EMILY.
But what then would be the electric spark which is visible, and must therefore be really material?
MRS. B.
What we call the electric spark, may, Sir H. Davy says, be merely the heat and light, or fire produced by the chemical combinations with which these phenomena are always connected. We will not, however, enter more fully on this important subject at present, but reserve the princ.i.p.al facts which relate to it to a future conversation.
Before we part, however, I must recommend you to fix in your memory the names of the simple bodies, against our next interview.
CONVERSATION II.
ON LIGHT AND HEAT OR CALORIC.
CAROLINE.
We have learned by heart the names of all the simple bodies which you have enumerated, and we are now ready to enter on the examination of each of them successively. You will begin, I suppose, with LIGHT?
MRS. B.
Respecting the nature of light we have little more than conjectures. It is considered by most philosophers as a real substance, immediately emanating from the sun, and from all luminous bodies, from which it is projected in right lines with prodigious velocity. Light, however, being imponderable, it cannot be confined and examined by itself; and therefore it is to the effects it produces on other bodies, rather than to its immediate nature, that we must direct our attention.
The connection between light and heat is very obvious; indeed, it is such, that it is extremely difficult to examine the one independently of the other.
EMILY.
But, is it possible to separate light from heat; I thought they were only different degrees of the same thing, fire?
MRS. B.
I told you that fire was not now considered as a simple element. Whether light and heat be altogether different agents, or not, I cannot pretend to decide; but, in many cases, light may be separated from heat. The first discovery of this was made by a celebrated Swedish chemist, Scheele. Another very striking ill.u.s.tration of the separation of heat and light was long after pointed out by Dr. Hersch.e.l.l. This philosopher discovered that these two agents were emitted in the rays of the sun, and that heat was less refrangible than light; for, in separating the different coloured rays of light by a prism (as we did some time ago), he found that the greatest heat was beyond the spectrum, at a little distance from the red rays, which, you may recollect, are the least refrangible.
EMILY.
I should like to try that experiment.
MRS. B.
It is by no means an easy one: the heat of a ray of light, refracted by a prism, is so small, that it requires a very delicate thermometer to distinguish the difference of the degree of heat within and without the spectrum. For in this experiment the heat is not totally separated from the light, each coloured ray retaining a certain portion of it, though the greatest part is not sufficiently refracted to fall within the spectrum.
EMILY.
I suppose, then, that those coloured rays which are the least refrangible, retain the greatest quant.i.ty of heat?
MRS. B.
They do so.
EMILY.
Though I no longer doubt that light and heat can be separated, Dr.
Hersch.e.l.l"s experiment does not appear to me to afford sufficient proof that they are essentially different; for light, which you call a simple body, may likewise be divided into the various coloured rays.
MRS. B.
No doubt there must be some difference in the various coloured rays.
Even their chemical powers are different. The blue rays, for instance, have the greatest effect in separating oxygen from bodies, as was found by Scheele; and there exist also, as Dr. Wollaston has shown, rays more refrangible than the blue, which produce the same chemical effect, and, what is very remarkable, are invisible.
EMILY.
Do you think it possible that heat may be merely a modification of light?
MRS. B.
That is a supposition which, in the present state of natural philosophy, can neither be positively affirmed nor denied. Let us, therefore, instead of discussing theoretical points, be contented with examining what is known respecting the chemical effects of light.
Light is capable of entering into a kind of transitory union with certain substances, and this is what has been called phosph.o.r.escence.
Bodies that are possessed of this property, after being exposed to the sun"s rays, appear luminous in the dark. The sh.e.l.ls of fish, the bones of land animals, marble, limestone, and a variety of combinations of earths, are more or less powerfully phosph.o.r.escent.
CAROLINE.
I remember being much surprised last summer with the phosph.o.r.escent appearance of some pieces of rotten wood, which had just been dug out of the ground; they shone so bright that I at first supposed them to be glow-worms.
EMILY.
And is not the light of a glow-worm of a phosph.o.r.escent nature?
MRS. B.
It is a very remarkable instance of phosph.o.r.escence in living animals; this property, however, is not exclusively possessed by the glow-worm.
The insect called the lanthorn-fly, which is peculiar to warm climates, emits light as it flies, producing in the dark a remarkably sparkling appearance. But it is more common to see animal matter in a dead state possessed of a phosph.o.r.escent quality; sea fish is often eminently so.
EMILY.
I have heard that the sea has sometimes had the appearance of being illuminated, and that the light is supposed to proceed from the sp.a.w.n of fishes floating on its surface.
MRS. B.
This light is probably owing to that or some other animal matter. Sea water has been observed to become luminous from the substance of a fresh herring having been immersed in it; and certain insects, of the Medusa kind, are known to produce similar effects.
But the strongest phosph.o.r.escence is produced by chemical compositions prepared for the purpose, the most common of which consists of oyster sh.e.l.ls and sulphur, and is known by the name of Canton"s Phosphorus.
EMILY.
I am rather surprised, Mrs. B., that you should have said so much of the light emitted by phosph.o.r.escent bodies without taking any notice of that which is produced by burning bodies.