LXXII. Some philosophers, who were aware of this serious difficulty, upon reflexion, conjectured that some portion of ethereal matter or pure air, might adhere to each particle of vapour; and in consequence of this conjunction of the two, the whole might become lighter than an equal quantity of the inferior gross air of our atmosphere; and on this account, might mount on the top of it; for although iron is much heavier than water, if we fasten a small portion of iron to a bit of deal board, it will swim, because the quantity of iron and board together, is lighter than an equal bulk of water. Francis Bayle, adopts this opinion of a portion of the ethereal matter adhering to the vapour; and on the other hand, Father Pardies a French jesuit supposes, that a particle of extended vapour, in the form of an air bubble, contains in its cavity the ethereal matter. All this is far from certain; but I shall not detain myself in controverting, either the one or the other of these modes of reasoning. Others again imagine, that various igneous particles, which arise from the earth, after separating from the water or some other liquid those small particles which we call vapour, with their continual impulse and agitation, force them upwards. But neither does this system appear to me very defensible, any more, than that of the vulgar philosophers, who say, the sun by its activity attracts the vapours; for if this was so, the vapours would not stop till they reached the sun, or at least till they were obstructed in the heaven of the moon, by the moon itself, or some other solid body; for the attractive power is stronger, the nearer the thing attracted advances to that which attracts it; and the first would never cease to move towards the other, till it came in contact with it, if it was not interrupted in its course by some obstruction; and besides, the attractive virtue is a certain something, which nobody can comprehend, and is therefore in a manner wholly banished from philosophy. LXXIII. Who would not be surprised, that physics should not have helped us to attain a knowledge of so common a phenomenon, as that of the ascent of vapours? but so far has it been from attaining it to any degree that comes near to truth or certainty, that hitherto, we have not been able to discover any thing upon the point, that is satisfactory to the understanding. And with respect to all other species of motion, we are in the same predicament. SECT. XVIII.