42 SCIENCE OF THE MIDDLE AGES. PT. n. 



like the spirit of man, which can do so much and yet cannot 

 be seen, and for this reason they called it ' spirit.' We know 

 now that when we heat substances we separate part of the 

 matter of which they are made into very small portions, 

 which float off as steam or gas into the air; so that this 

 spirit noticed by the Arabs was vapour or gas. 



It seems almost certain that the Arabs knew a great deal 

 about gunpowder and some other mixtures which explode 

 when they are set on fire. An Arab named Marcus Graecus, 

 who lived about the beginning of the ninth century, says that 

 if you mix together one pound of sulphur, two of charcoal, 

 and six of saltpetre, it will explode when you light it and 

 drive things into the air. This is one of the ways in which 

 gunpowder is still made. 



