20. There is one additional remark, which we may make, as to the resemblance of cometary[9] and nebular matter. That cometary matter is of very small density, we have many reasons to believe:--its transparency, which allows us to see stars through it undimmed;--the absence of any mechanical effect, weight, inertia, impulse, or attraction, in the nearest appulses of comets to planets and satellites:--and the fact that, in the recent remarkable event in the cometic history, the separation of Biela's comet into two, the two parts did not appear to exert any perceptible attraction on each other, any more than two volumes of dust or of smoke would do on earth. Luminous cometary matter, then, is very light, that is, has very little weight or inertia. And luminous nebulous matter is also very light in this sense: if our account of the cause of spiral nebulæ has in it any truth. But yet, if we suppose the nebulæ to be governed by the law of universal gravitation, the attractive force of the luminous matter upon itself, must be sufficient to bend the spirals into their forms. How are we to reconcile this; that the matter is so loose that it falls to the centre in rapid spirals, and yet that it attracts so strongly that there is a centre, and an energetic central force to curve the spirals thither? To this, the reply which we must make is, that the size of the nebular space is such, that though its rarity is extreme, its whole mass is considerable. One part does not perceptibly attract another, but the whole does perceptibly attract every part. This indeed need the less surprise us, since it is exactly the case with our earth. One stone does not visibly attract another. It is much indeed for man, if he can make perceptible the attraction of a mountain upon a plumb-line; or of a stratum of rock a thousand feet thick upon the going of a pendulum; or of large masses of metal upon a delicate balance. By such experiments men of science have endeavored to measure that minute thing, the attraction of one portion of terrestrial matter upon another; and thus, to weigh the whole mass of the earth. And equally great, at least, may be the disproportion between the mutual attraction of two parts of a nebulous system, and the total central attraction; and thus, though the former be insensible, the latter may be important.