Sept. 19, 1889] 



NATURE 



'511 



of phenomena connected witli magnetism, with induction, or the 

 action through space and through non-conducting bodies, which, 

 as in the case of radiant heat and light, seems to imply the 

 existence of an interatomic ether. Conversely, changes of mole- 

 cular equilibrium, brought about by the action of external forces, 

 produce corresponding changes in electrical currents : witness the 

 effects of heat, for example, on conductivity and the wondrous 

 revelations of molecular change obtained by the aid of Prof. 

 Hughes' induction balance. The behaviour of explosives illus- 

 trates also, and in a striking manner, the effects of disturbing 

 molecular equilibrium. Anexplosive is a substance which con- 

 tains in itself, in a solid or liquid form, all the elements necessary 

 to produce a chemical change by which it is converted into the 

 gaseous state. The application of heat, of pressure, or of im- 

 pact, causes, as in Prof Spring's experiments, chemical union 

 to take place, first at the spot where the equilibrium is disturbed 

 h)y the application of external force, and afterwards, with great 

 rapidity, throughout the mass, the disturbance being propagated 

 either by the air surrounding the particles or by the luminiferous 

 ether with all the rapidity of light ; the chemical reaction is 

 accelerated l)y the pressure which may arise, for example, if the 

 explosive be confined in the chamber of a gun or in the bore- 

 hole of a blast. High explosives, as they are termed, are com- 

 paratively inert to ordinary ignition ; but when the molecular 

 equilibrium is suddenly disarranged throughout the mass by the 

 detonation of a percussion fuse, combination takes place in- 

 stantly throughout, and violent explosion follows. In a similar 

 manner some gases, such as acetylene, cyanogen, and others, 

 can be decomposed by detonation and reduced to their solid 

 constituents. Prof. Thorpe has devised a very beautiful lecture 

 experiment, in which carbon disulphide is caused to fall asunder 

 into carbon and sulphur by the detonation of fulminate of mer- 

 cury fired by an electric spark. In these cases a reverse action 

 takes place, but illustrates equally well the conversion of one form 

 of energy into others, and the consequer;t disturbance of molecular 

 equilibrium in the s.ibstaiices affected. It seems to me clear, there- 

 fore, that the time has come when the conception of dynamic equi- 

 librium in the ultimate particles of matter in all its forms must 

 take the place of the structural system of inert particles. I 

 cannot conceive how the phenomena which I have enumerated 

 can be explained on the supposition that matter is built up of 

 motionless particles^ — how, for example, a stack of red andyelk)w 

 bricks could ever change the order of arrangement without being 

 completely pulled asunder and built up again, in which case an 

 intermediate state of chaos would exist ; but I can easily com- 

 prehend how a dense crowd of people may appear as a compact 

 mass, streaming, it may be, in a definite direction, and yet how 

 each member of that mass is endowed with limited motion, by 

 virtue of which he tnay push his way through without disturbing 

 the general appearance ; how the junction of two crowds would 

 form one whole, though, perchance, altered in character ; and 

 how even Prof. Spring's experiments may be explained by the 

 supposition that bystanders on the edge of a crowd would be 

 forced, by external pressure, to form part of it and partake of 

 its general movements. 



It is a suggestive fact that the product of the atomic weight of 

 certain groups of substances and their specific heats is a constant 

 quantity which, for the greater number of the elements, does 

 not differ much from 6"5. This implies that the quantity of 

 heat necessary to raise the temperature of the atoms of any one 

 of the groups to any given extent is the same ; hence these 

 atoms will be endowed with the same amount of energy at any 

 given temperature, and therefore would be competent to replace 

 each other without disturbing the general dynamic equilibrium. 



"When it is conceded that molecular motion pervades matter 

 in all its forms, and that the solid passes, often insensibly, into 

 the fluid, or even direct into the gaseous, it follows, almost of 

 necessity, that there must be a border-land, the limits of which 

 are determined by temperature and pressure, in which substances 

 are constantly changing from one state to another. This is 

 observable in fusion, but to a more marked degree in evapora- 

 tion, where the particles are being incessantly launched into 

 space as gas and return as constantly to the liquid state. Henri 

 Ste. Claire Deville has investigated similar phenomena in 

 cheiuical reactions ; he has found that at certain temperatures 

 and pressures substances fall asunder and combine much in the 

 way in which evaporation lakes place, and has given the name 

 of "dissociation "to this property of matter Prof. Mendeleeffand 

 others have extended the great French chemist's observations, 

 and have formulated the gener.il law that substances are capable 



of dissociation at all temperatures, not only in the case of 

 chemical unions, but also in that of solutions. 



If steel be looked upon as a solution of carbon and iron, then 

 the hardening of steel is explained by the theory that dissocia- 

 tion has taken place at the temperature at which it is suddenly 

 cooled, the sudden cooling fixing the molecular motion at sucn 

 an amplitude or phase that it gives a characteristic structure, 

 one of the properties of which is extreme hardness. In temper- 

 mg, the gradual communication of heat causes dissociation again 

 to take place, the molecular equilibrium is modified by the 

 increased energy imparted to the particles, and when suddenly 

 cooled at any point there remains again a distinct substance, 

 composed of iron and carbon, partly in various degrees of 

 solution and partly free, and again possessing special mechanical 

 qualities. That steel, and probably other alloys, differ in the 

 nature of their composition according to the way in which they 

 are worked, both with respect to heat and mechanical pressure, 

 has been abundantly proved by many eminent metallurgists, and 

 especially by Sir Frederick Abel, in the extended researches 

 which he has recently carried out, on the hardening of steel, 

 for the Institution of Mechanical Engineers, and it would 

 appear as a natural sequence that the properties of steel would 

 be greatly affected by the manner in which its temperature was 

 changed, as we indeed find that it is when these changes are 

 produced by baths of melted metals, by oil, or by water at 

 different temperatures. The action which takes place may be 

 illustrated by what would happen supposing that a complicated 

 dance, such as the lancers, were suddenly stopped in various 

 phases of the figures. The component parts would always 

 remain the same, but the relative distribution of the partners 

 would vary continually, and analysis would show that at one 

 time each gentleman was associated with a particular lady ; at 

 another, that two ladies were attached to a single gentleman, 

 while a number of gentlemen had no partners at all ; and yet 

 again, at another, that the movements which were once recti- 

 linear have become circular. In each case the groups would 

 assume a totally distinct appearance. 



In support of these views it may be stated that, as far as I 

 know, no pure element is capable of being hardened or 

 tempered, the reason being that no chemical change can take 

 place when there is only one substance ; the effect of heat or 

 pressure, however suddenly applied, produces merely a change 

 of form which does not appear to carry with it any correspond- 

 ing alteration of mechanical properties. 



It may be urged, however, that it is unlikely that alloys or 

 solutions could be affected in a manner so marked merely 

 by small changes at comparatively low temperatures ; but I 

 would observe that " great and little " are relative terms, and 

 we have abundant evidence of the immense effects produced by 

 \^hat would be called " little " causes. Sir Frederick Bramwell, 

 in his address last year, drew attention to the importance of the 

 "next to nothing." It is not so very long ago that anyone 

 would have been considered a dreamer for propounding a theory 

 that the presence of the fraction of a per cent, of carbon, phos- 

 phorus, or sulphur would totally alter the character of iron ; it 

 is known that the addition of one two-thousandth part of alu- 

 minium to molten iron makes the pasty ma s as fluid as water ; 

 that the presence of the smallest impurity in copper has a 

 disastrous effect on its powers of conducting electricity ; and 

 that the addition of one thousandth part of antimony converts 

 the "best selected " copper into the worst conceivable. I need 

 hardly allude to the great part played in Nature by microscopic 

 organisms, and how much of the beauty of our seas and rivers is 

 derived from substances so minute that nothing but the electric 

 beam of Prof. Tyndall is capable of revealing their presence. 



There is one more circumstance connected with my subject to 

 which I must draw your attention, because, though its application 

 to the mechanical properties of substances is very recent, it 

 promises to be of great importance. I allude to the periodic 

 law of Dr. Mendeleeff. According to that law, the elements, 

 arranged in the order of their atomic weights, exhibit an evident 

 periodicity of properties, and, as Prof. Carnelley has observed, 

 the properties of the compounds of the elements are a periodic 

 function of the atomic weights of their constituent elements. 

 Acting on these views, Prof. Roberts Austen has recently devoted 

 much time and labour to testing their exactness with reference 

 to the mechanical properties of metals. The investigation is 

 surroimded by extraordinary difficulties, because one of the 

 essential features of the inquiry is that the metals operated on 

 should be absolutely pure. For chemical researches a few grains 



