in the case of those substances which have been compacted from unctuous matter, which when the atmosphere is very thin, when the North winds, and amongst us (English) the East winds, are blowing, have a surer and firmer effect, but during South winds and in damp weather, only a weak one; so that those * substances which attract with difficulty in clear weather, in thick weather produce no motion at all; both because in grosser air lighter substances move with greater difficulty; and especially because the effluvia are stifled, and the surface of the body that has been rubbed is affected by the spent humour of the air, and the effluvia are stopped at their very starting. On that account in the case of amber, jet, and sulphur, because they do not so easily take up moist air on their surface and are much more plenteously set free, that force is not so quickly suppressed as in gems, crystal, glass, and substances of that kind which collect on their surface the moister breath which has grown heavy. But it may be asked why does amber allure water, when water placed on its surface removes its action? Evidently because it is one thing to suppress it at its very start, and quite another to extinguish it when it has been * emitted. So also thin and very fine silk, in common language _Sarcenet_, placed quickly on the amber, after it has been rubbed, * hinders the attraction of the body; but if it is interposed in the intervening space, it does not entirely obstruct it. Moisture also from spent air, and any breath blown from the mouth, as well as water put on the amber, immediately extinguishes its force. But oil, which is light and pure, does not hinder it; for although amber * be rubbed with a warm finger dipped in oil, still it attracts. But * if that amber, after the rubbing, is moistened with _aqua vitæ_ or spirits of wine, it does not attract; for it is heavier than oil, denser, and when added to oil sinks beneath it. For oil is light and rare, and does not resist the most delicate effluvia. A breath therefore, proceeding from a body which had been compacted from humour or from a watery liquid, reaches the body to be attracted; the body that is reached is united with the attracting body, and the one body lying near the other within the peculiar radius of its effluvia makes one out of two; united, they come together into the closest accord, and this is commonly called attraction. This unity, according to {57} the opinion of Pythagoras, is the principle of all things, and through participation in it each several thing is said to be one. For since no action can take place by means of matter unless by contact, these electricks are not seen to touch, but, as was necessary, something is sent from the one to the other, something which may touch closely and be the beginning of that incitement. All bodies are united and, as it were, cemented together in some way by moisture; so that a wet body, when it touches another body, attracts it, if it is small. So