262 



NA TURE 



[August 26, 1909 



electric method. The effect to be anticipated for a single 

 & particle is much smaller than that due to an a particle, 

 but not too small for measurement. In this connection 

 it is of iiUerest to note that Regener has observed evidence 

 of scintillations produced by the /3 particles of radium 

 falling on a screen of platinocyanide of barium, but the 

 scintillations are too feeble to count with certainty. 



E.xperiment has shown that the apparent mass of the 

 electron varies with its speed, and, by comparison of 

 theory with experiment, it has been concluded that the 

 mass of the electron is entirely electrical in origin, and 

 that there is no necessity to assume a material nucleus on 

 which the electrical charge is distributed. While there' 

 can be no doubt that electrons can be released from- the 

 atom or molecule by a variety of agencies, and, when in 

 rapid motion, can retain an independent existence, there 

 is still much room for discussion as to the actual con- 

 stitution of electrons, if such a term may be employed, 

 and of the part they play in atomic structure. There can 

 be little doubt that the atom is a complex system, consist- 

 ing of a number of positively and negatively charged 

 masses which are held in equilibrium mainly by electrical 

 forces; but it is difficult to assign the relative importance 

 of the r6le played by the carriers of positive and negative 

 electricity. While negative electricity can exist as a 

 separate entity in the electron, there is yet no decisive 

 proof of the existence of a corresponding positive electron. 

 It is not known how much of the mass of an atom is 

 due to electrons or other moving charges, or whether a 

 type of mass ^ quite distinct from electrical mass exists. 

 Advance in this direction must be delayed until a clearer 

 kno\vledge is gained of the character' and structure of 

 positive electricity and of its relation to the negative 

 electron. 



.The general experimental evidence indicates that 

 electrons play two distinct roles in the structure of the 

 atom, one as lightly attached and easily removable 

 satellites or outliers of the atomic svstem, and the other 

 as integral constituents of the interior structure of the 

 atom. The former, which can be easily detached or set 

 in vibration, probably play an important part in the 

 combination of atoms to form molecules, and in the spectra 

 of the elements : the latter, which are held in place by 

 much stronger forces, can only be released as a result 

 of an atomic explosion involving the disintegration of the 

 atom. For example, the release of an electron with slow 

 velocity by ordinary laboratorv agencies does not appear 

 to endanger the stability of the atom, but the expulsion 

 of a high-speed electron from a radio-active substance 

 accompanies the transformation of the atom. 



The idea that the atoms of the elements may be com- 

 plex structures, made up either of lighter atoms' or of the 

 atoms of some fundamental substance, has long been 

 familiar to .science. So far no direct evidence has been 

 obtained of the possibility of building up an atom of higher 

 atomic weight from one of lower atomic weight, but in 

 the case of the radio-active substances we have decisive 

 and definite evidence that certain elements show the con- 

 verse process of disintegration. It mav be significant that 

 this process has only been observed in the atoms of 

 highest atomic weights, like those of uranium thorium 

 and radium. With the exception possibly of potassium, 

 there is no trustworthy evidence that a similar process 

 takes place in other elements. The transformation of the 

 atom of a radio-active substance appears to result from 

 an atomic explosion of great intensity in which a part 

 of the atom is expelled with great speed. In the maloritv 

 of cases an a particle or atom of helium is ejected, in 

 some cases a high-speed electron, while a few substances 

 are transformed without the appearance of a detectable 

 radiation. The fact that the o particles from a simole 

 substance are all ejected with an identical and very high 

 y»locitv suggests the probability that the charged helium 

 atom before its expulsion is in rapid orbital movement In 

 the atom. There is at present no definite evidence of the 

 causes operative In these atomic transformations. 



Since in a large number of rases the transform.ntlons of 

 the atoms are accompanied bv the expulsion of one or 

 more charged atoms of helium; it is diflRcuIt to avoid the 

 ennrlusion that the atoms of the radio-active elements are 

 built UP, in part at least, of helium atoms. It is certainly 

 NO. 2078, VOL. 81] 



very remarkable, and may prove of great significance, that 

 helium, which is regarded from the ordinary chemical 

 standpoint as an inert element, plays such an important 

 part in the constitution of the atoms of uranium, thorium, 

 and radium. 



The study of radio-activity has not only thrown great 

 light on the character of atomic transformations, but it 

 has also led to the development of methods for detecting 

 the presence of almost infinitesimal quantities of radio- 

 active matter. It has already been pointed out that two 

 methods — one electrical, the other optical — have been 

 devised for the detection of a single o particle. By the 

 use of the optical or scintillation method, it is possible 

 to count with accuracy the number of a particles when 

 only one is expelled per minute. It is not a difficult 

 matter, consequently, to follow the transformation of any 

 radio-active substance in which only one atom breaks up 

 per minute, provided that an a particle accompanies the 

 transformation. In the case of a rapidly changing sub- 

 stance like the actinium emanation, which has a half 

 period of 37 seconds, it is possible to detect with certainty 

 the presence, If not of a single atom, at any rate of a. 

 few atoms, while the presence of a hundred atoms would 

 in some cases give an inconveniently large effect. The 

 counting of the scintillations affords an exceedingly 

 powerful and direct quantitative method of studying the 

 properties of radio-active substances which expei o par- 

 ticles. Not only Is it a simple matter to count the number 

 of a' particles which are expelled in any given interval, 

 but it Is possible, for example, by suitably arranged ex- 

 periments to decide whether one, two, or more a particles 

 are expelled at the disintegration of a single atom. 



The possibility of detection of a single atom of matter 

 has opened up a new field of investigation in the study 

 of discontinuous phenomena. For example, the experi- 

 mental law of transformation of radio-active matter ex- 

 presses only the average rate of transformation, but by 

 the aid of the scintillation or electric method it is possible 

 to determine directly by experiment the actual Interval 

 between the disintegration of successive atoms and the 

 probability law of distribution of the a particles about the 

 average value. 



Quite apart from the Importance of studying radio-active 

 changes, the radiations from active bodies provide very 

 valuable information as to the effects produced by high- 

 velocity particles In traversing matter. The three types 

 of radiation, the a, /3, and 7 rays, emitted from active 

 bodies, differ widely in character and their power of pene- 

 tration of matter. The o particles, for example, are com- 

 pletely stopped by a sheet of notepaper, while the 7 rays 

 from radium can be easily detected after traversing twenty 

 centimetres of lead. The differences in the character of 

 the absorption of the radiations are no doubt partly due 

 to the difference In type of the radiation and partly due to 

 the differences of velocity. 



The character of the effects produced by the a and 3 

 particles is most simply studied in gases. The a particle 

 has such great energy of motion that It plunges through 

 the molecules of the gas in Its path, and leaves In its 

 train more than a hundred thousand Ionised or dissociated 

 molecules. .After traversing a certain distance, the a par- 

 ticle suddenly loses Its characteristic properties and vanishes 

 from the ken of our observational methods. It no doubt 

 quickly loses its high velocity, and after its charge has 

 been neutralised becomes a wandering atom of helium. 

 The ionisatlon produced by the a particle appears to 

 consist of the liberation of one or more slow-velocitv 

 electrons from the molecule, but in the case of complex 

 gases there is no doubt that the act of ionisatlon is 

 accompanied by a chemical dissociation of the molecule 

 Itself, although it Is difficult to decide whether this dis- 

 sociation is a primary or secondary effect. The chemical 

 dissociation produced bv o particles opens up a wide field 

 of Investigation, on which, so far, only a beginning has 

 been made. 



The 6 particle differs from the a particle in its much 

 drentfr power of penetration of matter, and the very small 

 number of molecules II Ionises compared wilh the a partlrlo 

 (rpverslncf the same path in the gas. Tt is very easily 

 deflected from its pa(h bv encounters with the fas mole- 

 rules, and there is strong evidence that, unlike the a pai- 



