[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:255|3|0]]

Passing on now to the question of the nature of comets, I assume as established that they all have as their nucleus a more or less dense swarm of meteors. This conclusion rests partly on the clearly proved connection of the Leonid, Perseid, and Andromedid systems with the comets of 1866, 1862 and Biela’s comet, partly on the impossibility of conceiving that such comets as Halley’s could persist for so many returns if they were mere bunches of vapour. According to the generally accepted views of Dr Johnstone Stoney, even the moon and the smaller planets are incapable of permanently retaining atmospheres, in consequence of the rapid motion of the gaseous molecules. Since we are certain that the mass of Halley’s comet is much less than that of the moon, it is evident that its gravitational power would be too weak to hold it together if wholly gaseous. It is probable that the meteors are continually giving off small quantities of gas (at least while in the neighbourhood of the sun) since otherwise we should expect the vapourous envelope to be dissipated with fair rapidity. The fact that Halley’s comet has been emitting such large tails at every return for at least 2000 years makes it probable that in its case the meteoric masses are of considerable size, perhaps larger than the large masses in our museums, since these must have suffered loss in their passage through the air. For we should expect small lumps, under a foot in diameter to give up their whole supply of gas at a single apparition.

I put forward here the conjecture that, since it is only near perihelion that the loss of gas occurs, a large periodic time is favourable to a long life of the comet; hence the prevalence of nearly parabolic orbits may be a case of «Survival of the fittest», the comets with shorter periods having already exhausted their supply of gas, and therefore ceased to exist as visible comets. The disappearance of Biela’s comet presumably means only the loss of the gas contained in its meteors; these are still moving in the same orbit, as is shown by the many showers of Andromedids that have been observed since the comet disappeared.

I now pass to an independent proof of the meteoric constitution of a comet’s nucleus. This rests on the extremely close agreement between theory and observation in the date of the return of Halley’s comet to perihelion. The