250 



NA TURE 



\_Jidy 12, 1883 



review of cranial measurements. The material seems 

 insufficient and fragmentary, and affords imperfect means 

 forjudging in a satisfactory way of the exact status and 

 organisation of these people. A final contribution to the 

 linguistics of the subject, by A. S. Gatschet, closes the 

 volume, with a compendious statement of the relations of 

 the tribes of the western coast with a list of forty vocabu- 

 laries of western languages. 



Finally, this handsome volume, in typography, paper, 

 and illustrations, is of irreproachable beauty, and it treats 

 of a field in archaeological study of deep interest and wide 

 import. L. P. Gratacap 



THE SIZE OF ATOMS * 

 II. 



IN making brass, if we mix zinc and copper together we 

 find no very manifest signs of chemical affinity at all ; 

 there is not a great deal of heat developed : the mixture 

 does not become warm, it does not explode. Hence we 

 can infer certainly that contact-electricity action ceases, 

 or does not go on increasing according to the same 

 law, when the metals are subdivided to something like 

 1/100,000,000 of a centimetre. Now this is an exceed- 

 ingly important argument. I have more decided data as 

 to the actual magnitude of atoms or molecules to bring 

 before you presently, but I have nothing more decided in 

 giving for certain a limit to si/pposable small/less. We 

 cannot reduce zinc and copper beyond a certain thickness 

 without putting them into a condition in which they lose 

 their properties as wholes, and in which, if put together, 

 we should not find the same attraction as we should 

 calculate upon from the thicker plates. I think it is im- 

 possible consistently with the knowledge we have of 

 chemical affinities and of the effect of melting zinc and 

 copper together, to admit that a piece of copper or zinc 

 could be divided to a thinness of much less, if at all less, 

 than 1/100,000,000 of a centimetre without separating the 

 atoms or dividing the molecules, or doing away with the 

 composition which constitutes as a whole the solid metal. 

 In short, t lie structure as it were of bricks, or molecules, 

 or atoms, of which copper and zinc are built up ; cannot be 

 much, if at all, less than 1/100,000,000 of a centimetre in 

 diameter, and may be considerably greater. 



I will now read you a statement from an article which 

 was published thirteen years ago in Nature. 2 



" Now let a second plate of zinc be brought by a similar 

 " process to the other side of the plate of copper ; a second 

 " plate of copper to the remote side of this second plate of 

 " zinc, and so on till a pile is formed consisting of 50,001 

 " plates of zinc and 50,000 plates of copper, separated by 

 " 100,000 spaces, each plate and each space 1/100,000 of 

 "a centimetre thick. The whole work done by electric 

 "attraction in the formation of this pile is two centimetre- 

 " grammes. 



" The whole mass of metal is eight grammes. Hence 

 " the amount of work is a quarter of a centimetre-gramme 

 " per gramme of metal. Now 4030 centimetre-grammes of 

 " work, according to Joule's dynamical equivalent of heat, 

 " is the amount required to warm a gramme of zinc or 

 " copper by one degree Centigrade. Hence the work done 

 " by the electric attraction could warm the substance by 

 " only 1/16,120 of a degree. But now let the thickness of 

 " each piece of metal and of each intervening space be 

 " 1/100,000,000 of a centimetre instead of 1/100,000. 

 "The work would be increased a millionfold unless 

 " 1/100,000,000 of a centimetre approaches the small- 

 " ness of a molecule. The heat equivalent would therefore 

 "be enough to raise the temperature of the material by 



1 A a lecture delivered by Sir William Thomson .-it the Royal Institution, 

 on Friday. February 2. Revised by the Author. Continued from p. 205. 



■ See article *' On the Size of Atoms, "published in NATURE, vol. i. f. 551; 

 printed in Thomson and T.-ufs "Natural Philosophy," second edition, 

 1883, vol. i. part 2, Appendix F. 



"62°. This is barely, if at all, inadmissible, according to 

 " our present knowledge, or, rather, want of knowledge, 

 " regarding the heat of combination of zinc and copper. 

 " But suppose the metal plates and intervening spaces to 

 " be made yet four times thinner, that is to say, thethick- 

 " ness of each to be 1/400,000,000 of a centimetre. The 

 " work and its heat equivalent will be increased sixteen- 

 "fold. It would therefore be 990 times as much as that 

 " required to warm the mass by i° C., which is very much 

 " more than can possibly be produced by zinc and copper 

 " in entering into molecular combination. Were there in 

 " reality anything like so much heat of combination as this, 

 " a mixture of zinc and copper powders would, if melted in 

 "anyone spot, run together, generating more than heat 

 " enough to melt each throughout ; just as a large quantity 

 " of gunpowder if ignited in any one spot burns throughout 

 " without fresh application of heat. Hence plates of zinc 

 " and copper of 1/300,000,000 of a centimetre thick, 

 " placed close together alternately, form a near approxima- 

 " tion to a chemical combination, if indeed such thin plates 

 " could be made without splitting atoms." 



Similar conclusions result from that curious and most 

 interesting phenomenon, the soap-bubble. Philosophers 

 old and young who occupy themselves with soap-bubbles, 

 have one of the most interesting subjects of physical 

 science to admire. Blow a soap-bubble and look at it, 

 — you may study all your life perhaps and still learn 

 lessons in physical science from it. You will now see on 

 the screen the image of a soap-film in a ring of metal. 

 The light is reflected from the film filling that ring, and 

 focused on the screen. It will show, as you see, colours 

 analogous to those of Newton's rings. As you see the 

 image it is upside down. The liquid streams down (up 

 in the image) and thins away from the highest point of 

 the film. First we see that brilliant green colour. It 

 will become thinner and thinner there, and will pass 

 through beautiful gradations of colour till you see, as 

 now, a deep red, then much lighter, till it becomes a 

 dusky, yellowish white, then green, and blue, and deep 

 violet, and lastly black, but after you see the black spot 

 it very soon bursts. The film itself seems to begin to 

 lose its tension, when it gets considerably less than a 

 quarter of the wave-length of yellow light, which is 

 the thickness for the dusky white, preceding the final 

 black. When you are washing your hands, you may 

 make and deliberately observe a film like this, in a ring 

 formed by the forefingers and thumbs of two hands, and 

 watch the colours. Whenever you begin to see a black 

 spot or several black spots, the film soon after breaks. 

 The film retains its strength until wc conic to the black 

 spot, where the thickness is clearly much less than 1/60,000 

 of a centimetre, which is the thickness of the dusky white. 



Newton, in the following passage in his " Optics" (pp. 

 187 and 191 of edition 1721, Second Book, Part I.), tells 

 more of this important phenomenon of the black spot, 

 than is known to many of the best of modern observers. 



" Obs. 17. — If a bubble be blown with water, first made 

 " tenacious by dissolving a little soap in it, it is a common 

 " observation that after a while it will appear tinged with a 

 " variety of colours. To defend these bubbles from being 

 " agitated by the external air (whereby their colours are 

 " irregularly moved one among an other so that no accurate 

 " observation can be made of them), as soon as I had 

 " blown any of them I covered it with a clear glass, and by 

 " that means its colours emerged in a very regular order, 

 " like so many concentric rings encompassing the top of 

 " the bubble. And as the bubble grew thinner by the con- 

 " tinual subsiding of the water, these rings dilated slowly 

 " and overspread the whole bubble, descending in order to 

 " the bottom of it, where they vanished successively. In 

 " the meanwhile, after all the colours were emerged at the 

 " top, there grew in the centre of the rings a small round 

 " black spot like that in the first observation, which con- 

 " tinually dilated itself, till it became sometimes more than 



