454 



Of the Propagation of Heat 



the bottom of the basin by the naked hand without 

 wetting the hand ; which is one of the tricks commonly 

 shown by the jugglers in the country: this meal covers 

 the hand, and, descending along with it to the bottom of 

 the basin, defends it from the water. This substance 

 has the appearance of an exceeding fine, light, and very 

 moveable yellow powder, and it is very inflammable ; so 

 much so, that, being blown out of a quill into the flame 

 of a candle, it flashes like gunpowder, and it is made use 

 of in this manner in our theatres for imitating lightning. 

 Conceiving that there must have been a strong attrac- 

 tion between this substance and air, and suspecting, from 

 some circumstances attending some of the foregoing ex- 

 periments, that the warmth of a covering depends not 

 merely upon the fineness of the substance of which the 

 covering is formed, and the disposition of its parts, but 

 that it arises in some measure from a certain attraction 

 between the substance and the air which fills its inter- 

 stices, I thought that an experiment with semen lycopodii 

 might possibly throw some light upon this matter; and 

 in this opinion I was not altogether mistaken, as will 

 appear by the results of the three following experi- 

 ments. 



