950 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[May 25, 1872. 
green is not heated in green; hut red is heated in green, 
and green in red. We have heating only where we 
have absorption; and the heat generated is the equiva¬ 
lent of the light absorbed. Black absorbs all the rays of 
light, hence, indeed, its blackness ; and if it could speak, 
it might tell us the warmth of every colour. But warmth 
exists outside the colours. Beyond the red, where 
nothing is seen, the force acting on the retina is far 
greater than when the eye is plunged in the red. The 
objective here is entirely out of proportion to the sub¬ 
jective. 
The existence of this heat was thus proved. All the 
colours but the red were cut off by a red glass, and with 
a diaphragm having a circular opening, a well-defined 
red circle was produced. This was refracted by a prism, 
still remaining a circle. A thermo-pile with its face 
towards the lamp was then caused to approach the path 
of the beam. It would have been seen by its shadow on 
the screen if the light had been at all invaded ; but with 
a considerable interval between the pile and the light, a 
large defection of the galvanometer testified to the 
presence of heat beyond the luminous circle. An opaque 
solution* was substituted for the red glass. A circle re¬ 
mained, but it was an invisible circle of radiant heat 
instead of a circle of light, and the needle of the gal¬ 
vanometer did not fall, though the visible image had 
vanished. 
Thus, as regards refraction, we have radiant heat be¬ 
having like light. And now for reflection. A horizontal 
beam of light was reflected upwards by a plane mirror, 
.and when the light was cut off' by the introduction of the 
opaque cell, a powerful beam of reflected heat was proved 
still to remain. The luminous beam was then totally 
orjlected by a prism to a horizontal direction; the light 
was again cut off, and a powerful deflection of the gal¬ 
vanometer needle was obtained by the residual heat- 
beam. Thus, in respect to common and total reflection 
±he behaviour of light and heat is the same. 
The action of lenses on light and heat was then demon¬ 
strated, the invisible heat-rays being brought to a focus 
as readily as the rays of light. 
A beam of light was then made to strike a concave 
mirror, and at the focus, which was strikingly visible in 
the dust of the room, the thermo-pile was placed, having 
its face covered. The light being cut off by the dark cell, 
and the covering screen drawn away, the needle of the 
galvanometer at once flew to its stops, 
g Double refraction by Iceland spar was next described 
and explained. It was illustrated by passing through 
the spar a circular beam of light, which, on the screen, 
gave two images. The places on the screen where these 
two images fell were marked, and the light was cut off 
by the iodine cell. On introducing the thermo-pile with 
its face towards the lamp, when it occupied the position 
of either light-image, a deflection of the needle was 
obtained. Of the two images, one is the ordinary, the 
other the extraordinary. Is the same true of the heat P 
Placing the pile in the place of the ordinary image, cutting 
off’ the light, and turning the spar, the deflection of the 
needle remained unchanged; but when the spar was 
turned round, while the pile occupied the place of the 
extraordinary image, the needle instantly fell. Why ? 
Removing the dark cell and rotating the spar, the 
extiaordinary light-image was seen to rotate round the 
ordinary one, which remained fixed. The heat-beam 
did the same and thus quitted the pile. Here then we 
prove that the heat-beam also has its ordinary and 
extraordinary image. This, it was believed, was the 
first time the effect had been obtained with purely in¬ 
visible heat. Knoblauch had demonstrated the double 
refraction of heat, using the total beam, luminous and 
non-luminous of the Sun. 
Some of the phenomena of polarization were next 
touched on. Light is propagated by the undulations of 
an etherial medium, the direction of vibration being per¬ 
pendicular to the direction of propagation. A crystal of 
tourmaline has the property of quenching all vibrations 
except those which are parallel to the axis of the crystal; 
hence, a plate of tourmaline cut parallel to the axis will 
allow all vibrations in that direction to pass through it, 
but will stop all others. A beam of light which has 
passed through one plate of tourmaline is therefore unable 
to pass through another placed transversely to it, whereas, 
if the axes are parallel the light is but little dimmed by 
the second plate. The black space due to the superposi¬ 
tion of the crossed plates of tourmaline was shown, as 
also the abolition of the darkness by a thin film of mica 
introduced between tbe plates. 
A beam with all its vibrations reduced to the same 
plane is called a beam of plane polarized light. 
The two beams emergent from double-refracting spar 
are thus polarized. Nicol got rid of one. He cut a 
parallelopiped of spar into two by a very oblique section, 
polished the two surfaces, and united them by Canada 
balsam. The ordinary or more powerfully refracted ray, 
at the surface of the balsam is, in consequence of its 
obliquity, totally reflected, and the extraordinary ray 
passes on alone. In this w T ay we obtain an intense beam 
of polarized light. 
A beam' of light was sent through two large Nicol 
prisms, and shown to be entirely extinguished when the 
principal sections of the prisms crossed each other. The 
introduction of a plate of mica between them caused, as 
in the case of the crossed tourmalines, the instant re¬ 
appearance of the light. The opaque cell was then 
placed in front of the lamp, all visible rays being thus 
intercepted. The thermo-pile was next placed so as to 
receive the beam after leaving the second Nicol prism. 
Causing one of the crossed prisms to rotate, a path was 
opened for the heat exactly as for the light, the deflection 
of the needle speedily bearing witness to the fact. The 
prisms being again crossed, the heat-beam was again 
quenched; but, as in the case of light, the introduction 
of a piece of mica restored the heat and caused a large 
deflection of the galvanometer. 
Faraday’s great experiment was next performed. A 
beam of light, polarized by one Nicol’s prism, was made 
to pass through a piece of heavy glass placed between 
the perforated poles of an electro-magnet, and afterwards 
through another Nicol, 60 placed that the beam was 
extinguished. When the magnet was excited, the plane 
of polarization was caused to rotate and a luminous image 
flashed instantly out upon the scene. 
The effect of magnetization is greatly augmented by 
adopting the device of MM. De la Provostaye and 
Desains, of causing the principal sections of the Nicol’s 
prism to enclose, not a right angle, but an angle of 45°. 
This was done, the heat falling on the pile being neutra¬ 
lized by the method of compensation. On sending a 
current round the magnet a considerable deflection of 
the needle was obtained, the direction of the deflection 
depending on that of the magnetizing current. 
De la Provostaye and Desains thus obtained with 
luminous solar heat a deflection of two or three degrees. 
With the iodine filter and the electric lamp a deflection 
equivalent to 150 of the lower degrees of the galvanometer 
was obtained from purely non-luminous heat. 
ADULTERATION OF CHEMICALS IN THE 
UNITED STATES. 
The following particulars as to adulterations of che¬ 
micals met with in commerce in the United States, are 
taken from the “ Report on Adulterations and Sophisti¬ 
cations” presented at the last meeting of the American 
Pharmaceutical Association.* m 
Acetic Acid .—Frequently weakened with water and 
* Iodine in bisulphide of carbon. 
* See also ante, p. 353. 
