[ 474 3 
veflels, there muft be great Fri&ion excited, and from 
that Fri&ion Heat generated 5 as is eafily done by rub* 
bing two Pieces of Wood together, or a Piece of 
Wood and a Piece of Metal, or two Pieces of Metal, 
or hard Stones : but it is known, by daily Experience, 
that either any watery Fluid, or oily or greafy Sub- 
ftance, applied to thefe Bodies while rubbed, will 
prevent the Excitation of Heat ; as for Inftance, the 
Ufe of Water in polifhing of Glafs or Marble, and 
the greafmg or oiling all manner of Wheel-Machines, 
many of which, for want of that Application, have 
heated, taken Fire, and been even confumed in 
Flames of their own exciting. I know of no Expe- 
riment, whereby it appears, that any the leaft Degree 
of Heat has been generated by the fimple or mere 
mechanical Agitation or Fri&ion of the Particles of 
any Fluid, either by itfelf, or mixed with various 
Fluids} Water, Wine, vinous Spirits, Oils, Quick- 
filver, either agitated flngly or mixed, will by no 
Force, or Velocity of Motion I ever heard of, pro- 
duce Heat ; nor can the Blood of Animals, when 
once let out of the Body, be kept either fluid or 
warm by any the mod violent Agitation. Indeed 
Heat is generated in Fluids in fome particular Cir- 
cumftances, as in thofe two fo commonly known 
Cafes of Fermentation and Effervejcence ; which, as 
they are frequently confounded by Perfons not tho- 
roughly verfed in chemical Matters, I fhall beg Leave 
to explain the Difference. Fermentation is that fpo.n- 
taneous inteftine Motion, which, in the Degree of 
Heat of the univerfal Temperature of fubterraneous 
Caverns, will, in a few Hours, bring on fuch a 
Change in vegetable Juices, or in Water charged 
with 
