488 
The adjoining figure gives a view of the interior of the 
Lecture Theatre as seen from its large oriel window in 
the front of the building. It is a lofty apartment lighted 
munly by two large windows, one the oriel just referred 
to, the other, seen in the picture, looking into the west 
quadrangle. 
experiments, in illustration of those lectures on the “ Pro- 
perties of Matter,” which have always formed a most 
interesting and suggestive part of Lord Kelvin’s course, 
and which, to every one who has heard them, have inten- 
sified the regret, felt by so many, that the second volume 
of Thomson and Tait’s “ Natural P hilosophy,” in which 
this subject was to be specially treated, is not to 
appear. 
In one of these experiments a slab of pitch, or of shoe- 
makers’ wax in water in a glass jar, is made to confine 
a number of common corks below it, while in the water 
From 
“Good Words." | 
Fic. 2.—Lecture Theatre, from front window. 
above the jar are placed a few lead bullets. After a 
month or 
above the pitch, while the lead bullets have sunk down 
through the pitch to the bottom of the jar. Other corks 
are on their way through, and, being imbedded in the 
pitch, are lost to view ; and of the paths followed by the 
corks and bullets, which have made the passage, no trace 
remains. All the time the pitch or wax is so brittle as to 
fly to pieces if thrown down ona table, or violently struck 
with a hammer. 
To the small continuously applied forces due to the 
corks and bullets the pitch has behaved like a fluid ; in- 
deed, its properties have been precisely those of a highly 
viscous liquid. 
shape, but the resisting force has depended on the rate 
of progress of the change, not, as elastic resistance 
would, on the amount of change already accomplished. 
On the other hand, a piece of the same pitch melted 
into the form of a bell, and struck with a hammer in the 
NO. 1430, VOL. 55] 
NATURE 
On the sill of the latter window, which is 
passed each day by every student entering or leaving 
the room, are usually arranged a series of semi-secular 
[From a photograph by T. and R, 
two some of the corks are found in the water | 
It has offered resistance to change of 
[Marcu 25 
5, 1897 
ordinary way, would give out a musical note, showing 
that for rapidly alternating changes of shape the forces 
excited in the pitch are proportional to the strains pro- 
duced ; which indicates that the material under the latter 
conditions possesses the properties of a solid. Thus one 
and the same substance may, according to the circum- 
stances in which it is placed, behave either as a viscous 
liquid or as an elastic solid. 
This result is important as bearing upon the difficulty 
as to how the luminiferous ether, under any conceivable 
estimate of its density, can possess so high a degree of 
rigidity as to transmit the waves of transverse oscillation, 
which, according to the elastic solid theory of the ether, 
we have in light, with a velocity of 3 x 10!° centimetres 
per second ; while the planets and the components of 
double or multiple stars move freely through it. The 
difficulty (if it is a real difficulty, and is not to be got rid 
of in a new view of the propagation of light based on 
electromagnetic theory) is not explained by this experi- 
ment ; but it is reduced by it 
to an affair of properties of 
matter, by being shown to 
have a parallel in a pheno- 
menon of which we have 
undoubted experience. 
Another piece of apparatus 
in the window is a model 
glacier in which a slope of 
wood takes the place of the 
sloping bed on the moun- 
tain-side, and shoemakers’ 
wax that | of Mice: [See 
Dr. Bottomley’s descrip- 
tion in NATURE for De- 
cember 18, 1879 (vol. xxi. 
p. 159).] 
In the window also are 
generally displayed tubes 
illustrating the diffusion of 
liquids into one another, and. 
the osmotic passage of a 
sugar solution through a 
diaphragm. 
On the other side of the 
room is a large oriel window, 
which is partly visible in 
the view of the class-room 
table and lecture appar- 
atus given in Fig. 3. Set 
up in this oriel window 
are two tall tubes running 
nearly the whole height 
of the room, and _pro- 
tected by wooden cases fitted 
with glass doors. One of 
these tubes illustrates the diffusion of sulphate of copper 
solution upwards into water, and the water itself in the 
opposite direction. The other tube shows the same 
thing for water and alcohol. These tubes were set up 
nearly a quarter of a century ago, soon after the new 
building was taken possession of by the University; and 
the original surfaces of separation, with the dates, are 
marked upon them. This is, perhaps, the longest ex- 
periment on diffusion that has ever been carried on ; 
but of course it is capable of infinite duration, as an in- 
finite time would have to elapse before the liquids in 
the tubes were completely mixed by this process. In 
his lectures Lord Kelvin is fond of accomplishing the 
work of an infinite time in diffusion, by reversing two 
or three times.a closed tube in which the liquids have 
innan and Sons, Glasgow. 
| been originally separated by their different specific 
gravities. 
The progress of diffusion in the secular experiments is 
shown by the motion of specific gravity beads (small 
