66 
Si ? * Benjamin Thompson's 
The next experiment I made was with semen lycopodii, 
commonly called witch-meal, a substance which possesses very 
extraordinary properties. It is almost impossible to wet it ; a 
quantity of it strewed upon the surface of a basin of water, not 
only swims upon the water without being wet, but it prevents 
other bodies from being wet which are plunged into the water 
through it ; so that a piece of money, or other solid body, may 
be taken from the bottom of the basin by the naked hand, with- 
out 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 appear- 
ance 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 attraction 
between this substance and air, and suspecting, from some cir- 
cumstances attending some of the foregoing experiments, that 
the warmth of a covering depends not merely upon the fine- 
ness 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 interstices, 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 ap- 
pear by the results of the three following experiments. 
