256 Mayow 



aerial spirit is wont to assume diverse states, for at 

 one time it is in the most complete rest, and at 

 another with its greatest agility it enters on most 

 excited motion. For nitro-aerial spirit, when mixed 

 with sulphureous particles, is moved with most rapid 

 and sometimes quite igneous motion ; but when 

 loosed from the company of sulphureous particles, 

 it attains a state of extreme quiet, as we have else- 

 where shown. 



Lastly, let us here note that the nature of the 

 brain seems to fit it in a high degree for collecting 

 and storing nitro-aerial particles, as the brain in 

 comparison with other organs lacks the sulphureous 

 particles which agitate and waste the nitro-aerial 

 spirit ; and that it has a sort of saline quality which 

 specially fits it for retaining the nitro-aerial spirit. 



From this hypothesis of ours the reason may be 

 sought, why, above all other sensibles, light affects 

 the animal spirits, and, by exciting them to motion, 

 produces wakefulness, so that to induce sleep animals 

 must close their eyes and so shut out the external 

 light, while yet to the other sensoria the way lies 

 open for external objects even in sleep. For I have 

 elsewhere tried to show that light depends on the 

 motion of nitro-aerial particles disseminated through 

 the air. Therefore, as animal spirits consist of nitro- 

 aerial particles, it comes to pass that they easily 

 follow the motion of the luminous particles which are 

 of the same kind as themselves. It tells also in the 

 same direction that the eye, when struck with a pretty 

 strong blow, seems to see a flame in front of it. For 

 it is probable that the nitro-aerial particles, thickly 

 occupying the optic nerves, are excited by the violent 

 concussion to the motion requisite for the production 

 of light. But whether some animals, such as the cat, 



