not at all to be wonder’d, if a Person, much accustom’d to Exercises, notwithstanding the daily Expence of a greater Perspiration, should have his Blood of a better Condition, and more Rich than that of another Person living a sedentary Life, by reason of the greater Impression, the greater Tincture (if I may be allow’d so to speak) of this most exquisite and inimitable Fluid. These Things are not to be stated exactly, and yet they are not to be accounted altogether precarious; for tho’ we shall never perhaps be able to know exactly what the Animal Spirits are, yet we may make a shift to distinguish what they are not. According to the common Notions, a well-prepar’d volatile Salt, after it has pass’d the Lacteals, and comes into the Blood, might be taken to be a pure Animal Spirit; and yet, undoubtedly, the Fluid, prepar’d by the Glands of the Brain, has something in it transcendently preferable to any thing that can be the Effect of Art. Whether those Glands are so dispos’d, as to unite some Nitro-Aerial Particles with others proper to serve as a Vehicle to ’em, is not to be determin’d by me or any Body else; but it may not be altogether so absurd, to guess at some such thing, since we know nothing in Nature that can afford Particles of that Elasticity as Nitre does; and we may discern, that the Animal Spirits seem to consist of a _Fulgur_, an _Impetum faciens_, something that is Irraditating; and yet withal there seems to be something extremely Mild and Plastick, and as it were Tenacious, combin’d with the Elastick. I hope I don’t run into an _Hypothesis_; I would carry this no farther than it can be kept in Countenance by _Phænomena_ arising in the Cure of Distempers; for thus we see in the Nervous Atrophy, tho’ the Spirits, taken as _Impetum facientes_, pass freely, and are not obstructed, as in the Palsie, yet the Benign Plastick Quality seems to be wanting, because the Habit of Body does not thrive, tho’ the Spirits are brought all over it; and that the Spirits, when they are in their true Purity, are concern’d in Nutrition, is plain enough; because the intercepting of ’em, by cutting off a Nerve, always causes the wasting of the Part to which that Nerve lead. Thus I have endeavour’d to shew some of the secret Advantages accruing to us from the frequent Use of Exercise; and by which it will appear, that the Fluids of the Body are of such a Texture, as will admit of Improvement from the greatest Rapidity of their Current, that is consistent with the Organs thro’ which they are convey’d.