846 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[April 26, 1S7&. 
I will now cause the rays to converge to a focus, by the 
aid of a concave mirror. The mirror, silvered in front, is 
placed in this tin camera behind the electric lamp. A 
cone of rays, reflected, passes through the aperture in 
front, closed by a window of rock salt, and forms a focus 
outside the camera. 
If I place paper in the focus formed by this mirror it 
will ignite. Zinc placed in it will not only fuse but also 
bum and vaporize. Magnesium will do the same. 
Even such refractory metals as copper and silver will 
be fused on being brought within the focus. The same 
effects may be produced by the concentration of purely 
radiant heat. For this purpose convex lenses are em¬ 
ployed to convey the purely radiant heat proceeding from 
the carbon points. Even a lens of so cold a substance as 
ice may be employed for this purpose. I have here a lens 
of ice brought to such a shape by being rubbed in a 
mould. At the focus to which the rays are brought by 
means of this lens, paper may be ignited and metals fused, 
and gun-cotton may be exploded. 
This mode of concentrating the rays of heat—namely, 
by lenses—is the same as that first employed by the cele¬ 
brated Academy of Florence. But the employment of a 
lens of ice is something remarkable. It was performed 
by Scoresby in the Arctic regions with solar light. It 
has, however, never before been performed with artificial 
light. The lens may be cold, as the ice is, and yet trans¬ 
mit sufficient heat to ignite various substances. This 
looks odd, but it is easy of explanation when we reflect 
that it is only those rays that are absorbed that produce 
the impression of heat. I shall now endeavour to ignite 
a diamond by the heat radiated from the carbon points. 
This experiment is a difficult one, and requires great cau¬ 
tion. The diamond, as you see, is suspended in a jar. 
The jar is filled, by displacement, with oxygen ; and the 
diamond is then brought within the focus of the light 
from the mirror. 
Looking at these effects, the question naturally arises, 
What constituent of the radiation produces them ? I 
introduce a cell of alum in front of the aperture of the 
camera, causing it to interfere with the light passing 
from the mirror to the focus. As you observe, the light 
remains as it was, but upon placing, as I now do, some 
gun-cotton in the focus, it shines brightly, but does not 
explode. As soon, however, as the alum cell is removed, 
the gun-cotton explodes. 
This experiment shows us that there must be some 
constituent of the beam of light which, being transmitted, 
produces the calorific effect. What is this constituent ? 
If we bring the light to a focus by means of the mirror 
within the camera, and then cut off all light, certain 
effects will follow which will give us a clue. Chemical 
combination exerts an enormous influence on the lumini¬ 
ferous ether. Every person in this brilliant audience is 
radiant with these rays, and the reason why we cannot 
see them is that the vibrations are too slow to awaken 
sensation in the retina. The atmosphere, which is a 
mechanical mixture of nitrogen and oxygen, is a practical 
vacuum to the passage of heat rays. When the atmos¬ 
phere is dry there is an enormous reduction of temperature 
because of the amount of heat radiated by the earth into 
space. The elementary bodies, such as oxygen, hydrogen, 
chlorine, bromine, etc., are remarkably transparent to the 
rays of heat. Let us see what will result from using with 
our apparatus iodine dissolved in bisulphide of carbon. 
By means of the iodine solution employed in this 
manner, we may intercept all the luminous rays of the 
spectrum and then examine the calorific rays alone. I 
now place a piece of paper where the focus of light was, 
and, as you observe, it instantly ignites. 
I will now employ a sheet of platinum, coated with pla¬ 
tinum black, instead of the paper used in the first experi¬ 
ment. Platinum, with its surface thus prepared, acts in 
regard to the invisible heat rays just as the sulphide of 
quinine paper did to the invisible light rays. A visible 
image of the carbon points appears on the surface of the 
platinum. The temperature of the air surrounding the 
glowing platinum has nothing to do with this circum¬ 
stance of heating. The air at the focus may be of a 
frosty temperature, since no heat is communicated to it 
from the mirror. The heat raises the platinum to a 
white heat while not affecting the air. This, of course, 
is owing to the fact that while the air cannot absorb the 
heat rays, the platinum can and does. Still keeping the 
solution of iodine in bisulphide of carbon in front of the 
camera, and thus cutting off all the luminous rays, I will 
now proceed to burn at the perfectly dark focus paper 
and several of the metals. 
What agent produces these effects ? Let us heat a 
platinum wire by means of the voltaic current. The wire 
soon reddens, and finally glows with a white heat. Dr. 
Draper, to whom I am glad to pay a tribute of respect, 
has shown by prismatic analysis of the light emitted by 
the glowing wire that the light at first is a pure red. As 
the glow augments the red becomes more brilliant, but 
at the same time orange rays are added to the emission. 
Augmenting the temperature still further, yellow rays 
appear beside the orange, after the yellow green rays 
are emitted, and after the green come in succession blue, 
indigo, and violet rays. To display all these colours 
at the same time the platinum wire must be white-hot : 
the impression being in fact produced by the simultaneous, 
action of all these colours on the optic nerve. 
Let us reason backward from this experiment of Dr. 
Draper. At the beginning, and even before the electric, 
current had acted at all upon the wire, it emitted in¬ 
visible rays. The vibrations or waves causing these were 
too long and too slow to excite vision. What becomes of 
these waves ? They are not destroyed, but their intensi¬ 
ties are augmented as the shorter are introduced. It is 
the wave amplitude and not the wave length which 
determines the intensity of heat as well as of light and 
also of sound. The rule is that the square of the maximum 
velocity of the wave determines its intensity. All these 
waves existed simultaneously. Without such a provision 
there could be no sun. There cannot be light rays without 
dark rays also being found. This, Herschel’s great dis¬ 
covery has shown. How shall we discover these dark 
rays ? The human eye is delicate as regards the visual 
rays. But when we come to other rays, not capable of 
exciting visual impressions, the eye will not suffice, and 
we must employ an instrument of greater nicety. 
I notice in the poetry of Emerson many allusions and 
expressions which find their application. For instance, 
he speaks at one place of the discontent felt by the 
human mind for what we may call the statical condition. 
Like an organic growth is the growth of science. Some¬ 
thing seems an end, but when that end is reached the 
process does not stop, the bud expands and opens and 
fruit is bom ; and so in science, an investigation of a 
certain subject discovers facts which in the investigator’s 
own and others’ hands ripen into yet greater discoveries. 
In 1821 Seebeck of Berlin discovered that heat applied at 
the junction of two metals of unequal heat-conducting 
power soldered together at one end, the other extremities 
of the bars being connected with a galvanometer, would 
give rise to electric currents. Oersted, Nobili, and others 
caught at the idea and turned it to account in constructing 
a thermometer on this principle, by means of which most 
delicate changes of temperature may be determined. I 
place the instrument here and cast an image of the gal¬ 
vanometer dial on the screen. On approaching it with 
the hand the heat of the latter incites a current which 
deflects the needle and its image on the screen. 
With this instrument we may discover the seat and 
intensity of the invisible rays of heat in the spectrum. 
This is done by carrying the thermo-electric pile through 
the spectrum many successive times, and recording its 
indications. The chart above the screen represents the 
range of the heat rays. 
By this and other modes of experiment it has been, 
shown that the heat radiated from the non-luminou» 
