JULY 22, 1920] 



NATURE 



643 



choosing the concentrations, for if we re-write the 

 reversible reaction 



CU+C1',:XC1.C1'+C1.C1' 



it transpires that we have chosen for the concentration 

 of the resultants, because they are the same, the sum 

 of their individual concentrations, although for the 

 reactants, which also are chemically the same, the 

 individual concentrations have been taken. It is clear 

 that it is the individual concentrations in both cases 

 that have to be taken, and therefore that one-half of 

 the CI. CI' concentration is involved. Then ki^k^. 

 So with any reaction of this type involving two mole- 

 cules, apart from the question of isotopes altogether, 

 the 4 that always appears in the conventional text- 

 book examples is merely a consequence of a loose and 

 physically unjustifiable mode of representing the con- 

 centrations. Writers of future text-books might 

 ponder a little over this. If the same change in the 

 choice of the concentrations is made in the thermo- 

 dynamical argument, the difference of entropies, 

 R logg 4, reduces apparently to R log^ i =0. 



I have now made some progress in the application 

 of probability considerations to the kinetics of the 

 reaction. The distribution already given (i) has refer- 

 ence merely to the manner in which the two kinds of 

 atoms will arrange themselves among the three kinds 

 of molecules, assuming promiscuous combination be- 

 tween the two kinds, the two kinds being identical. 

 But the particular distribution obtained does, I find, 

 depend upon the kind of recombination assumed. As 

 regards the dissociation of the molecules into atoms 

 prior to their jecombination, the matter appears 

 straightforward, at least so far as I have got. Thus 

 whether one supposes that in a certain time a certain 

 fraction of collisions, the same on the average for 

 each kind of molecular collision, is fruitful in dis- 

 sociating the two molecules into four atoms, or one 

 regards the dissociation as monomolecular, as pre- 

 sumably it would be if light were the dissociating 

 agent, one arrives at the same result, that if x, y, and 

 z denote fractional proportions of Cl2,Cl'2 and CI. CI' 

 molecules respectively (x + y+s=i), the relative rate 

 of disappearance of each by dissociation is similarly 

 denoted. By equating this rate of disappearance to the 

 rate of formation for the three kinds of molecules, one 

 gets the equilibrium distribution. The distribution 

 given by (i) is got in this way, whether (i) all the 

 atoms of the two kinds recombine promiscuously or 

 (2) the four atoms formed in a single fruitful col- 

 lision recombine again only among themselves. If 

 a similar limitation be applied to a monomolecular 

 "dissociation, obviously the reaction cannot affect the 

 distribution at all, which remains unchanged whatever 

 the initial distribution. But I also found by in- 

 advertently applying the law of promiscuous recom- 

 bination separately to each of the nine cases that have 

 to be taken into account on the collision view— since 

 there are three tvpes of molecules which mav collide 

 with any one of the three types — instead of to the 

 sums of each of the two kinds of atoms produced, 

 that a very extraordinary equilibrium distribution 

 resulted, given by 



CL : CI', : Cl.Cl' : : in(i -h2n) : i(3-2«)(i-„) :|n(i-n), 



which leads to the curious concentration equation 



[CI jci',]=j( [ci.ci']»+ [CLcn }. 



This in the case n = o-^ happens to reduce to Mr. 

 Chapman's relation (i) (Nature, June 17, p. 487). 



The case, of course, has no phvsical meaning, but 

 \t may serve to show that the equilibrium distribution 

 Is sensitive to the particular assumptions made as to 

 NO. 2647, VOL. 105] 



the type of reaction which occurs. 1 do not imagine 

 I have exhausted the physical possibilities, but, so far 

 as I can see, my distribution relation (i) covers the 

 physically conceivable cases, and therefore the half, 

 not the whole, concentration of the substance under- 

 going a bimolecular reaction with itself ought to enter 

 into the equilibrium equation. 



Frederick Soddy. 



Science in Medical Education. 



The discussion at the British Medical Association 

 on July I on the plac^ of preliminary science in the 

 medical curriculum seemed to indicate practical 

 unanimity on some points, such as the need for a 

 higher minimum standard of general education, the 

 raising of the minimum age tor the registration of 

 medical students to seventeen years, and the neces- 

 sity for the maintenance of a high standard of instruc- 

 tion in physics, chemistry, and biology. There was 

 no indication of the desire on the part of any one of 

 the speakers to reduce the present standard of require- 

 ments in any one of these three fundamental sciences, 

 and several suggestions were put forward for extend- 

 ing the courses of each of them into the later years 

 of medical study. 



Particularly welcome to many of the science teachers 

 who were present were the remarks of Dr. Bracken- 

 bury, who insisted that a high standard of scientific 

 education was just as necessary for the general prac- 

 titioner as it is in the case of any specialist, and that 

 consequently, in so far as the preliminary science 

 courses are concerned, there should be no division of 

 the courses into a higher and lower standard for 

 different classes of medical students. 



On the question of the relegation of the science 

 courses to the school period of the student's educa- 

 tion there were some minor points of difference of 

 opinion, and there is need for further consideration 

 of this matter and for the development of a common 

 plan of action. If by raising the age of registration 

 to seventeen years the school period is increased by 

 an average of one year, there will be time for some 

 school instruction in the fundamental sciences after 

 the student has passed a matriculation examination 

 without science, and there can be no doubt that if 

 this time is profitably used, so that the student gains 

 some knowledge of the elementary facts and prin- 

 ciples of the sciences, the courses in the first year 

 of study at the universities can be so modified in form 

 as to bring home to the student much more forcibly 

 than the courses do at present the relation of pure to 

 applied science in medicine. The very prevalent idea 

 that a great deal oi. time is wasted in the first year 

 at the university in learning science that has no 

 application to medicine arises entirely from the fact 

 that the majority of the students come to the uni- 

 versity so ignorant of elementary science and so un- 

 trained in scientific thought that the time of the 

 university teachers is wasted in teaching the most 

 elementary principles that could and should be taught 

 at school. It seemed, however, to be the general 

 opinion of those who were present at the meeting 

 that the teaching of chemistrv, physics, and biologv 

 should not cease at the end of the school period, but 

 be extended into the first year of university study in 

 a form which would be more general as regards prin- 

 ciples, and more specialised as regards its application 

 to the medical sciences. The suggestion made by Prof. 

 Lorrain Smith and other sneakers, that the teaching 

 of science should be extended into the later voars of 

 the medical curriculum so that the links that bind 

 the Dure sciences to the medical sciences should be 

 continuously presented to the medical student, does 



