experiments, by which the laws of the activity of this spirit could be accurately determined."

In Sir Isaac Newton's Optics, the same thought appears in the modest form of queries.

"Is not heat conveyed through a vacuum by the vibrations of a much more subtile medium than air? Is not this medium the same by which light is refracted and reflected, and communicates heat to bodies, and is put into fits of easy reflexion and transmission? Do not hot bodies communicate their heat to cold ones by the vibrations of this medium? And is it not exceedingly more rare and subtile than the air, and exceedingly more elastic and active? And does it not readily pervade all bodies? And is it not, by its elastic force, expanded through all the heavens?"

Since the days of Newton, science has done much to supply the deficiency of experiment to which he alludes. The Undulatory Theory of Light, in displacing his own, has lent new evidence of the truth of his modest conjectures. In the hands of Young, Fresnel, Malus, Cauchy, Airy, Lloyd, and Stokes, it has come very near to astronomy in the singular and beautiful triumphs of its analysis, and the large variety of curious phenomena which have been explained. But its first postulate is the existence of an ether, such as the sagacity of Newton divined long ago, and indeed the same by which Huyghens, his contemporary, had already solved the phenomena of double refraction.

This ether must be diffused through all known space. For light is caused by its undulations, and this reaches us from stars immensely distant. Its elasticity, also, implies its diffusion in all directions, and not merely in the lines of the transmission of stellar light.

Again, this ether must be self-repulsive. If its particles were mutually attractive, they would evidently condense around centres, where there was any excess of density at first, and light could not pass from one of these condensing systems to another. Mutual repulsion, therefore, must plainly be one of its fundamental laws.

The denial of the existence of this ether, when confirmed by so many discoveries of modern times, though M. Comte in his Lectures ventures to style it a mark of superior wisdom, is a step backwards into the nonage of science. To recognise a cause, which is pointed out by many classes of phenomena converging together, is just as imperative a law of sound philosophy, as to reject and refuse every really superfluous element.

8. No second fluid, of caloric, electricity, or magnetism, ought to be recognised, until it can be proved that the action and reaction of common matter and a luminous ether are incapable of supplying the required explanation.

This results at once from the second Axiom. The

simplest hypothesis may justly claim to be the first examined. That which recognises matter alone, is disproved by the phenomena of cohesion and of light. Next in order of simplicity is that which recognises both matter and a luminiferous and elastic ether, but no other fundamental variety. The ready adoption of so many hypothetical fluids, as have been often proposed, repels thoughtful students, and makes them ready to question the existence of any ethereal medium whatever, distinct from common or ponderable matter. The line of safe induction lies between these two extremes; and the most able analysts and experimentalists already tend to this middle view.

9. The existence of matter and ether requires the ad

mission of three, and only three, laws of force for their mutual action.

First, matter acts on matter, and the law of its action is already known, being one of mutual attraction, inversely as the square of the distance.

Secondly, matter must act on ether, and ether on matter. This force must also be attractive, since otherwise the ether would not be condensed around the material atoms, nor give rise to cohesive affinity between neighbouring particles. Its exact law of force is hitherto unknown.

Thirdly, ether acts on ether. This force must be repulsive; since otherwise the ether would converge into patches, and not be diffused through space, as the transmission of light from the stars in every direction evidently proves it to be.

Fourthly, these two unknown laws cannot be the same with that of gravitation, or vary only as the inverse square. In this case all three forces would increase and diminish together, and cohesive attraction and ethereal repulsion would have the same relative amount at great as at small distances. There could thus be neither increase of cohesion, nor of resistance to pressure, however closely particles were packed together. These two forces, then, must follow some higher law than the inverse

square.

10. The ether of the universe greatly exceeds in quantity, or in the number of its atoms, the amount of ponderable

matter.

The force of cohesion, by the hypothesis, must depend on the presence of ether along with the material atoms. Hence, even in solids and liquids, the number of ether

atoms must exceed that of the material atoms. But the planetary and stellar spaces are all filled with ether, and nearly void of matter. The space of the solar system half way to the nearest stars, exceeds the bulk of the sun more than 1022 to 1, or ten thousand trillion times. It does not seem likely that the mean distance of the monads of matter can be much less than that of the free ether in space. We may conclude that the number of ethereal is immensely great, compared with that of the material

atoms.

11. The mean distance of the particles of free ether must be less, and is probably far less, than one ten millionth of an inch.

The violet rays of light make 60,000 undulations in one inch. With the limit assumed above, there would be only 167 ether particles in the length of a wave. In like manner 250 would be the number in the length of a red ray, and the difference about 80. It seems plain that, in a discontinuous medium of the kind suppposed, no vibration can be propagated to any distance, of which the length is not some multiple of the mean interval from atom to atom. But the black lines of the speculum divide it into a much greater number of different shades or kinds of light. Hence it seems to follow that the distance of the atoms must be less than 1000000 in. and may be very greatly less.

12. The pressure of the ether on any surface must be immensely great.

The pressure of common air, at the earth's surface, is nearly fifteen pounds per square inch. Now when two media are compared, the velocity of a vibration varies as the square root of the pressure, or modulus of the elasticity.

The velocity of sound, however, apart from its increase by the momentary heat, is only 916-3 feet per second, while that of light is 192,000 miles. The ratio is thus 1,106,360 to 1. The ethereal pressure, then, must be 1,224037,000000 times greater than that of atmospheric air, or 18 billions of pounds to the square inch. It is plain, then, how powerful must be the action of what are usually called the imponderable elements, or the mechanical forces, which light, heat and electricity, bring continually into play.

13. The action of matter on ether must vary as the inverse cube or some higher law, and the repulsion of ether on ether, as the inverse fourth, or some higher integer power.

First, the principle of the second Axiom requires us to assume integer, rather than fractional powers, for the two unknown laws of force, until decisive evidence to the contrary can be found, since the assumption is evidently far simpler. The nature of cohesion, also, and ethereal action, evidently requires inverse powers. But the action of matter on ether must increase more rapidly than gravitation for small distances, or else cohesion would not be limited to such distances. Hence the inverse cube is the lowest admissible power.

Again, the repulsive force of the ether must vary more rapidly than the affinity of matter for ether, since otherwise there would be no limit to the condensation, and attraction would increase the fastest with the density of The inverse fourth is the lowest power which can satisfy this first condition. Reasons, however, will presently appear for assuming still higher powers for the true laws of nature.

the mass.

14. The Three Laws of Force imply two independent