On Wetter and Air, 
[March, 
182 
atoms and molecules of bodies. I have explained to you 
already what I mean by atoms and molecules. As 1 said, 
this opinion is universally entertained at the present day , 
but it is curious to observe, in the course of history, how 
this opinion has developed itself, and h °w early the mmds 
of sagacious, penetrating men saw and seized upon this 1 
that heat was what we call atomic or molecular m . 
At the time of Sir Isaac Newton there were men who seized 
this idea that heat was a motion ; but there was, in par- 
ticular, one man of the greatest genius, named Robert 
Hooke, who endeavoured to give his mind a definite 1 g 
to rest upon with regard to what we call fluidity— that con- 
dition which we get when this substance, ice, is converted 
into liquid water. Hooke’s image is so quaint and so pene- 
trative that I think I will read his account of his conception 
in his own words. He says First, what is the cause of 
fluidness ? This I conceive to be nothing else but a certain 
pulse or shake of heat; for heat being nothing else but a 
very brisk and vehement agitation of the parts of a body (as 
I have elsewhere made probable), the parts of a body 
thereby made so loose from one another, that they easily 
move any way and become fluid.” So with the particles of 
our ice when they are converted into water. He continue. 
“That I may explain this a little by a S ros * 
let us suppose a dish of sand set upon some body that is 
very much agitated and shaken with some quick and strong 
vibrating motion, as on a millstone turned round upon t 
under-stone very violently whilst it is empty, or on a y 
stiff drum-head which is vehemently or very mmWy beaten 
with the drumsticks. By this means the sand m the dish, 
which before lay like a dull and inadtive body, ^omes a 
perfect fluid, and ye can no sooner make a hole in it with 
vour finger but it is immediately filled up again, and the 
upper surface of it levell’d. Nor can you bury a light body, 
as P a piece of cork, under it, but it presently emerges or 
swims P as ’twere on the top; nor can you lay a heavier on 
the top of it, as a piece of lead, but it is immediately bune 
in sand, and (as ’twere) sinks to the bottom. Nor can you 
make a hole in the side of the dish but the sand shall run 
out of it to a level. Not an obvious property of a fluid body, 
as such, but this does imitate ; and all this meerly caused 
hv the vehement agitation of the conteinmg vessel, for, by 
this means^each land becomes to have a vibrating or 
dancing motion, so as no other heavier body can rest on it 
unless sustain’d by some other on either side : nor will it 
suffer any body to be beneath it, unless it be a heavier than 
