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172, has produced in a dark chamber, by means of the sun's rays, a phenomenon precisely similar to the corona. These experiments are exceedingly interesting when viewed in connexion with the total solar eclipse of the 18th of July 1860. It is confidently expected that this debated point will finally be settled by the observations to be then made. The representation of comets by the same means is very interesting and striking. Observation confirms the supposition that comets are not composed of a continuous substance like a gas, but of a mass of discrete particles, like a cloud of dust. This is known by its not refracting light like our gaseous atmosphere. Some comets, when very near the sun, seem to lose their envelopes, though they should now expand to their largest dimensions. The explanation of Herschel is, that the heat converts the visible discontinuous particles into an invisible gas. A comet would thus correspond to a cloud which consists of watery particles, but which become invisible on the application of heat—the watery particles being converted into the gaseous form of invisible vapour. So attenuated is the matter of comets, that, if condensed into a solid body, the largest might probably be packed into a ship's hold. This is deduced from the fact that the largest comets do not, in the slightest degree, affect the motion of planets or satellites, however close they may approach; and we have the case of one comet which