J. J. Thomson

“ We may, I think, get a useful lesson by considering for a moment from this point of view a theory of the atom which, though it is not in very close touch with physical phenomena, has yet the advantage of being so precisely defined that the properties of its atoms can be deduced by purely mathematical principles. The theory to which I allude is that known as the 'Vortex Atom Theory of Matter', which supposes that the Universe consists of an ideal substance known to mathematicians as a perfect fluid. ”

“ Confident as one may be in the truth of a theory, there are few whose faith is so robust that they do not feel relieved when they see the conclusions to which they have been led by theory verified by experiment. ”

“ As the forces which one atom exerts on another depend on the rearrangement of the electrons in the atom, the forces which a neutral atom exerts on other atoms—what we may call the social quantities of the atom—will depend mainly on the outer belt of electrons. Now these forces are the origin of chemical affinity, and of such physical phenomena as surface tension, cohesion, intrinsic pressure, viscosity, ionising power, in fact of by far the most important properties of the atom; and the most interesting part of the atom is the outside belt of electrons. ”

“ The laws of electric and magnetic action which we use in our theoretical investigations are based on the results of experiments, made not with a single unit of electricity, but with collections of millions of such units; they represent in fact the average effect of millions of individuals. When, however, we come to the atom, we have to deal with the effects produced by the individual electron or positive charge, and not with the average effect produced by countless numbers of such charges. ”

“ By means of the electric spectrum we can prove in a very direct and striking way some of the fundamental truths of the Atomic Theory. For example, when we form the electric spectrum of a mixture of gases, such as the air, we get a limited number of sharply-divided streams, which show no tendency to merge into each other. This shows that the gas contains only a few kinds of particles, and that all the particles of one kind have exactly the same mass, for if there had been any variation in the masses the streams would have been fuzzy. This shows that all the atoms of an element are alike ”

“ We must, therefore, proceed in a somewhat tentative fashion, and try if our statistical knowledge, which is all we have at present, will ensure a fit for the atom; we need not, however, be very much surprised if the fit is not perfect, and we must, by the means which fortunately are now at our disposal for the study of the properties of the electron and the positive charge, endeavour to supplement our statistical knowledge by the knowledge of the effect produced by each individual. ”

“ It is, I think, possible that an atom may be able to give out vibrations of almost any period if these are excited in the proper way, say by the impact of cathode rays possessing a suitable amount of energy, and that the lines which are actually observed in the spectrum of an element may be determined by the energy which can be given to the electrons, which are sucked into the atom by the attraction the atom exerts upon them, rather than by the inability of the atom to vibrate in other periods. We may compare an atom to an orchestra with a complete set of instruments ”

“ There is a widespread belief that the mind itself is desperately speculative, that it is only kept from wild imaginings by the control of its stolid and prosaic partner, the physical facts. The true state of affairs is, I think, that it is the mind which acts as the brake in this combination, that the impulsive partner is the facts, and that these spur on the mind to take leaps which it would shudder at when not under the influence of this stimulus. ”

“ We have seen that the view, so strongly supported by recent experiments, that the atoms of the elements are aggregations of simpler systems, involves the admission that losses or gains of mass or weight must occur in the formation of the heavier atoms. But we know that the ratio of mass to weight is the same for all substances, from hydrogen, the lightest, up to uranium, the heaviest, and even, as Southern's experiments on uranium and my own on radium have shown, for radio-active substances. ”

“ The advance made through the electrification of the atom has been most remarkable; it is due to the fact that an unelectrified atom is so elusive that unless more than a million million are present we have no means sufficiently sensitive to detect them, or, to put it in another way, unless we had a better test for a man than we have for an unelectrified molecule, we should be unable to find out that the earth was inhabited. ”

“ If we could be sure that we had a complete list of the elements, that few, if any, had escaped the vigilance of the chemist, and that all the elements were members of one family, the atomic number would be the quantity with which we should naturally connect the number of electrons in the atom: for we may regard each element as derived from the preceding one by the addition of a primordial atom containing one electron. ”

“ As facts play such a large part in stimulating our imagination and suggesting new ideas, every mechanical improvement in our apparatus, every new method which makes it easier to investigate physical phenomena, affects not merely the technique of the science, but may originate ideas which will ultimately revolutionize our philosophy of the universe. ”

“ When we know the work spent or liberated in any change of condition, we can calculate the consequent increase or decrease in mass. In chemical combination heat is liberated, and there is, therefore, a change in mass, but a calculation shows that even in the cases when the greatest amount of heat is produced, as for example in the burning of coal, the change in mass is too small to be detected by our most sensitive balances, and though some chemists have devoted a lifetime to the investigation, no change in mass has ever been established as the result of chemical combination. ”

“ I have alluded to several ways of investigating the structure of the atom; they one and all involve great labour, and any one who has used them must often have felt what a boon it would have been if we had an eye which would enable us to have a good look at an atom and have done with it. ”

“ When a particle describes a circular orbit the 'circulation' is proportional to its moment of momentum, and we see, that in a theory of this kind the moment of momentum of particles describing circular orbits would always be an integral multiple of a definite unit. We see from this example that when we have a structure as fine as that associated with atoms, we may find dynamical quantities such as moment of momentum, or it may be kinetic energy, assuming the atomic quality and increasing or decreasing discontinuously by finite jumps. ”

“ With such penetrating agents as Röntgen and cathode rays at our disposal, other properties which the atom retains unaltered have been brought to light, such, for example, as the absorption of these rays when they pass through atoms; the absorption by a given atom is quite independent of any other atoms with which it may happen to be associated, and depends only on the quality of the atom itself. ”

“ We see that changes in the atom large enough to change the chemical character of the atom, i.e. to split an atom of one element up into different kinds of atoms, involve enormous transformations of energy; in fact the explosion of the atom in a few pounds of material might be sufficient to shatter a continent. We are living in the midst, nay, are made up of quiescent volcanoes ”

“ By observing the behaviour of the electron under electric and magnetic forces, the values of its mass and electric charge—the quantities which determine its behaviour under specified conditions—have been measured; indeed, though the electron has only lately come under our notice, we know a good deal more about it than we do of many things which have been discovered centuries ago. ”

“ Since the atomic weights of the elements show that in their formation a measurable change of mass has taken place, the changes of energy involved in the formation of the elements must be enormous compared with those liberated in any chemical changes with which we are acquainted. ”

“ The theory that matter in spite of its apparent continuity is in reality made up of a great number of very small particles, is as old as the science of Physics itself, and was enunciated almost as soon as men began to reason about physical phenomena. It would, however, be misleading to suppose that there is any very close connexion between the modern Atomic Theory and the views of Democritus and Lucretius. ”

“ The hydrogen atom, minus its electron, is the simplest atom we can conceive; it is much simpler than the normal hydrogen atom, with its electron intact, and essentially different from it. The investigation of its properties is a matter of very great interest. The comparison of the spectrum of a hydrogen atom which has lost its electron with that of one which has not, is a matter of very great importance ”

“ In one form of a theory which has rendered great service to physical science—I mean Planck's theory of the 'quantum'—the changes from radiant to kinetic energy are supposed to occur not continuously, but by definite steps, as would inevitably be the case if the energy were atomic in structure. ”

“ The electron and the positive charge produce an electric field all round them, and an electric field is probably a very complicated piece of mechanism. We may picture it in this case as consisting of a large number of lines of force, with one end on the electron and the other on the positive charge, spreading out into the space round the atom, and we may also suppose that these lines of force may move about even though their ends are at rest, and thus vibrate independently of the electrons. ”

“ The great and invaluable service which the Greek atomists have rendered to science is that they were the first to attempt on mechanical principles to explain complicated physical phenomena as the result of combinations of simpler ones; they pointed out the goal which science is still struggling to reach. ”

“ It can be shown that any portion of this fluid which once possesses rotatory motion will never lose it, while if it does not at any instant possess it, it can never acquire it; the atoms on this theory possess at any rate some of the characteristics of real atoms, as they can neither be created nor destroyed. The atoms of one substance on this theory are differentiated from those of another, not merely by the quantity of the rotating liquid, but also by the speed with which it is rotating. ”

“ The bombardment by α rays seems to be the most promising means of producing atomic transformation, for in this case the energy of the rays comes from these transformations themselves—''tis its own pinion that impels the steel.' They do not, however, appear to produce any appreciable effect, for the life of a radio-active substance in a dilute solution, where it is only exposed to a few α rays, seems to be no longer than the life in a strong solution, where the substance is bombarded by many rays. ”

“ Though what we know about the atom is but a minute fraction of what there is to know, some very important conclusions about atoms have been established on what seems strong evidence in the course of the last few years. ”

“ We can produce a system which is still simpler than the ordinary hydrogen atom, for we can extract the electron from the atom and get merely the positive charge left: these positively charged hydrogen atoms exist in large numbers in the positive rays. ”

“ The electrified atom or molecule, on the other hand, is much more assertive, so much so that it has been found possible in some cases to detect the presence of a single electrified atom; a billion unelectrified atoms may escape our observation, whereas a dozen or so electrified ones are detected without difficulty.
One reason why electrified atoms and molecules are so much easier to study is that we can subject them to forces far more intense than any we can apply to unelectrified ones ”

“ A diminution of mass means a liberation of energy proportional to it, so that the amount of energy liberated in the formation of these lighter elements will be proportional to the defect of this atomic weight from the nearest integer. ”