1 96 Newton's Letters, Hypothesis and Experiments 



saltpetre, or otherwise. And so lead melted with silver quickly 

 pervades and liquifies the silver in a much less heat than is 

 required to melt the silver alone; but if they be kept in the 

 test till that little substance that reconciled them be wasted or 

 altered, they part again of their own accord. And in like 

 manner the aetherial animal spirit in a man may be a mediator 

 between the common aether, and the muscular juices, to make 

 them mix more freely, and so by sending a little of this spirit 

 into any muscle, though so little as to cause no sensible tension 

 of the muscle by its own force, yet by rendering the juices 

 more sociable to the common external aether, it may cause that 

 aether to pervade the muscle of its own accord in a moment 

 more freely and more copiously than it would otherwise do, 

 and to recede again as freely, so soon as this mediator of so- 

 ciableness is retracted ; whence, according to what I said 

 above, will proceed the swelling or shrinking of the muscle, 

 and consequently the animal motion depending thereon. 



Thus may therefore the soul, by determining this aetherial 

 animal spirit or wind into this or that nerve, perhaps with as 

 much ease as air is moved in open spaces, cause all the mo- 

 tions we see in animals; for the making which motions strong, 

 it is not necessary that we should suppose the aether within the 

 muscle very much condensed, or rarefied, by this means, but 

 only that its spring is so very great that a little alteration of its 

 density shall cause a great alteration in the pressure. And 

 what is said of muscular motion may be applied to the motion 

 of the heart, only with this difference; that the spirit is not 

 sent thither as into other muscles, but continually generated 

 there by the fermentation of the juices with which its flesh is 

 replenished, and as it is generated, let out by starts into the 

 brain, through some convenient ductus, to perform those mo- 

 tions in other muscles by inspiration, which it did in the heart 

 by its generation. For I see not why the ferment in the heart 

 may not raise as subtile a spirit out of its juices, to cause those 

 motions, as rubbing does out of a glass to cause electric at- 

 traction, or burning out of fuel to penetrate glass, as Mr. 

 Boyle has shown, and calcine by corrosion metals melted 

 therein. 



Hitherto 1 have been contemplating the nature of aether and 

 aetherial substances by their effects and uses, and now I come 

 to join therewith the consideration of light. 



In the fourth place, therefore, I suppose light is neither aether, 

 nor its vibrating motion, but something of a different kind 

 propagated from lucid bodies. They that will may suppose 

 it an aggregate of various peripatetic qualities. Others may 

 suppose it multitudes of unimaginable small and swift corpus- 



