136 Mayow 



even when they move with the swiftest and most 

 fiery motion. How much less then will sulphureous 

 particles, which seem to be grosser than nitro-aerial 

 spirit, penetrate such bodies in an instant ? 



I may therefore maintain with the distinguished 

 Descartes that light consists in motion or impulse 

 alone, which, because of the continuity of the lumi- 

 nous medium, is transmitted to the greatest dis- 

 tance without any delay. For certainly impulse or 

 motion is eminently adapted to the laws which are 

 followed in the propagation of light. For the nature 

 of impulse is such that it will promptly cease when 

 the impelling force is withdrawn, and it advances only 

 in straight lines. Further the force of impulse is 

 propagated instantanepusly to the greatest distances 

 through solid bodies. For the case here, owing to 

 the continuity of the medium, is just as if one end of 

 a rod being moved, the blow impressed on it were 

 transmitted almost instantaneously to the other very 

 remote end. 



As to the medium by the impulse of which the 

 rays of light are transmitted, it is not to be believed 

 that it is air itself, since light can be propagated very 

 intensely even in a glass vessel containing no air. 

 And therefore it is probable that besides the nitro- 

 aerial particles fixed in the aerial particles, other nitro- 

 aerial particles are interspersed among them and fill 

 all their interstices ; which we infer from this, that 

 solar rays, if collected by means of a burning-glass, 

 actually ignite even in a glass from which the air is ex- 

 hausted. For gunpowder can be ignited by them there, 

 and sulphureous matter can also be sublimed by their 

 heat ; but I have already attempted to show that heat 

 and fire do not arise except from nitro-aerial particles 

 set in motion. Thus it would seem that even in a 



