusion to potato producers and 

 merchants. The synonym committee of the institute, 

 consisting of experts both scientific and practical, 

 makes an annual report which declares which of the 

 varieties entered for the immunity trials under new 

 and distinct names prove to be identical with varieties 

 already in the market. 



During their visit the King and Queen were able 

 to see the normal work of the Official Seed Testing 

 Station at progress in most of the laboratories. The 

 exhibits of cereals and potatoes arranged in two of the 

 laboratories enabled the visitors to realise the scope 



of the other branches of the institute's work. 'Iheir 

 Majesties were keenly interested in everything that 

 was shown to them, inquiring minutely into the pro- 

 cesses of seed-testing, and pa\-ing special attention 

 to the methods of potato-breeding and the measures 

 taken to check the spread of wart disease. At the 

 close of their visit they expressed to Sir Lawrence 

 Weaver their complete satisfaction with all that they 

 had seen and their admiration of Mr. Border's 

 planning of the buildings and the arrangements- 

 made to secure the -efficiency and comfort of the 

 staff. 



Chemical Reactivity and the Quantum Theory. 

 By Dk. Eric K. Rideal. 



THE recent discussion held by the Faraday Society 

 on modern developments in the theories of 

 catalytic chemistry gave rise to an important debate 

 concerning what lias been termed the radiation theory 

 of chemical action. It is now almost generally 

 accepted, both by the protagonists and by some of the 

 opponents of this theory, that molecules of the same 

 species in a reacting system may differ from one 

 another in what is termed chemical "■ activity." Thus 

 in a mixture of hydrogen and oxygen a certain 

 fraction, both of the hydrogen and of the oxygen 

 molecules, are "active." Collision between active 

 molecules of the two species results in chemical com- 

 bination ; collision between inactive molecules pro- 

 duces no chemical change. 



It is further argued by the supporters of the theory 

 that true monomolecular chemical reactions exist, 

 e.g. the conversion of allotropes, the dissociation of 

 a diatomic gas, or the decomposition of substances 

 like phosphine or ammonia ; consequently, as pointed 

 out by Perrin, "'active " molecules must 'exist per 

 se, and reaction is not the result of a particular kind 

 of directive collision, or, indeed, of a collision taking 

 place at some particular phase of the molecular vibra- 

 tion ; the decomposition of phosphine thus finds an 

 analogy in the disruption of radium. 



Granting that this assumption is correct, atten- 

 tion has to be directed to the source of energv of 

 activation. The opponents of the theory, who go as 

 far as to admit the validity of the first postulate, 

 affirm that this energy resides within the molecule 

 itself, and may possibly be identified with the "null 

 punkt " energy at absolute zero. 



The supporters of the radiation theory adopt the 

 hypothesis that the energy of activation is acquired 

 from the circumambient radiation, and that in con- 

 sequence all reactions are in the broadest sense of 

 the term photochemical. The energy supplied to one 

 molecule so as to make it '"active" to undergo the 

 given reaction, whether it be explosion or combina- 

 tion with another active molecule, is assumed to be 

 supplied by radiation of a particular frequency, and 

 in amount equal to ]iv, where h is Planck's constant. 



For all the ordinary chemical reactions the amount 

 of energy of activation to be supplied, as calculated 

 by application of the fundamental equation of 

 Arrhenius to the temperature coefficient of the 

 reaction, is sufficiently small as to permit of the 

 utilisation of quanta in the infra-red portion of the 

 spectrum ; for some reactions, however, visible or 

 ultra-violet light will be necessary, whilst for accelerat- 

 ing the rate of change of radio-active decomposition 

 ultra-X-rays would be required. 



The equation of Wien on radiation intensitv, and 

 of Arrhenius on the temperature coefficient of chemi- 

 XO. 2712, VOL. 108] 



cal reactions bear a formal resemblance to one an- 

 other, and it is not doubted that the same funda- 

 mental properties of matter and of radiation account 

 for the similarity. It is further admitted that the 

 quantum theory- of J'lanck, applied by Bohr to the 

 internal structure of the atom, is likewise valid in 

 manv physical and chemical operations, such as cal- 

 culation of the latent heats of change of state, the 

 heats of formation of chemical compounds, including 

 heats of ionisation and the photoelectric effect. More 

 recently the quantum theory has been applied with 

 success to a general study of reaction kinetics, and 

 it is now evident that there is no essential difference 

 between a typical monomolecular chemical reaction, 

 such as the decomposition of phosphine, and a 

 phvsical reaction like evaporation. It cannot be 

 doubted that both physical and chemical forces are 

 identical in their nature and also in their mode of 

 action. 



The opponents of the theor}- admit these premises, 

 but see no reason to assign to radiation the important 

 role given to it by its adherents, and prefer to attri- 

 bute the two phenomena to some common, but as yet 

 unknown, property, giving rise to these apparent 

 similarities. 



The supporters of the theory point out that in 

 fact many photochemical reactions do exist, and, thus 

 admitting the possibility of the direct action of radia- 

 tion on matter in causing both physical and chemical 

 change, there is^ no reason why this property should 

 not be universal. 



In the development of the theory in its quantitative 

 aspects, however, certain difficulties have arisen 

 necessitating a modification of the simple theor}' 

 originally proposed ; thus the rate of decomposition 

 of phosphine has been accurately determined over a 

 wide range of temperature, the frequency of the radia- 

 tion necessar\" to bring about its decomposition cal- 

 culated from the reaction temperature coefficient, and 

 the amount of energy flowing into the reaction 

 chamber per c.c. per second at the observed tem- 

 perature calculated from Planck's law. It has been 

 found that there is not enough energy supplied by 

 radiation to account for all the explosions actually 

 observed. 



To account for this serious discrepancy several 

 hvpotheses may be advanced. Thus we may assume 

 that during the explosion ot one phosphine molecule, 

 which has already been activated by the absorption of 

 one quantum of radiant energy, energy is radiated 

 and absorbed by another molecule or bv other mole- 

 cules. Since it is not permissible to assume ab- 

 sorption in fractions of a quantum, it is necessary to 

 adopt an hypothesis of activation of the phosphine 

 molecule - by a number of smaller quanta (infra-red) 



