vn] of the Seventeenth Century. 187 



It is this which he calls sal-nitro-aereum or spiritus nitro- 

 aereus, or sometimes igneo-aereus. 



Mayow was a chemist, far more of a chemist than Lower 

 or Hooke, perhaps even more distinctly a chemist than Boyle ; 

 he had doubtless read carefully the works of Sylvius and 

 van Helmont, and had probably repeated many of their ex- 

 periments. He uses the current terms of the chemists of the 

 period; and in reference to this it may be well to note that 

 the word ' salt ' had not yet acquired the limited meaning 

 afterwards attached to it ; it was still often used to denote 

 any substance (whether elementary, as we should now say, 

 or compound) not distinctly metallic, or distinctly liquid. 



Mayow first expounds the nature of nitre. The common 

 method at that time of preparing nitre was to allow heaps of 

 vegetable matter to decompose with exposure to the air, and 

 then to dissolve out and crystallize the potash nitre so formed. 

 He says that it is derived partly from the air and partly from 

 the earth. There is the sal fixum or sal alkali, or as we now 

 say, potash ; this distinctly comes from the earth. Besides 

 this there is the spiritus acidus, or as we now say, nitric acid. 



For a while he thought that the whole of this spiritus 

 acidus was derived from the air, that it existed distributed 

 throughout the atmosphere divided into extremely minute 

 particles. But this seemed incompatible with the well-known 

 fact that the spiritus acidus is a corrosive liquid, extinguishing 

 flame and destructive to life. He concludes therefore that part 

 only of the acid exists in the atmosphere, and that part is his 

 sal-nitro-aereum or spiritus nitro-aereus. 



In attempting to lay hold of the nature of this nitro-aereal 

 agent he first reminds his readers that a certain part of the 

 air is necessary for the maintenance of combustion. He says : 



"In the first place it must, I take it, be granted that 

 "something in the air, whatever it be, is necessary for the 

 "burning of every flame. This Boyle's experiments have 

 "placed beyond doubt. For these shew that a lighted candle 

 "goes out much more quickly in a glass flask empty of air 

 "than in the same vessel full of air, a clear proof that the 

 " flame, enclosed in the flask, goes out not because it is 



