touching his Theory of Light and Colours. 195 



subject is a deserving one, I shall not stick to tell you how I 

 think it may be done. 



First, then, I suppose there ft such a spirit ; that is, that 

 the animal spirits are neither like the liquor, vapour, or gas, of 

 spirits of wine; but of an aetherial nature, subtile enough to 

 pervade the animal juices as freely as the electric, or perhaps 

 magnetic, effluvia do glass. And to know how the coats of 

 the brain, nerves, and muscles, may become a convenient ves- 

 sel to hold so subtile a spirit, you may consider how liquors 

 and spirits are disposed to pervade, or not pervade, things on 

 other accounts than their subtilty; water and oil pervade wood 

 and stone, which quicksilver does not; and quicksilver, metals, 

 which water and oil do not; water and acid spirits pervade 

 salts, which oil and spirit of wine do not; and oil and spirit 

 of wine pervade sulphur, which water and acid spirits do not; 

 so some fluids (as oil and water), though their parts are in 

 freedom enough to mix with one another, yet by some secret 

 principle of unsociableness they keep asunder; and some that 

 are sociable may become unsociable by adding a third thing to 

 one of them, as water to spirit of wine by dissolving salt of 

 tartar in it. The like unsociableness may be in aetherial na- 

 tures, as perhaps between the aethers in the vortices of the 

 sun and planets ; and the reason why air stands rarer in the 

 bores of small glass pipes, and aether in the pores of bodies, 

 may be, not want of subtilty, but sociableness* ; and on this 

 ground, if the aetherial vital spirit in a man be very sociable to 

 the marrow and juices, and unsociable to the coats of the brain, 

 nerves, and muscles, or to any thing lodged in the pores of 

 those coats, it may be contained thereby, notwithstanding its 

 subtilty ; especially if we suppose no great violence done to it 

 to squeeze it out, and that it may not be altogether so subtile 

 as the main body of aether, though subtile enough to pervade 

 readily the animal juices, and that as any of it is spent, it is 

 continually supplied by new spirit from the heart. 



In the next place, for knowing how this spirit may be used 

 for animal motion, you may consider how some things unso- 

 ciable are made sociable by the mediation of a third. Water, 

 which will not dissolve copper, will do it if the copper be 

 melted with sulphur. Aquafortis, which will not pervade 

 gold, will do it by addition of a little sal-ammoniac or spirit of 

 salt. Lead will not mix in melting with copper; but if a lit- 

 tle tin, or antimony, be added, they mix readily, and part again 

 of their own accord, if the antimony be wasted by throwing 



* In the third book of the Optics, Newton states more accurately the 

 true theory of capillary attraction ; but the germ of that theory is certainly 

 contained in these expressions. 



