tion of this subject, in 1838, we have had frequent opportunities to consult the oldest and most observing practical surveyors, and they have, without an exception, in every instance, corroborated the statements of Mr. Harris, and Lieutenant Johnson. One whose name, for particular considerations, we shall omit, but who was as good authority, probably, as any one we have consulted, not only testified his firm belief in the cause assigned by Mr. Harris, but suggested the thought, that both diurnal and annual variations of the needle might, possibly, be determined by the variations even of the thermometer.

This,

But some may, perhaps, be willing to acknowledge the premises, from which we started, but deny the validity of our conclusions.-They may assent to the proposition, that electricity causes such variations of the needle, as we have been contemplating, and that electricity may be produced by the mere friction of the sun's rays upon the glass cover of the compass, but, that it cannot be the sun-light itself. however, would be an assumption altogether unreasonable and unphilosophical. Even if produced by the friction of the rays, (which cannot be the case, since light passes so readily through a transparent medium, without friction,) either the light or the glass must give out the electricity; for, in all cases, where electricity is developed by friction, either the rubber or substance rubbed produces it. The one substance, that affects the other, is, uniformly, the substance that

is the generating agent. Even if light, then, produces electricity by friction upon the cover, it, after all, develops it from its own substance, and so, nothing is gained by the objector, nor are our conclusions at all impaired.

LECTURE VIII.

THE SINGULAR

PROPERTIES AND ELECTRIC QUALITIES

OF

LIGHT AND HEAT ILLUSTRATED BY ARGUMENTS DERIVED FROM VARIOUS SOURCES.

In addition to the facts, which were introduced into our last Lecture, and which were of themselves sufficient to demonstrate that light is electricity, it has been ascertained by the celebrated Mrs. Somerville, of England, that by passing the sun's rays through a prism, and separating them, by analyzation, into the seven primitive colors, the blue color possesses the power of imparting magnetism or polarity to the needle, and magnetism, we now know, to be electricity by experiments too conclusive to be controverted.

There is another very important fact respecting the organic laws of the constitution of both light and electricity, which furnish additional and weighty testimony in favor of identity. The attractions of electricity decrease in exact proportion as the squares of the distance increase, in receding from an electrified body. This is precisely, (as we should suppose,) the law of the divergence of light, and this law, which runs throughout all the imponderables, has its origin in the law of solar emanation or divergence, and the simple reason why the attraction of all bodies decreases in proportion as the squares of the distance increase, is

because the emanating influence of all bodies, which constitutes attraction, obeys this law.

The very strongest testimony, nowever, in proof of our proposition, is contained in the phenomena of the polarization of light, by which it is demonstrated, that every particle of light, as well as of electricity, has opposite polarities. This curious subject will be more fully examined, when we come to the discussion of the subject of caloric.

By a fair logical deduction, then, with facts amply to sustain it, we unhesitatingly infer that light is electricity.

Heat or caloric comes next in the order of remark, and, in the investigation of this subject we shall accumulate such an additional array of facts as shall establish our proposition beyond the shadow of a doubt.

The same arguments which would prove that light is an electric fluid are applicable, also, to the agent of heat. Heat like light is imponderable, subtle, ethereal and all-prevading. No obstacle can stay its passage. It insinuates itself between the particles of the densest bodies as though it were immaterial. Its power is prodigious-irresistible in its energies. It generates the tremendous power that propels the steam boat; and were it, or could it, by any means, be confined in subterranean volcanic caverns with bands strong enough, and there accumulated, it would, by the power of its expansive and explosive force, burst the solid globe to atoms, and send its shattered fragments in every direction through the vacuum that surrounds it.

With a glass bulb and tube, for instance, one of the energies of heat, can be forcibly demonstrated. By inverting it, and inserting the open end in a basin of water in its natural state, you will perceive no effect whatever. But, by passing into the tube the subtle agent of caloric from a spirit lamp, and again inverting it, you will see the water rise with great rapidity, and fill more than nineteen twentieths of the tube. This shows that heat has the power to expel the atmosphere and occupy its stead. But the moment you attempt to confine it there, by closing the tube, it is gone, like a flash-gone like a viewless, incorporeal, intangible thing, and the water rushes up to fill that

vacuum.

If all the imponderables, as we have assumed, be identical, then Light and Heat are the same-they coexist and are inseparable. But, it may occur to some one, that those phosphorescent substances, which emit light, do not, also, emit heat, and that our position is, therefore, untenable. This conclusion is, however, altogether too hastily formed. It will be seen, by the following lucid extract from Turner, that heat is always necessary to make substances phosphorescent.

"The chemical agency of artificial light is analogous to that from the sun. In general the former is too feeble for producing any visible effect; but light of considerable intensity, such as that from ignited lime, darkens chloride of silver, and seems capable of exerting the same chemical agencies as solar light, though in a degree proportionate to its inferior brilliancy.