On Sal Nitrum and Nitro-A'erial Spirit 79 



it is probable that some subtle and nimble matter 

 interspersed with aerial particles and continually 

 agitating them conduces not a little to the expansive 

 force of the air. And in this way the elastic force of 

 the air seems to be increased when heat is com- 

 municated to it, for the nitro-aerial particles (from 

 whose agitation we have concluded elsewhere that 

 heat arises) strike the aerial particles and keep them 

 in motion ; but these when moved tend to unfold. 

 But indeed it is scarcely credible that the elasticity of 

 air depends on this cause alone ; because when the 

 lamp or the animal is shut up in the aforesaid glasses, 

 the aerial particles there, being heated by the flame or 

 by the presence of the animal, would necessarily be set 

 in motion, and therefore the elastic force of that air 

 would be rather increased than diminished, if it 

 resulted merely from the movements of aerial 

 particles. 



2. Elastic force, or the power of recoil, arises from 

 the bending of rigid bodies inasmuch as these when 

 bent strive to return to their original form. And 

 indeed it is probable that the elasticity of the air 

 results mainly from this very cause. It would certainly 

 be reasonable to suppose that nitro-aerial and fiery par- 

 ticles are fixed in the aerial particles themselves and 

 constitute the more active part of them. For although 

 aerial particles are very minute and are commonly re- 

 garded as most simple and elementary, still it seems to 

 me necessary to suppose that they are compound and 

 that some of their parts are branchy and adhere firmly 

 to each other as if by mutually clasping hooks ; while 

 others are extremely subtle, solid, smooth, agile, fiery 

 and truly elementary, and that these when firmly 

 fixed among the other particles make them rigid in 

 much the same way as rigidity and elasticity are in- 



