48 Mayow 



tides which, when whirled round and hot, separated 

 from each other the particles of these glowing sub- 

 stances and opened up their structure, now, when they 

 cease to move in consequence of encountering cold, 

 are fixed like wedges or very solid spikelets in their 

 pores. Things are hardened by them when fixed in 

 this manner, and indeed cold seems to close the pores 

 of things in this way only. 



But that nitro-aerial particles reside in iron made 

 rigid in this way, is evident from the fact, that this iron 

 acquires the property of giving out fire when struck 

 by a flint. For we must suppose that the sparks 

 struck out from steel are caused by igneo-nitrous 

 particles of the steel bursting forth with extreme 

 velocity on account of the violence of the blow. 

 Indeed we perceive that the fire of these sparks is 

 very like burning nitre, and that they burn very 

 readily though struck out from the steel in a place 

 where there is no air — which is certainly a clear 

 proof that there are nitro-aerial particles in steel. So 

 that to strike fire from it there is no need — as in 

 other cases where fire is produced — for nitro-aerial 

 particles to be supplied from the air. It corrobo- 

 rates this view that if heated iron cools slowly, the 

 igneo-nitrous particles gradually extricate themselves 

 and escape through the open pores of the iron (for these 

 are not, as in the previous case, contracted by the cold) ; 

 so that the iron, from want of igneo-nitrous par- 

 ticles, becomes less rigid, and unfit to give out fire 

 when struck. And what the ingenious Dr R. Hooke 

 has set down in his Micrographia is not opposed 

 to this — to wit, that the sparks of steel, after their 

 extinction, are nothing but small globules or minute 

 vitrified bits of steel. For it must not on that account 

 be thought that little morsels of metal take the form 



