Now in applying this to comets' tails it is only necessary to remark that they are composed of either gaseous or dusty particles, or both, rising from the nucleus, probably under the influence of the heat or the electrical action of the sun, and these particles, being below the critical size, are driven away from the sun, and appear in the form of a tail following the comet. It may be added that the same principle has been evoked to explain the corona of the sun, which may be composed of clouds of gas or dust kept in suspension by the pressure of light.