April 19, 1873 ] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS 
829 
The results of our observations conclusively show that 
the density of potassium-vapour, as produced in the pro¬ 
cess described, cannot exceed 45 times that of hydrogen, 
and that therefore the molecule of potassium consists of 
two atoms (K 2 ). 
We intend to prosecute our research in other directions, 
proposing to ascertain, if possible, the densities of the 
iodides of caesium, rubidium, and potassium, these being, 
according to Bunsen’s experiments, the most volatile of 
the haloids of the alkali metals. 
PROFESSOR TYNDALL ON LIGHT * 
(Continued from p. 807.) 
And now we have to push these considerations to a final 
illustration. Polarized light may be turned to account in 
various ways as an analyser of molecular condition. I hold 
in my hand a strip of glass six feet long, two inches wide, 
and a quarter of an inch thick. Holding it by the centre 
between my finger and thumb, I sweep over one of its halves 
a wet woollen rag ; you hear an acute sound, due to the 
vibrations of the glass. What is the condition of that 
glass while the sound is heard ? This : its two halves 
lengthen and shorten in quick succession. Its two ends, 
therefore, are in a state of quick vibration ; but what 
is the condition of the centre ? Here the pulses from 
the two ends alternately meet and retreat. Between 
their opposing actions the glass at the centre is kept 
motionless; but on the other hand it is here alter¬ 
nately strained and compressed. The state of the glass 
may be illustrated by spots of light, as the propagation of 
a sonorous pulse was illustrated in a former lecture. Here 
is a row of spots, which by a simple mechanical contriv¬ 
ance are made to vibrate to and fro. You notice the ter¬ 
minal dots have the largest amplitude of vibration, while 
those at the centre are alternately crowded together and 
drawn apart. The condition of this sounding strip of 
glass is here correctly represented. 
Now, supposing we introduce the glass between the 
crossed Nicols, taking care to keep the strip oblique to 
the direction of vibration of the Nicols ; and that we 
sweep our wet rubber over the glass, what will occur ? 
At every moment of compression the light will flash 
through; at every moment of strain the light will flash 
through; and these states of strain and pressure will follow 
each other so rapidly that we may expect a permanent 
luminous impression upon the eye. By pure reasoning, 
therefore, we reach the conclusion that the light will be 
revived upon the screen whenever the glass is sounded. 
That it is so, experiment testifies : at every sweep of the 
rubber a fine luminous disk flashes out upon the screen. 
The experiment may be varied in this way : Placing in 
front of the polarizer a plate of ornamented glass, you 
have those beautiful coloured rings, intersected by a black 
cross ; every sweep of the rubber not only abolishes the 
rings, but introduces complementary ones, the black cross 
being for the moment supplanted by a white one. This 
is Biot’s experiment, though his apparatus confined the 
observation of it to a single person at a time. But we 
have to follow the ether still further. Suspended before 
you is a pendulum, which, when drawn aside and then 
liberated, oscillates to and fro. If, when the pendulum 
is passing the middle point of its excursion, I impart a 
shock to it tending to drive it at right angles to its present 
course, what occurs ? The two impulses compound them¬ 
selves to a vibration oblique in direction to the former 
one. But the pendulum, as you see oscillates, in a plane. 
But if the rectangular shock be imparted to the pendu¬ 
lum when it is at the limit of its swing, then the com¬ 
pounding of the two impulses causes the suspended ball 
* Abstract of a series of lectures delivered in the Cooper 
Institute, New York, and reported in the New York 
Tribune . 
to describe, not a straight line, but an ellipse ; and if my 
shock be competent of itself to produce a vibration of the 
same amplitude as the first one, the ellipse becomes a 
circle. But why do I dwell upon these things ? Simply 
to make known to you the exact resemblance of these 
gross mechanical vibrations to the vibrations of light. I 
hold in my hand a plate of quartz cut from the crystal 
perpendicular to its axis. This crystal thus cut, possesses- 
the extraordinary power of twisting the plane of vibration 
or a polarized ray to an extent dependent on the thickness 
of the crystal. And the more refrangible the light, the 
greater is the. amount of twisting, so that where white 
light is employed its constituent colours are thus drawn 
asunder. Placing the quartz between the polarizer and 
the analyser you see this splendid colour, and turning the 
analyser in front, from right to left, the other colours ap¬ 
pear in succession. Specimens of quartz have been found 
which require the analyser to be turned from left to right, 
to obtain the same succession of colours. Crystals of the 
first class are therefore called right-handed, and of the 
second class, left-handed crystals. 
With profound sagacity Fresnel, to whose genius we 
mainly owe the expansion and final triuumph of the un- 
dulatory theory of light, reproduced mentally the me¬ 
chanism of these crystals, and showed their action to be 
due to the circumstance, that in them the waves of 
ether so act upon each other as to produce the condition 
represented by oar rotating pendulum. Instead of being 
plane polarized the light in rock crystal is circularly pola¬ 
rized. Two such rays transmitted along the axis of the 
crystal, and rotating in opposite directions, when brought 
to interference by the analyser, are demonstrably com¬ 
petent to produce the observed phenomena. 
I now abandon the analyser and put in its place the 
piece of Iceland spar, with which we have already illus¬ 
trated double refraction. The two images of the carbon 
points are now before you. Introducing a plate of quartz 
between the polarizer and the spar, the two images glow 
with complementary colours. Employing the image of 
an aperture instead of that of the carbon points, we have 
two complementary coloured circles. As the analyser is 
caused to rotate, the colours pass through various changes : 
but they are always complementary to each other. If the 
one be red, the other will be green ; if the one be yellow, 
the other -will be blue. And here we have it in our 
power to demonstrate a statement made in a former lec¬ 
ture, on the authority of Helmholtz, that, although the 
mixture of blue and yellow pigments produces green, the 
mixture of blue and yellow lights produces white. By 
enlarging our aperture, the two images produced by the 
spar are caused to approach each other, and finally to 
overlap. The one is now a vivid yellow, the other a vivid 
blue, and you notice that where the colours are super¬ 
posed we have a pure white. 
This brings us to a point of our inquiries which though 
not capable of brilliant illustration, is nevertheless, so- 
likely to affect profoundly the future course of scientific 
thought that I am unwilling to pass it over without re¬ 
ference. I refer to the experiment which Faraday, its 
discoverer, called the magnetization of light. The arrange¬ 
ment for this celebrated experiment is now before you.. 
We have put our electric lamp, then a Nmol's prism to 
polarize the beam emergent from the lamp ; then an 
electro-magnet, then a second Nicol’s prism, and finally 
our screen. At the present moment the prisms are 
crossed and the screen is dark. I place from pole to pole 
of the electro-magnet a cylinder of a peculiar kind of 
glass, first made by Faraday, and called Faraday’s heavy 
glass. Through this glass the beam from the polarizer 
now passes, being intercepted by the Nicol’s in front. I 
now excite the magnet, and instantly light appears upon 
the screen. On examination we find that by the action 
of the magnet upon the ether contained within the heavy 
glass the plane of vibration is caused to rotate, and thus 
get through the analyser. 
I have already mentioned the two classes into which.- 
