lOO Mayow 



then the space left by the impaired air would have been 

 filled by the vapours emitted, and consequently the 

 water would not have risen in the glass except in sO' 

 far as these underwent condensation. But the water 

 rose in the glass much faster than vapours of that 

 kind usually condense, so that we must believe that it 

 ascended not only into the place of the condensed 

 vapours but also into that of the air at that time 

 impaired. Indeed we must suppose that particles of 

 the liquid and of the metal, dispersed through the air 

 and fermenting there, gradually rub its particles and 

 diminish their elastic force. Hence if the vapours 

 produced by any action are of such a sort as cannot 

 last a good while in the air, the water will only ascend 

 into the space left by them on their contraction by 

 condensation. If therefore they are of such a kind 

 that they do not undergo notable condensation, the 

 water in the said glass will not rise above the limit 

 first marked. Whenever, I say, the vapours generated 

 will occupy more space than is left by the impaired air,, 

 then, however much the elastic force of the air is 

 diminished, it will not be observed in the said glass. 

 And hence it is that if action is caused in the glass by 

 spirit of nitre and a fixed salt being mixed, as also by oil 

 of vitriol and iron acting on each other, in the manner 

 aforesaid, the water will not rise above its original level. 

 It is to be noticed in passing that although the 

 exhalation produced by the spirit of nitre and iron^ 

 when acting on one another, will never pass into a 

 liquid condition, as will be shown below, still it can 

 scarcely be thought that it is really air. For when the 

 fermentation was first produced in the aforesaid glass,, 

 the air in it was for the most part impaired — at least 

 as regards elastic force — as was previously pointed out ; 

 yet when the glass was almost filled with the exhala- 



