LXXVIII. Finally, there is no motion whatever, about the cause of which, the philosophers do not altercate. What contests have there been among them, to explain how the movements of rarefaction, and condensation are performed? Some fancy, that rarefaction consists, in the same quantity of matter occupying a larger space; which explanation, others looking upon as unintelligible, assert, it consists in the opening of the pores, and the extension in consequence of that opening, of various parts of the body, by the introduction of a subtil liquid substance, in the same manner that water enters a spunge, or as rarefied air enters water, or the ætherial matter insinuates itself into the air. This is the doctrine of the Cartesians; but in the opinion of the Maignanists, and Gassendists, such reasoning is nothing to the purpose; for these, as they admit in nature, not only of the possibility, but of the necessity of a vacuum, or many voids distributed in small interstices, find no inconvenience in supposing, that in bodies, there are small empty spaces, unpossessed by any matter whatever. LXXIX. Fermentation, this solemn instrument of nature for the performance of an infinite number of her works; consists in nothing, but an intestine motion of the insensible particles of the mixtures, by which is solicited, a new combination of their elements. But from whence does this motion proceed? The Moderns, since the discovery of the acid and the alkali by Otho Takenius, attribute all the fermentations, to the meeting of these two substances. But this is only pointing out the matter, on which this motion has its effect, and we do not enquire here for the material cause, but for the efficient one. Who, or what, impels the acid and the alkali to this conflict? The juice newly expressed from the grape, will quiet this combustion for some little time; but after that, the tumult begins again. What new agent can we discover here, to excite the second fermentation? This is a secret, which only the Cartesians have presumed to unravel, by recurring to their invisible fairy, _the subtil matter_; whom they make the author of all this domestic sedition. It is with some propriety that I call it a fairy; for as the vulgar attribute to the fairies, all the nocturnal noises and commotions, whose causes they are ignorant of; so the Cartesians impute all those motions, which on account of the darkness that obscures their causes, may be truly termed nocturnal, to the impulse of the subtil matter.