February 9> 191 1] 



NATURE 



491 



about 4460 metres, and above this the mountain rose to 

 4750 metres. A former extension of glaciation in this 

 part of the range down to the altitude of about 4000 

 metres was shown by striae on the rock surfaces and the 

 presence of a small, typical glacier lake, though no 

 glaciers are existing in this part of the range now. The 

 •'xpedition encountered many difficulties in the journey 

 from the coast to the mountain ranges in the interior, but, 

 raught by the experiences of the first expedition, special 

 irrangements were made to push up the North river so 

 rar and so rapidly as possible to avoid the delays and 

 ickness incidental to a prolonged stay in the low and 

 narshy region. 



Besides the geographical information obtained, much 

 vork was also done in zoology and botany. A thousand 

 birds' skins and ten thousand insects collected during the 

 expedition are now being studied at Leyden, and numerous 

 new species have been obtained ; the Australian character 

 of the fauna is well marked, and especially so among the 

 fishes captured in the North river. 



The botanical collection, ranging from the tropical to 

 the Alpine flora,, shows a majority of plants having a 

 Malayan character, but there are so large a number of 

 endemic forms that New Guinea and the adjacent islands 

 seem to be separable as a botanical region from the 

 Soenda Islands. Savannas, consisting largely of intruding 

 species from North Australia, occur ; but the Alpine flora, 

 on the other hand, is said to be of a northern character, 

 resembling that of the mountains of Java, Sumatra, and 

 the Himalayas. 



The Wilhelmina Peak is stated to consist of Alveolina 

 limestone, and generally the geological age of the forma- 

 tions traversed was of comparatively recent date ; eruptive 

 rocks were only met with near Geelvink Bay. .\ very 

 interesting collection of ethnological objects was obtained, 

 and many observations were made concerning the Papuans 

 living in the plains and those of the mountains. 



The results of this expedition, together with those of 

 the British expedition now in New Guinea, should greatly 

 extend our knowledge of this region. 



RADIO-ACTIYITY AS A KINETIC THEORY 



OF A FOURTH STATE OF MATTER.^ 

 'T'HERE are many points of resemblance between the 

 movements of the molecules of a gas and the move- 

 ments of those corpuscular radiations with which we have 

 become acquainted in following up the discovery of radio- 

 activity. In both cases we find that things of extremelv 

 minute dimensions are darting to and fro with great 

 yelocit>', and in both cases the path of any one individual 

 is made up of straight portions of various lengths, along 

 which it is moving uniformly and free from external in- 

 fluence, and of encounters of short duration with other 

 individuals, when energy is exchanged and directions of 

 motion are altered. There is even a resemblance in 

 the universality of each movement. The motion of mole- 

 cules is a fundamental fact throughout the whole of 

 our atmosphere, and, indeed, in all material bodies; the 

 motion of the radiant particles emitted by radio-active 

 substances is also widely distributed, and of great import- 

 ance. Taking Eve's estimate of the usual ionisation of 

 the air, we can calculate that in this room, in everv 

 second, some thousands of a and $ particles enter into 

 existence, complete their paths through all the atoms thev 

 meet, and sink into obscurity ; some of them, viz. the o 

 particles, as atoms of helium. These last move through 

 definite and well-known distances in the air. For ex- 

 ample, a third of those which are due to radium pro- 

 ducts move through a range of just above 4 cm., an equal 

 number have a range of just below 5 cm., and again an 

 equal number move through 7 cm., and the speed is so 

 great that the life of each a particle as such is completed 

 in about a thousandth-millionth of a second. They leave 

 their mark behind them in the ionisation of the air 

 through which they have passed, and in the heat into 

 which their energy has been commuted. The former effect 

 1.S easily detected by the sensitive measuring instruments 



1 Discourse delivered at t^e Royal In«t tution on Friday, January 27, by 

 ProC William H. Bragg, F.R S. y ^. "X 



NO. 2154, VOL. 85] 



which we now possess ; the latter is too small to measure, 

 and must be greatly increased by the aid of radium itself 

 before it can be investigated. But on a large scale, which 

 takes into account the distribution of radio-active material 

 through the earth, the sea, and the air, the effects are of 

 first-rate importance to the physical conditions of our 

 earth. 



If we compare the movements a little more closely, w^e 

 find differences as interesting as the resemblances. The 

 motions which the kinetic theory of gases considers are 

 those of the molecules of which gases consist ; in the case 

 of radio-activity, the things which move are quite different. 

 They are sometimes electrons, which have come .to be 

 called 3 rays when their speed is great, and kathode rays 

 when it is somewhat less ; or they are y or X-rays, which 

 are new things to us ; or if as a particles they are helium 

 atoms, such as we have known before, they move with 

 excessive speeds which give them quite new properties. 

 In general, the radiant particles move hundreds of 

 thousands of times as fast as the gas molecules do, and 

 it is, no doubt, on account of this fact, as well as through 

 their usually extreme minuteness, that their power of 

 penetrating matter is so great. When two molecules of 

 a gas collide, they approach within a fairly definite dis- 

 tance, which we call the sum of the radii of the molecules, 

 and the approach is followed by a recession and new con- 

 ditions of motion. Each molecule has, as it were, a 

 domain into which no other molecule can penetrate. But 

 the defences which guard the domain are of no account 

 to the vigorous movements which we are considering now. 

 The radiant particles pass freely through the atoms, and 

 their encounters are rather with one or other of a number 

 of circumscribed and powerful centres of force which 

 exist within the atomic domain, and act with great power 

 when, and only when, approached within distances which 

 are small in comparison with the atomic radius. It is on 

 this account that the new theory opens out to us such 

 possibilities of discovering the arrangement of the interior 

 of the atom. Never before have we been able to pass 

 anything through an atom ; our spies have always been 

 turned back from the frontier. Now we can at pleasure 

 cause to pass through any atom an a particle, which is 

 an atom of helium, or a ^ particle, which is an electron, 

 or a 7 or X-ra\% and see what has happened to the particle 

 when it emerges again, and from the treatment which it 

 seems to have received we must tr>' to find out what it 

 met with inside. 



The newer movement exists superimposed upon the 

 other. Its velocities are so great that the gas (or liquid 

 or solid) molecules are, in comparison, perfectly still. 

 There is, as it were, a kinetic theory within a kinetic 

 theor)' ; there is a grosser movement of gas molecules 

 which has long been studied, and in the same place and 

 at the same time there is a far subtler and far more lively 

 movement which is practically independent of the other. 

 Your vice-president. Sir William Crookes, was the first 

 to find any trace of it. The behaviour of the kathode rays 

 in the vacuum tubes which he had made showed him that 

 he was dealing with things in no ordinary condition. 

 Whatever was in motion was neither gas, nor solid, nor 

 liquid, as ordinarily known, and he supposed it must be 

 possible for matter to exist in a fourth state. We have 

 gone far since Sir William's first experiments. The X-ray 

 tube and radium have widely increased our knowledge of 

 phenomena parallel to those of the Crookes tube. But I 

 think we may still be glad to use Sir William's definition. 



There ' is another ver\' striking characteristic of the 

 newer kinetic theory which differentiates it sharply from 

 the older. The experiences of any one of the radiant 

 particles in an atom which it crosses are quite unaffected 

 by any chemical combination of that atom with others ; 

 that is to say, by any molecular associations it may have. 

 Naturally, this simplifies investigation. We may, no 

 doubt, ascribe this state of things to the fact that a radiant 

 particle is concerned rather with the interior of the atom 

 than with the exterior, and that it is the latter which is 

 of importance in chemical action. 



Let us take notice of one more important difference. 

 The molecules of a gas move with velocities which vary 

 at every collision, yet vary about a certain mean. But 

 the peculiar motion of the radiant particle is only tem- 



