3io 



NA TURE 



[September 2, 1922 



his proper training ; and when he comes to use larger 

 quantities of materials and more expensive appar- 

 atus he is more likely to do so with economy and 

 care. 



In the early part of the work many simple substi- 

 tutes for the test tube may be used, and there is an 

 advantage in variety, the chief of many considera- 

 tions being the ease with which such things are washed : 

 a glass slide, as used for the microscope, or any small 

 piece of glass is suitable — for it may be rapidly 

 cleaned after each drop - experiment — and the drop 

 may be studied with the pocket lens, enabling the 

 student to distinguish between crystalline and amor- 

 phous precipitates and to note whether the colour is 

 m the precipitate or the solution ; indeed the training 

 of the student in the use of the pocket lens from the 

 very outset is highly desirable. Other materials are 

 porcelain tiles or broken china, readily replaced by 

 the student himself. A third class of "material, very 

 convenient and possessing certain advantages in 

 chemical tests, is filter-paper or some form of absorbent 

 paper, or in its absence white paper of any kind. 

 When a reaction takes place in a drop on filter-paper, 

 the separation of the precipitate from the solution 

 becomes even more obvious than when the same re- 

 action takes place on a tile or piece of glass. He may 

 learn something also about surface tension, adsorption, 

 and. the difference between crystalloid and colloid, 

 and many elementary physical facts which he would 

 never learn by the test tube method. 



There are very few reactions commonly carried out 

 in the chemical laboratory which cannot be carried 

 out just as well or better in the microchemical way ; 

 for example, the reduction of a copper solution by an 

 aldehyde. Let the student place several separate 

 drops of Fehling's solution on a glass slide and to 

 each add a drop of some different aldehyde solution ; 

 let him warm the slide gradually over a small flame 

 and make comparison of the changes in the several 

 drops. This is an example of an experiment of a 

 simple kind where an attempt is made to obtain 

 constant conditions, varying only one factor. Such 

 instructive little tests as these are surely worth far more 

 than mere colour or precipitation tests in test tubes. 

 Many more such tests could be described, but each 

 teacher will probably prefer to devise experiments of 

 his own, suited to the needs of his class. 



It is submitted, therefore, that microchemical 

 methods form not only an excellent introductory 

 training for the student but mean a great saving in 

 time, labour, and money. We have saved in one 

 year at least 1,000/., enabling us to buy more ap- 

 paratus of a permanent kind for teaching and research. 

 Egerton C. Grey. 



Government Medical School, 

 Cairo, July 1922. 



An Atomic Model with Stationary Electrons. 



Bohr's atomic model with its circling electrons 

 appears at first sight quite incompatible with Lang- 

 muir's model, in which the electrons are stationary 

 or oscillate about fixed positions of equilibrium. Dr. 

 Langmuir himself, however, has pointed out that a 

 statu atom possessing many of the properties of the 

 Bohr atom is possible provided a force of repulsion 

 equal to F= ijmr* . (nhlnr)" act between an electron 

 (mass m, charge e) and a nucleus. Here n is an 

 integer and h is Planck's constant. The distance r 

 of the electron from the nucleus in stable equilibrium 

 is the same as the radius of a circular orbit correspond- 

 ing to a stationary state in Bohr's theory. The total 

 energy of the electron is also the same as that given 

 by Bohr's theory. The frequency of oscillation about 



NO. 2757, VOL. 1 10] 



the position of equilibrium is identical with the 

 frequency of revolution of the electron in the Bohr 

 atom. Thus the Rydberg constant and the Balmer 

 series can be deduced without assuming moving 

 electrons. 



I wish to direct attention to the fact that a force 

 of exactly the type required in Langmuir's theory 

 is provided by the quantum mechanism recently 

 described by Prof. E. T. Whittaker in the Proceedings 

 of the Royal Society of Edinburgh. The mechanism 

 may be pictured as a magnetic wheel consisting of a 

 number of magnetic poles (total strength M) revolving 

 in a circle of radius a. When this magnetic wheel is 

 rotating about its axis with angular velocity u, it 

 sets up an electric field such that an electron situated 

 at a point on the axis at a distance r, large in com- 

 parison with a, is acted on by a force Mea-u/r 3 along 

 the axis. Prof. Whittaker has shown that the 

 angular momentum of the magnetic wheel in its 

 steady state (after the passage of an electron com- 

 pletely through it) is determined by Aw = 2eM, where 

 A is the moment of inertia of the wheel. We shall 

 assume that, in general, the angular momentum is 

 given by Nicholson's quantum relation, so that 



Aw= 2fM = nhJ2.Tr. 



Substituting the values of Me and a thus found in the 

 expression for the force on the electron, we find that 

 the " quantum force " is given by 



a 2 (nh\ * 



Ar 3 \2rrJ 



2Ay 3 \27r 



This agrees precisely with Langmuir's expression for 

 the force of repulsion, provided we make the single 

 additional assumption that A = \wia? L . 



Thus we see that by means of Prof. Whittaker's 

 quantum mechanism it is possible to construct an 

 atomic model which will yield many of the results of 

 Bohr's theory, without employing moving electrons. 

 I have discussed the question more fully in a paper 

 to be published by the Physical Society of London. 



H. S. Allen. 



August 15. 



The Variable Depth of Earthquake Foci. 



Prof. H. H. Turner has given reasons (Mon. Not. 

 R.A.S., Geophys. Suppt. No. 1) for believing that the 

 depths of the foci of earthquakes differ among them- 

 selves by quantities up to about 300 km. It mav 

 be pointed out that this is precisely what may be 

 expected from the theory of a cooling earth. The 

 available information concerning the thermal state of 

 the earth indicates that the rocks in the asthenosphere, 

 at depths of 400 km. and more, must be very much 

 weaker than those at the surface ; this is amply 

 confirmed by the geodetic evidence collected by 

 Barrell, which also suggests that the rocks at depths 

 comparable with 100 km. are considerably stronger 

 than those at the surface. Accordingly, whatever 

 may be the cause of crustal deformation in the earth, 

 yield will occur in the asthenosphere for smaller 

 stresses than are necessary to produce it 111 the 

 upper parts of the crust. Thus the earthquakes 

 arising from fractures below 400 km. would be more 

 numerous but much less violent than those occurring 

 at higher levels, and the greatest earthquakes should 

 have their foci at the depth of greatest strength. We 

 should therefore expect that the depths of earthquake 

 foci may range from zero to 200 or 300 km. 



Dorothy Wrinch. 



Harold Jeffreys. 

 August 21, 1922. 



