Oil Sal Nitrum and Nitro- Aerial Spirit 147 



luminous particles, impinging on a white surface, 

 acquire the new motion by which the white colour 

 is propagated, they lose entirely their luminous and 

 fiery impulse ; but it is otherwise when they fall on 

 a black surface. 



Finally, we note also here that the impulses of light 

 and of colours follow almost the same laws ; for colours 

 and images of things (although they are propagated 

 laterally and in all directions, as was shown above) 

 proceed, like the rays of light, only in straight lines, 

 and further, like light, they undergo reflection and 

 refraction. 





CHAPTER XIII 



OF LIGHTNING 



Having now treated of fire and light, let me add a 

 few remarks on lightning, since it seems to waver 

 between flame and light. 



In the first place then as to lightning, it is not to 

 be supposed that any flame discharged from a flashing 

 cloud reaches the eye. For who can conceive a flame 

 so vast and swift as to spread in a moment over 

 almost the whole hemisphere. Nor can it be said that 

 the sulphureous exhalations which are raised by the 

 sun's heat and widely dispersed through the atmo- 

 sphere are all kindled together by the flame of the 

 flashing cloud, for if it were so these sulphureous 

 exhalations, when once kindled, would burn till they 

 were totally consumed ; and, consequently, since the 

 sulphureous particles would be used up the first time, 



