April 7, 1887J 



NA TURE 



549 



has, it seems to me, an important bearing upon a question which 

 has been raised by Marignac, Schutzenberger, and others, and 

 which hxs again been raised by Mr. Crookes in the address I have 

 aheady referred to. Mr. Crookes thinks that it may well be ques- 

 tioned whether there is an absohue uniformity in the mass of 

 every ultimate atom of the same chemical element, and that it is 

 probable that our atomic weights merely represent a mean value, 

 around which the actual atomic weights of the atoms vary 

 within certain narrow limits, or in other words that the 

 mean mass is a statistical constant of great stability. The 

 facts of diffusion would seem to lend no support to such a 

 supposition. 



Graham was still living when Loschmidt published what Prof. 

 Kxner calls his "epoch-making paper" on the " .Size of the Air 

 Molecule." Although the numerical estimate which Loschmidt 

 deduced from the mean free path of the molecules and their 

 volume has now only an historical interest, it has exercised a pro- 

 found influence on the development of molecular physics in de- 

 monstrating that in dealing with molecules we are dealing with 

 masses of finite dimensions, and further that these dimensions 

 are by no means immeasurably small. The very manner in 

 which Loschmidt stated his conclusions was well calculated to 

 rivet attention. He showed that these magnitudes, small as they 

 are, are yet comparable with those which can be reached by 

 mechanical skill. The German optician Nobert has ruled lines 

 on a glass plate so close together that it requires the most perfect 

 microscopes to observe the intervals between them : he has 

 drawn, for example, as many as 4000 lines in the breadth of a 

 millimetre, that is about 112,000 lines to the inch. Now, if we 

 assume, with Maxwell, that a cube whose side is 1/4000 of a 

 millimetre is the smallest volume observable at present, it would 

 follow that such a cube would contain from 60 to too millions of 

 molecules of oxygen or nitrogen, and if we further assume that 

 the molecules of organised bodies contain on an average 50 

 " elementary " atoms it further follows that the smallest organised 

 particle visible under the microscope contains about two million 

 molecules of organic matter. And as at least half of every living 

 organism is made up of water, we arrive at the conclusion that 

 the smallest living being visible under the microscope does not 

 contain more than about a million organic molecules. I could 

 have wished, had time permitted, to have dwelt a little upon the 

 intensely interesting questions which such a conclusion at once 

 raises. In the article "Atom" in the present edition of the 

 " Encyclopaedia Britannica," from which I have quoted, you 

 will find Clerk Maxwell points out its relation to physiological 

 theories, and especially to the doctrine of Pangenesis. Mole- 

 cular science, says Maxwell, " forbids the physiologist from 

 imagining that stractural details of infinitely small dimen- 

 sions can furnish an explanation of the infinite variety which 

 exists in the properties and functions of the most minute 

 organisms." 



In the year following Graham's death Sir William Thomson 

 still further developed the modes of molecular measurement, and 

 from a variety of considerations based upon the kinetic theory of 

 a gas, upon the thickness of the films of soap-bubbles, and from 

 the electrical contact between copper and zinc, he arrived at esti- 

 mates which, although sensibly different from that of Loschmidt, 

 are still commensurable with it. In a subsequent lecture to 

 the Koyal Institution, given about four years ago, he extended 

 the lines of his argument and arrived at the conclusion that in 

 any ordinary liquid, transparent solid, or seemingly opaque solid, 

 the mean distance between the centres of contiguous molecules 

 is less than 1/5,000,000 and greater than 1/1,000,000,000 

 of a centimetre. And in order to give us some conception of 

 the degree of coarse-grainedness implied by this conclusion he 

 asks us to imagine a globe of water or glass as large as a foot- 

 ball to bs magnified up to the size of the earth, each constituent 

 molecule being magnified in the same proportion. The magnified 

 structure would be more coarse-grained than a heap of small shot, 

 but probably less coarse-grained than a heap of footballs 

 (Nature, vol. xxviii. p. 278). 



Here I think we may leave the subject, at all events for to- 

 night. I am painfully conscious that I have left unpaid much 

 th.at ought to have been said, and possib'y said some things that 

 might well have been left unsaid. But my main purpose will 

 have been served if I have succeeded in indicating to you 

 Graham's position as an atomist, and in showing you how his 

 ideas respecting the constitution of matter have germinated, and, 

 like the seed which fell upon good ground, have borne fruit an 

 hundredfold. 



SOCIETIES AND ACADEMIES 



London 



Royal Society, March 17. — "On the Total Solar Eclipse of 

 August 29, 1SS6 (Preliminary Account)." By Arthur Schuster, 

 E.R.S. 



The instrument intrusted to me by the Eclipse Expedition was 

 similar to that employed in Egypt during the eclipse of 1882. 

 The equatorial stand carried three cameras, one of which was 

 intended for direct photographs of the corona, while the twtt 

 others were attached to spectroscopes. 



Pliotogi;i/^/is of the Coronu .—The lens had an aperture of 

 4 inches, and a focal length of 5 feet 3 inches ; giving images of 

 the moon having a diameter of about o'6 of an inch. 



During the first minute of totality the corona was covered by 

 a cloud, which was, however, sufficiently transparent to allow 

 the brightest parts of the corona to show on the ten photographs 

 exposed during that time. 



During the remaining time, that is to say, during about two- 

 minutes and a half, the sky was clear, but there were clouds in 

 the neighbourhood of the sun. 



The time of exposing the photographs which had been fixed 

 beforehand had to be altered in consequence of the uncertainty 

 of the weather, and for this reason I can only give the actual 

 times of exposures very approximately and from memory. One 

 photograph on sensitive paper shows only little detail ; but three 

 photographs on glass were obtained, which, as regards definition, 

 I believe to be equal to those obtained in Egypt. The approxi- 

 mate exposures were 15 to 20 seconds, 10 to 15 seconds, and 

 about 5 seconds. 



With the view of possibly increasing the amount of detail 

 which it has hitherto been possible to obtain on the photographs 

 of the corona, I have, on this occasion, given considerable 

 attention to the different adjustments, so as to fix the cause which 

 at present limits the definition, and I have come to the conclusion 

 that, if we are to obtain better photographs of the corona, we 

 can only hope to do so by means of a better mechanical 

 arrangement for moving t!ie camera. 



Photographs of the spectrum of the corona were obtained by 

 means of two instruments, one being identical with that employed 

 at Caroline Island in 1883. This spectroscope has two prisms 

 of 62' refracting angle, the theoretical resolving power being 

 about 10 in the yellow. (The unit of resolving power is the 

 resolving power which allows of the separation of two lines 

 differing by the thousandth part of their own wave-length.) The 

 slit of this spectroscope was placed so that it w as tangential to- 

 the image of the sun formed by the condensing lens. One plate 

 was exposed during the whole of totality. The results are good ; 

 a number of lines belonging to the prominences and to the 

 corona are very distinct and can be measured w ith fair accuracy. 

 The difficulty of measurement lies in the multitude of lines. I 

 have measured at present between forty and fifty distinct corona 

 lines between the solar lines F and H, and a number remain 

 unmeasured. 



A comparison between the photographs of 1882 and 1886 

 shows that, although the lines seem to be in the same position, their 

 relative intensity has greatly altered. The strongest corona line 

 during the last eclipse had a wave-length of about 4232 ; it is 

 slightly but distinctly less refrangible than the strong calcium 

 line at 4226. 



The second spectroscope had its slit placed so as to take a 

 radial section of the corona. It had one large prism giving a 

 theoretical resolving power of 1 1 "4 ; slightly larger therefore 

 than the two-prism spectroscope. 



The film was one prepared by Capt. Abney so as to be 

 more sensitive in the green than the ordinary plates. 



The photograph obtained is faint, but I believe will ultimately 

 give good results. 



A good drawing of the corona was obtained by Capt. 

 Maling at the station occupied by Capt. Darwin and myself 



The plates were prepared by Capt. .\bney, whose valuable 

 help I have had in the whole of the preliminary arrangements. 



March 24. — "Preliminary Note on the ' Radio- Micrometer, "^ 

 a New Instrument for measuring the most Feeble Radiation."' 

 By C. Vernon Boys. 



The author considered that, if the thermo-electric power of a 



'I have learnt that an instrument e.ssentially the same in principle as the 

 radio-micrometer was made and shown by M. D'Arsonval to the French 

 Physical Society ; it is hardly necessa'-y to -ay that I was not aware of this 

 before reading the paper. — C V. B. 



