306 
The vibrations of light are not such as can be transmitted by 
a set of disconnected molecules; if by molecules at all, it must 
be by molecules connected into a solid, z.e, by a body with 
rigidity. Rigidity means active resistance to shearing stress, 7.¢. 
to alteration in shape ; it is also called e/astictty of figure ; it is 
by the possession of rigidity that a solid differs from a fluid. 
For a body to transmit vibrations at all it must possess inertia ; 
transverse vibrations can only be transmitted by a body with 
rigidity. All matter possesses inertia, but fluids only possess 
volume elasticity, and accordingly can only transmit longitudinal 
vibrations. Light consists of transverse vibrations ; air and water 
have no rigidity, yet they are transparent, 7.¢. transmit transverse 
vibrations ; hence it must be the ether in ide them which really 
conveys the motion, and the ether must have properties which, 
if it were ordinary matter, we should style zvertéa and rigidity. 
No highly rarefied air will serve the purpose; the ether must be 
a distinct body. Air exzs¢s indeed in planetary space even to 
infinity, but it is of almost infinitesimal density compared with 
the ether there. It is easy to calculate the density of the atmo- 
sphere at any height above the earth’s surface, supposing other 
bodies absent. 
The density of the air at a distance of » earth radii from the 
centre of the earth is equal to a quarter the density here divided 
by 10” #. Soata height of only 4000 miles 2bove the sur- 
face, the atmospheric density is a number with 127 ciphers after 
the decimal point before the significant figures begin. The 
density of ether, on the other hand, has been calculated by Sir 
William Thomson from data furnished by Pouillet’s experiments 
on the energy of sunlight, and from a justifiable guess as to the 
amplitude of a vibration, and it comes out about io a 
number with only 17 ciphers before the significant figures. In 
inter-planetary space, therefore, all the air that exists is utterly 
negligible ; the density of the ether there, though small, is 
enormous by comparison. 
Once given the density of the ether, its rigidity follows at 
once, because the ratio of the rigidity to the density is the 
square of the velocity of transverse wave propagation, viz. in the 
case of ether, 9 x 107°. The rigidity of ether comes out, there- 
fore, to be about g00. The most rigid solid we know is steel, 
and compared with its rigidity, vz. 8 X 101}, that of ether is 
insignificant. Neither steel nor gla:s, however, could transmit 
vibrations with anything like the speed of light, because of their 
great density. The rate at which transverse vibrations are pro- 
pagated by crown glass is half a million centimetres per second 
—a considerable speed, no doubt, but the ether inside the glass 
transmits them 40,000 times as quick, viz. at twenty thousand 
million centimetres per second. 
The ether outside the glass can do still better than this, it 
comes up to thirty thousand million, and the question arises 
what is the matter with the ether inside the glass that it can 
only transmit undulations at two-thirds the normal speed. Is it 
denser than free ether, or is it less rigid? Well, it is not easy 
to say ; but the fact is certain that ether is somehow affected by 
the immediate neighbourhood of gross matter, and it appears to 
be concentrated inside it to an extent depending on the density 
of the matter. Fresnel’s hypothesis is that the ether is really 
denser inside gross matter, that there is a sort of attraction 
between ether and the molecules of matter which results in an 
agglomeration or binding of some ether round each atom, and 
that this additional or bound ether belongs to the matter, and 
travels about withit. The s%gzazty of the bound ether Fresnel 
supposes to be the same as that of the free. 
If anything like this can be imagined, a measure of the density 
of the bound ether is easily given. For the inverse velocity-ratio 
is called uw (the index of refraction), and the density is inversely 
as the square of the velocity, hence the density-measure is 17. 
The density of ether in free space being called 1, that inside 
matter has a density u*, and the density of the bound portion of 
this is w?—1. 
This may all sound very fanciful, but something like it is 
sober truth; not as it is here stated very likely, but the fact that 
(: - : )ih of the whole ether inside matter is bound to it and 
ye 
travels with it, while the remaining ur th is free and blows 
freely through the pores, is fairly well established and confirmed 
by direct experiment. 
(Zo be continued.) 
NATURE 
UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE 
CAMBRIDGE.—The following further announcements of lec- 
tures have been made :— 
Prof. Humphry, Circulatory and Respiratory Systems, Jan. 25; 
senior class, Jan. 29; Demonstrations by the Demonstrator for 
Natural Science Tripos, Jan. 26; Osteology, for beginners, 
Jan. 17; Demonstrations for second year students, Jan. 18; 
Mr. McAlister will give six lectures later in the term, on the 
Mechanism of the Human Skeleton. Dr. Michael Foster’s 
course of Elementary Physiology, Jan. 23 ; Mr. Lea, Chemical 
Physiology, Jan. 24; Dr. Vines, Anatomy of Plants, advanced, 
with practical work, Jan. 24 (Chri-t’s College) ; General Ele- 
mentary course, New Museums, Jan. 23, to extend over two 
terms, and be illustrated by demonstrations. A class for the 
practical study of systen.atic botany, Mr. T. H. Corry, assistant 
curator of the Herbarium, will be formed. Dr. Hicks will 
lecture on the Morphology of Flowering Plants, with especiay 
reference to classification, including floral diagrams, in the Hall 
of Sidney College, beginning Jan. 26; Mr. Glazebrook, 
advanced Demonstrations in Electricity and Magnetism, Caven- 
dish Laboratory, Jan. 24; Mr. Shaw, Demonstrations in 
Mechanics and Heat, Jan. 23; if more students attend than 
can be accommodated in the laboratory at one time, the course 
will be repeated on the same days. Mr. Trotter, Trinity Col- 
lege, Physical Optics, Jan. 25. Mr. Pattison Muir, Non- 
metallic Klements, Elementary, Jan. 22, Caius College Labora- 
tory; General Principles of Chemistry, Advanced, Jan. 23. 
Mr. Solly will give Demonstrations on Minerals in the Lecture 
Room of the Mineralogical Museum, first lecture, Jan. 22. Prof. 
Stuart, Jacksonian Lecture Room, Theory of Structures, Jan. 30 ; 
the Demonstrator of Mechanism, Mathematics required for 
Engineering, Jan. 29. 
Christ’s College Open Scholarships, Natural Science ; E. L. 
Sortain, Bath College, 30/.; 3rd year, J. C. Bose, 30/. ; Caius 
College, Natural Science, Edgworth, Clifton College, 40/. 
Mr. MARSHALL WARD is giving a course of free publie 
lectures at Owens College, on the Nutrition of Plants, 
SCIENTIFIC SERIALS 
Fournal of the Franklin Institute, January.—Electric lighting 
in mills, by C. J. H. Woodbury.—Bricks and brick-making 
machinery, by C. Chambers, Jun.—Experiential principles of 
controlled combustion, by E. J. Mallett, Jun.—Olsen’s testing 
machines. 
Archives des Sciences Physiques et Naturelles, December 15, 
1882.—Meteorological 7észmé of the year 1881 for Geneva and 
the great St. Bernard, by A. Kammermann.— Observations on 
cometary refraction, by W. Meyer.—Development of the vege- 
table kingdom in different regions since the tertiary epoch, 
according to Dr. Engler’s work, by A. de Candojle.—Periodical 
movements of the air indicated by spirit levels, by Ph. Planta- 
mour.—On the same, by C. von Orff. 
SOCIETIES AND ACADEMIES 
LONDON 
Chemical Society, January 18.—Dr. Gilbert, president, in 
the chair.—It was announced that a ballot for the election of 
fellows wouid be held at the next meeting, February 1.—The 
following japers were read:—The fluorine compounds of 
uranium, by A. Sumithells. The author has investigated the 
action of aqueous hydrofluoric acid upon the green uranoso-uranic 
oxide. He finds that a voluminous green powder, uranium 
tetrafluoride, is left, and that a yellow solution is formed which 
contains uranium oxyfluoride. The author confirms the results 
previously obtained by Bolton, and proves those obtained by 
Ditte to be erroneous.— On a new method of estimating the 
halogens in volatile organic compounds, by Kk. T. Plimpton 
and K. E. Graves. The authors burn the vapour of the compound 
in a glass Bunsen burner, the products of the combustion are 
aspirated through caustic soda solution, which is heated with 
sulphurous acid aad the halogen precipitated by silver nitrate, 
&c., in the usual way. Good results were obtained with various 
liquids from ethy| bromide boiling at 39° to acetylene bromiodide 
boiling at 150°. On a modified Liebig’s coidenser, by W. A, 
Shenstone. The author has slizhtly modified a vertical con 
[ Fan. 25, 1883 | 
