July, 1911. 



KNOWLEDGE. 



263 



encounter some obstacle such as a second plate 

 which may be placed at any convenient distance to 

 catch them. The plate receiving the recoil may 

 subsequently be removed and actuall\' found to be 

 coated with a deposit of radium B by testing in the 

 usual manner with an electroscope. 



The same thing mav be accomplished in a some- 

 what different wa\-. as was first shown b}' Hahn, 

 b\- working at atmospheric pressure with an 

 electric field as shown in the accompanving diagram 

 (Figure 1). 



The positiveK' charged plate A. coated with radium 

 A. is placed below the plate B which is negatively 

 charged and insulated from the plate A. After a 

 suitable exposure the plate B is remo\ed and found 

 to be coated with radium B. 



Whichever mode of experimenting is adopted, it 

 will be seen that the method of recoil affords a con- 

 venient means of separating a radio-acti\e product 

 from the parent substance from which it is formed, 

 and this is perhaps the most important practical use 

 to which these phenomena have been put. Indeed 

 the method has alreadv rendered great service in 

 this direction, and its application has resulted in the 

 discover\- of new radio-active products. 



To take two examples of particular interest which 

 were first investigated bv Hahn. It is well known 

 that thorium possesses radio-active properties similar 

 to those of radium, and gives rise to a number of disin- 

 tegration products. In particular, at one stage of the 

 transformations a gaseous emanation is formed, which 

 was known to produce successivelv three radio-active 

 substances — thorium .\. thorium B, and thorium C. 

 It was, however, unknown what became of the last 

 of these products after disintegration. By using the 

 method of recoil, it was shown that a previously 

 undiscoxered product was formed which was called 

 thorium U. A similar case is that of radium C. 

 which produces a series of slowly-decaying radio- 

 active products. Bv exposing a plate to the recoil of 

 radium C, a new substance has been isolated, which 

 has been called radium C, to distinguish it from 

 radium C. from which it is produced. The nature 

 of the recoil has quite recently been studied by 

 Fajans, and it would seem that, from the results so 

 far obtained, an entireh" new t\pe of radio-active 

 process is invoh'ed : but it is, perhaps, somewhat 

 earlv to speak with absolute certainty on the matter 

 as yet. 



To turn again to the question of the dynamics of 

 the atom, which we have seen can be studied in the 

 case of radio-active recoil, and which has been 

 subjected to experimental test hv Russ. E\ans, and 

 the author. It has been mentioned that when the 

 radio-active atom breaks up with the expulsion 

 of an a particle, the residual atom recoils in 

 such a wav that its momentum is the same 

 as that of the a particle. Now a charged 

 particle in motion has been shown to act in manj- 

 ways as an electric current, and will therefore be 

 acted on by a magnetic field, just as is the case with 

 a wire carrying an electric current. Moreo\er. if 



the particle is caused to move through a strong 

 electric field applied at right angles to the direction 

 in which it is travelling, it will be deflected from its 

 course. This has been shown to be the case with 

 (I particles, and the same should be the case with a 

 ■■ recoil-stream " if this should prove to be charged. 



Now the magnetic deflection depends upon the 

 momentum of the particles, and therefore the 

 deflection of the n particle and recoiling atom in a 

 magnetic field of given strength should be the same 

 if thev carr}- the same charges. On the other hand, 

 the electric deflection of the recoiling atoms should 

 be greater than for the a rays, since this deflection 

 can be shown to depend on the energy of the 

 particles. These matters have been put to the 

 test of experiment in the case of the recoil of 

 radium B from radium A. and the magnetic and 

 electric deflections obser\ed show that the recoil- 

 stream carries with it an electric charge. It is, 

 however, remarkable that the deflections are such 

 as to indicate that the charge on the a rays 

 and recoil-stream is of the same sign — namely, 

 positive. This fact gives rise to some interesting 

 considerations as to what happens with the negative 

 charge when the atom of radium .-\ breaks up, but 

 the point has not yet been definitely settled. 



The magnetic deflection of the recoil-stream can 

 be observed in different wa\s. of which perhaps the 

 most interesting is to allow a narrow stream of 

 radium B particles to pass /;; vacuo through a 

 magnetic field and fall upon a brass surface. After 

 some minutes the field is reversed and the brass 

 subsequently removed and placed in contact with a 

 photographic plate. The radio-active matter on 

 the brass, photographs itself, giving two bands 

 as shown in Figure 2. the distance between the 

 bands representing twice the deflection suffered 

 by the stream in passing through the field. .\n 

 examination of the photograph shows that the 

 deflection is onl\- half that experienced b\- the a 

 particles under similar conditions. From the law of 

 momentum it may be inferred that the charge 

 carried b\' the recoiling atom is half that on an a 

 particle, or the atomic charge of electricity, since the 

 It particle is known to be associated with twice that 

 charge. The electric deflection of the recoil-stream 

 has been measured in a somewhat similar manner, 

 though not yet with the same accuracy. But though 

 the precise manner of obtaining this information 

 cannot be entered into, the results suffice to make 

 certain important deductions regarding the mass of 

 the atom of radium B — or. in other words, of its 

 atomic weight. The result of the calculation is to 

 show that the atomic weight of radium B is in the 

 neighbourhood of two hundred, a result which 

 had alreadv been predicted from radio-active theory 

 as follows : — The atomic weight of radium has 

 Iieen measured by several in\estigators, and 

 found to be two hundred and twenty-six. Now 

 radium gives rise to the emanation with the loss 

 of one atom of helium, whose atomic weight is four. 

 Thus the emanation should have an atomic weight of 



