June 28, 1883] 



NATURE 



20; 



and other artisans directly employed in producing the 

 metal. The author has a very high opinion of the 

 Chinese miners, who are described as sober, regular in 

 work, and accustomed to cooperative enterprises, against 

 which, however, must be set the defects of being addicted 

 to excess in opium and gambling, besides being very 

 quarrelsome and exceedingly superstitious. The latter 

 failing is, however, of interest as reproducing the old 

 European legends of guardian genii of the mine, the 

 "Kobads" of Germany and "Knockers" of Cornwall, 

 who require to be propitiated by sacrifices and kept in 

 good humour by orderly behaviour on the part of the 

 miners. Infractions of the last rules are punished by 

 the withdrawal of the guardian gnome, who takes all 

 the unwrought ore in the mine away with him. 



The execution of the work, both as regards illustration 

 and typography, are exceedingly good, and reflect great 

 credit upon the French National Printing Office. 



H. B. 



THE SIZE OF A TOMS * 



FOUR lines of argument founded on observation have 

 led to the conclusion that atoms or molecules are 

 not inconceivably, not immeasurably small. I u;e the 

 words " inconceivably" and "immeasurably" advisedly. 

 That which is measurable is not inconceivable, and there- 

 fore the two words put together constitute a tautology. 

 We leave inconceivableness in fact to metaphysicians. 

 Nothing that we can measure is inconceivably large or 

 inconceivably small in physical science. It may be diffi- 

 cult to understand the numbers expressing the magnitude, 

 but whether it be very large or very small there is nothing 

 inconceivable in the nature of the thing because of its 

 greatness or smallness, or in our views and appreciation 

 and numerical expression of the magnitude. The general 

 result of the four lines of reasoning to which I have re- 

 ferred, founded respectively on the undulatory theory of 

 light, on the phenomena of contact electricity, on capil- 

 lary attraction, and on the kinetic theory of gases, agrees 

 in showing that the atoms or molecules of ordinary matter 

 must be something like the 1/10,000,000, or from the 

 1/10,000 000 to the 1/100000,000 of a centimetre in dia- 

 meter. I speak somewhat vaguely, and I do so, not in- 

 advertently, when I speak of atoms and molecules. I 

 must ask the chemists to forgive me if I even abuse the 

 words and apply a misnomer occasionally. The chemists 

 do not know what is to be the atom ; for instance, whether 

 hydrogen gas is to consist of two pieces of matter in 

 union constituting one molecule, and these molecules fly- 

 ing about ; or whether single molecules each indivisible, 

 or at all events undivided in chemical action, constitute 

 the structure. I shall not go into any such questions at 

 all, but merely take the broad view that matter, although 

 we may conceive it to be infinitely divisible, is not infi- 

 nitely divisible without decomposition. Just as a building 

 of brick may be divided into parts, into a part containing 

 1000 bricks, and another part containing 2500 bricks, and 

 those parts viewed largely may be said to be similar or 

 homogeneous ; but if you divide the matter of a brick 

 building into spaces of nine inches thick, and then think 

 of subdividing it farther, you find you have come to some- 

 thing which is atomic, that is, indivisible without destroying 

 the elements of the structure. The question of the molecular 

 structure of a building does not necessarily involve the 

 question, Can a brick be divided into parts, and can those 

 parts be divided into much smaller parts ? and so on. It 

 used to be a favourite subject fur metaphysical argument 

 amongst the schoolmen whether matter is infinitely 

 divisible, or whether space is infinitely divisible, which 

 some maintained, whilst others maintained only that 

 matter is not infinitely divisible, and demonstrated that 



' A lecture delivered by Sir William Thomson at the Royal Institution, 

 on Friday, February 2. Revised by the Author. 



there is nothing inconceivable in the infinite subdivision 

 of space. Why, even time was divided into moments 

 (time-atoms !), and the idea of continuity of time was 

 involved in a halo of argument, and metaphysical — I will 

 not say absurdity — but metaphysical word-fencing, which 

 was no doubt very amusing for want of a more instruc- 

 tive subject of study. There is in sober earnest this very 

 important thing to be attended to, however, that in 

 chronometry as in geometry, we have absolute continuity, 

 and it is simply an inconceivable absurdity to suppose a 

 limit to smallness whether of time or of space. But on 

 the other hand, whether we can divide a piece of glass 

 into pieces smaller than the 1/100,000 of a centimetre in 

 diameter, and so on without breaking it up, and making 

 it cease to have the properties of glass, just as a brick 

 has not the property of a brick wall, is a very practical 

 question, and a question which we are quite disposed to 

 enter upon. 



I wish in the beginning to beg you not to run away 

 from the subject by thinking of the exceeding smallness 

 of atoms. Atoms are not so exceedingly small after all. 

 The four lines of argument I have referred to make it 

 perfectly certain that the molecules which constitute the 

 air we breathe are net very much smaller, if smaller at 

 all, than 1/10,000,000 of a centimetre in diameter. I was 

 told by a friend just five minutes ago that if I give you 

 results in centimetres you will not understand me. I do 

 not admit this calumny on the Royal Institution of Great 

 Britain ; no doubt many of you as Englishmen are more 

 familiar with the unhappy British inch ; but you all surely 

 understand the centimetre, at all events it was taught 

 till a few years ago in the primary national schools. Look 

 at that diagram (Fig. 1), as I want you all to understand an 



One centimetre. 



One millimetre. 



Fig. 



inch, a centimetre, a millimetre, the 1/10 of a millimetre, 

 and the 1/100 of a millimetre, the 1/1,000 of a millimetre, 

 and the 1/1,000,000 of a millimetre. The diagram on the 

 wall represents the metre ; below that the yard ; next the 

 decimetre, and a circle of a decimetre diameter, the 

 centimetre and a circle of a centimetre, and the milli- 

 metre, which is 1/10 of a centimetre, or in round numbers 

 1/40 of an inch. We will adhere however to one simple 

 system, for it is only because we are in England that 

 the yard and inch are put before you at all, among 

 the metres and centimetres. You see on the dia- 

 gram then the metre, the centimetre, the millimetre, 

 with circles of the same diameter. Somebody tells me 

 the millimetre is not there ; I cannot see it, but it cer- 

 tainly is there, and a circle whose diameter is a milli- 

 metre, both accurately painted in black. I say there is a 

 millimetre and you cannot see it. And now imagine 

 there is 1/10 of a millimetre, and there 1/100 of a milli- 

 metre and 1/1000 of a millimetre, and there is a round 

 atom of oxygen 1/1,000,000 of a millimetre in diameter. 

 You see them all. 



Now we must hare a practical means of measuring, 

 and optics supply us with it for thousandths of a milli- 

 metre. One of our temporary standards of measurement 

 shall be the wave-length of light ; but the wave-length is 

 a very indefinite measurement, because there are wave- 

 lengths for different colours of light, visible and invisible, 

 in the ratio of 1 to 16. We have, as it were — borrowing 

 an analogy from sound— four octaves of light that we 

 know of. How far the range in reality extends above 

 and below the range hitherto measured, we cannot even 

 guess in the present state of science. The table before 

 you (Table I.) gives you an idea of magnitudes of length, 



