384 
MEI MES ?, 
[7cd. 10, 1870 
same genus. 7ppagus is closely allied to Crened/a, as the latter 
is represented by C. g/andu/a of Totten and, indeed, I cannot 
detect any character to distinguish them generically. These agree 
in shape, sculpture, hinge, muscular impressions and inflexion 
of the beaks. The genus Verticordia of S. Wood is very different 
and belongs to another family, viz. that of Lyozsia. 
Mr. Arthur Adams informs me that he has several Japanese 
species, which he believes are also found in the Mediterranean, 
and I have identified many British species with those of North 
Japan. J. GWYN JEFFREYS 
An Oversight by Faraday 
Tr is not often that Faraday committed an oversight ; but such I 
think he must have done in his well-known paper concerning the 
existence of alimit to vaporisation. ( ‘‘ Experimental Researches 
in Chemistry and Physics,” p. 119.*) Faraday showed experi- 
mentally, that mercury emitted no appreciable vapour below 
20° F, and accounted for this on the ground that ‘‘the elastic 
force of any vapour which the mercury could have produced at 
that temperature, was less than the force of gravity upon it and 
that, consequently, the mercury was then perfectly fixed.” He 
adds, ‘‘I think we can hardly doubt that such is the case, at 
common temperatures, with respect to silver and with all 
bodies which bear a high temperature without appreciable loss 
by volatilisation, as platina, gold, iron, nickel, silica, alumina, 
charcoal, &c., and that, consequently, at common temperatures, 
no portion of vapour rises from these bodies or surrounds 
them.” 
Has not Faraday overlooked the fact that though gravity 
might prevent the 7/se of vapour, it would assist the /a// of 
vapour from the sides or under surface of a body suspended 
in vacuo? If Faraday’s theory had any grounds in truth, it 
would be possible to distil a substance from above downwards 
by the sole force of gravity, but I know of no experiment to 
support the idea. Were all the other forces which could act upon 
the molecules, exactly balanced in unstable equilibrium, the force 
of gravity might undoubtedly upset this equilibrium, so that 
vapour would be produced from the under surface of a suspended 
solid when it could not be produced from the upper surface. 
But a very slight estimation of the forces which may enter into 
such a problem, shows how unlikely it is that the case could ever 
happen. 
I do not mean to say that the force of tenacity of solid sub- 
stance, is identical with that which is opposed to volatilisation ; 
but it is possibly comparable with it in amount. Now a copper 
wire, having a section of one square millimetre, will beara weight 
of 90 lbs. and this force of tenacity only acts between portions 
of metal in absolute contact and continuity. Compare now the 
weight of a film of copper, say z+, part of a millimetre thick, with 
the force by which it adheres to the remainder of the mass of 
copper with which it is continuous. Taking the specific gravity at 
5:9, the weight will be, per square millimetre, 0‘000089 gramme ; 
the force which would be required to tear it from the remainder of 
the mass would be, for the same surface, 40819" grammes. The 
force of tenacity, therefore, exceeds the weight rather more than 
450,000,000 (450 million) times. This seems to show that 
the molecular forces which tend to maintain the integrity of a 
solid metal, are almost infinitely greater than the gravitation of 
the same molecules towards the earth. To cause these molecules 
to fly off in free vapours, we must call in the aid of forces of 
heat, electricity, or chemical affinity which can cope with the 
prodigious force of solid cohesion. It is practically impossible 
that we should ever meet with a case, where these forces were so 
exactly balanced that the exterior force of gravity, many million 
times less in amount, should produce a perceptible disturbance. \ 
These considerations do not, however, appear to affect the 
validity of Wollaston’s speculations concerning a definite limit to 
the earth’s atmosphere as caused by the gravity of the aérial 
particles. If 
Where are the Nebule ? 
I AM unafilectedly glad to find that one whose opinion has 
such weight as Mr. Spencer’s must have, should have anticipated 
me in the matters to which he directs your attention in his 
interesting letter. ‘There can be no question as to his priority ; 
since in 1863 I had not only formed no views respecting the 
nebule ; but had no further knowledge of astronomy than I 
derived from a very faint recollection of what I had learned in a 
hasty two hours’ perusal of Goodwin’s Astronomy (Course of 
Phil. Trans. 1826, p. 434. 
Mathematics) the night before our examination on the subject in 
the ‘Three days” at Cambridge. 
In considering the subject of the nebule recently, however, 
the points touched on by Mr. Spencer had not escaped me. In 
five papers in the Zyfellectual Observer and Student called ‘* Notes 
on Nebulz,” ** Notes on Star-streams,” and ‘‘ A New Theory of 
the Universe” (three parts), I touched on these and many other 
proofs that the nebulze are not external galaxies ; but part and 
parcel of the sidereal system. 
I have since found that Dr. Whewell, in his ‘‘ Plurality of 
Worlds,” had adopted the same view. But as a matter of fact, 
we owe the enunciation of clearly convincing evidence respecting 
the true nature of the nebula to Sir John Herschel ; while Sir 
W. Herschel, when as yet the available evidence was incomplete, 
indicated his belief that the Orion nebula (amongst others) is 
within the sidereal system. 
Strangely enough, the point first dwelt on by Mr. Spencer was 
boldly quoted by the Rev. C. Pritchard (V.P.R.A.S.) as a proof 
that the nebulz are external galaxies, immediately after I had 
read my communication on the distribution of the nebulee to the 
Astronomical Society. I asked at once if we were to regard 
those vacant spaces as the spy-holes, so to speak, of the sidereal 
system, through which alone the nebulz could be hopefully looked 
for and Mr. Pritchard said ‘‘ Yes.” Mr. Stone, also, pointed 
out subsequently, that the glare from the stars might elsewhere 
obliterate the nebulee (at least the fainter ones) from view. This 
would be a point to be attentively considered were it not that in 
the Nubeculze we have evidence that the glare from many stars 
does not obliterate faint nebula. 
The second point dwelt on by Mr. Spencer affords a remark- 
able instance of the way in which considerations that should be 
perfectly obvious, escape even practised astronomers. Strangely 
enough, I dwelt on this point, only three days ago, ina letter I 
addressed (not for publication) to the editor of the Spectator, It 
is commonly understood and stated that telescopes which are 
only just able to show stars of the tenth, twelfth, or fourteenth 
magnitude asthe case may be, are able to exhibit the component 
stars of certain external galaxies, which must (according to 
the theory) be thousands of times farther from us than the 
fourteenth magnitude stars. Not a thought has been given 
to the obvious conclusion that these component stars, to be thus 
visible, must be millions of times larger than the members of our 
galaxy. 
In a letter addressed last August to Sir John Herschel (a por- 
tion of the answer to which was quoted in my article in NATURE 
for January 27), I pointed out half-a-century of reasons for be- 
lieving that the sidereal system is differently constituted than has 
been supposed and that the nebule are not external to it. 
(This would have involved an egregiously long letter had I been 
writing to an ordinary correspondent, but in the actual case a 
few words served sufficiently to indicate each reason.) These 
reasons were not interdependent—each afforded good and 
most afforded perfectly sufficient ground for rejecting the ac- 
cepted theory. In his repiy, Sir John (always kind, courteous 
and encouraging) was good enough to speak of the ‘‘ number 
and variety of the striking facis brought together and the evi- 
dent bearing of a large proportion of them on the great problem 
offered by the sidereal system to man’s contemplation.”” Amongst 
the facts which afford the strongest evidence of all, are two I left 
unnoticed in my late paper, viz., (1) the relatively large proper 
motion of the fainter stars and (2) the drift of whole groups 
of stars in a definite direction. These facts apply to the struc- 
ture of the sidereal system, rather than to the position of the 
nebule ; but, as a matter of fact, the two matters are so closely 
related, that evidence bearing on one carries with it conclusions 
affecting the other also. Ricup, A. PRrocror 
February 4 
Analogy of Colour and Music 
I FIND in your number of January 13 an interesting paper by 
Mr. Barrett on the Correlation of Colour and Sound. It seems 
to me that Mr, Barrett depreciates the phenomenon of Newton’s 
rings by saying that the ‘* connection between the relative spaces 
occupied by each colour and the relative vibrations of the notes 
of the scale”... . ‘cannot be more than a coincidence.” The 
diameters of the rings are functions of the wave-lengths and, there- 
fore, expressions of a physical condition, Mr. Barrett's own 
process 15, to say the least, very rough and, after taking ‘* the 
mean of two limits,” rather wide apart for the length of the 
waves of each colour, he obtains a series of numbers which 
