F L Li 
the Cucumber, Melon, Gourd, T urky-Yv teat, Wal- 
nut, Oak, Beech, &c. 
FLUIDITY. [Fluiditas, of fluere, Let. to flow.] 
Having occaiion to mention fluids and fluidity, in 
fpeaking of the properties of the elements air, water, 
fire, &c. I thought it neceffary, in this place, to give 
the following account of that property, which I have 
extracted from the moft approved authors. 
A fluid,- or fluid body, is by feme defined to be a 
body, whofe particles are but weakly connected, their 
mutual cohefion being, in a great meafure, prevented 
from fome external caufe : in which fenfe, a fluid 
Hands bppofed to a folid ; and is, by the excellent Sir 
Ifaac Newton, defined to be one whole parts eafily give 
place, or move out of the way, on any force impelled 
upon them, and by that means do fo ealily move one 
over another. Which definition is much better than 
that of Defcartes, That a fluid is a body whofe parts 
are in continual motion, becaufe it is neither apparent 
that the parts of all fluids are fo, nor that the parts of 
fome folid bodies are not fo. 
Fluidity ,is the ftate or affection of bodies, which de- 
nominates or renders them fluid, and Hands in dired 
oppofltion to firmnefs and folidity. 
It- is diHinguifhed from liquidity and humidity, in 
that the idea of fluidity is abfolute, and the property 
contained within the thing itfelf ; whereas that of hu- 
midity is relative, and implies wetting, or adhering, 
i. e. fomething that gives us the fenfation of wetnels 
■or moiflure, and would have no exiflence, but for our 
fenfes. 
Thus melted metals, air, tether, and even fmoke, 
and flame itfelf, are fluid bodies, and not liquid ones ; 
the parts of them being actually dry, and not leaving 
any fenfe of moiflure. 
Fluidity feems to confifl in this, that the parts of 
any body, being fine and fmall, are fo difpofed by 
motion and figure, as that they can eafily Hide over 
one another’s furfaces all manner of ways. Mr. Boyle 
alfo obferves, That it is requifite they fhould be vari- 
oufly and feparately agitated to and fro, and that 
they fhould touch one another but in fome parts only 
of their furfaces. And the fame gentleman fays, in 
his Hiftory of Fluidity, That the conditions requifite 
to conflitute a fluid body, are chiefly the three fol- 
lowing. 
ifl. The minutenefs or fmallnefs of its parts : thus 
we fee the fire, by dividing metals into parts very fine 
and fmall, will melt them, and make them fluid ; and 
after the fame manner do acid menflruums difiolve 
them, fufpend their liquor, and render them fluid ; 
and that fire turns the hard body of common fait al- 
mofl wholly into a liquor by diflillation : though it 
is not improbable, but that the fhape and figure of 
thefe fmall parts may conduce much towards produ- 
cing this quality of fluidity ; for it is found in the 
diflillation of Olive oil (which is a fluid made only 
by preflure) that moft of the oil will, by the action 
of the parts of the fire (if it be done in a retort) be 
turned into a kind of confident fubftance like butter. 
Likewife mercury, whofe parts are, without doubt, 
much grofler than thofe of oil and water, is yet more 
fluid than either of them. 
2dly, It feems requifite to fluidity, that there be ftore 
of vacuities, or vacant fpaces, interfperfed between 
the corpufcles of the fluid body ; for elfe there will 
not be room for each particle to continue its motion 
and agitation on the furfaces of the neighbouring 
ones. For, 
gdly, The chief condition requifite to conflitute a 
fluid body is, that its particles be agitated varioufly 
and apart, either by their own proper motion, or by 
fomething of fubftance, that tumbles them up and 
down by its pafiage through them. 
That this qualification is chiefly requifite to fluidity, 
you may gather from that common experiment of 
putting a little dry powder of alabafter, or plafter of 
Paris, finely lifted ; in aflat-bottomed veffei over the 
fire j for in a little time it will boil like water, and 
imitate all the motions of a boiling liquor ; it will 
tumble varioufly over in great waves like that • it will 
bear flirting with a flick or ladle without refilling, as 
it will do when cold ; hay'j if it be flirted ftrongly neaf 
the fide of the veffei, its waves will apparently dafli 
up againft the fides : yet if any of i t be fpeedily taken 
out, and laid on a piece of paper, you will fee nothing 
but a dry powder. 
So that it is evident from hence, that there is a real 
difference between a fluid body and a wetting liquor ; 
for not only this boiling powder and melted metals* 
but the air and mther, and even flame itfelf, are pro- 
perly fluid bodies, though not iiioift liquors. 
This ingenious gentleman found alfo, that by blow- 
ing the fmoke of Rofemary into a glafs pipe, and then 
holding the pipe (when filled) upright, the lurface of 
the fmoke would accommodate itfelf to a level littia- 
tion ; and which way foever the tube was inclined, 
the luperficies of the fmoke would be parallel to the 
horizon ; and when the glafs was much inclined, 
would run along it like water. 
From whence he infers, that, in order to the render- 
ing a body fluid, there is no need that its parts fhould 
be clofely condenfed, as thofe of water are. 
And Dr. Hook, in his Micrograph, p. 12. prefents us 
with a very pretty experiment or two, to prove this 
account of fluidity, viz. That a difh of fand being let 
on a drum head, brifkly beaten by the flicks, or on 
the -upper ftone of a mill, turning fv/iftly round on 
the (empty) lov/er one, it in all refpedb, emulate the 
properties of a fluid body ; for a heavy body will 
immediately fink in it to the bottom, and a light one 
emerge to the top; each grain of fand hath a con- 
ftant vibrating, dancing motion ; and if a hole be 
made in the fide of the difh, the fand will fpin out 
like water. 
The corpufcular philofophy, before it was wonder- 
fully improved by Sir Ifaac Newton, did not go to 
the bottom of this matter ; for it gave no account of 
the caufe of the chief condition requifite to confti- 
tute a fluid body, viz. the various motions and agi- 
tations of its particles : but this may, in a great 
meafure, be accounted for, if it be fuppofed to be 
one of the primary laws of nature, That as all par- 
ticles of matter attrafl one another when they come 
within a certain diftance, fo likewife they fly away 
from, and avoid one another, at all greater diftances 
from one another. 
For then, though their common gravity may keep 
them together in a mafs (it may fometimes be) toge- 
ther with the preflure of other bodies upon them ; 
yet their continual endeavour to avoid one another 
fingly, and the adventitious impulfes of light, heat, 
or other external caufes, may make the particles 
of fluids continually move round about one another, 
and fo produce this quality. 
It is indeed a difficulty not eafily got over, to account 
for the particles of fluids always keeping at fuch a 
diftance from one another, as not to come within the 
fphere of one another’s, attra&ion. 
The fabric and conftitution of that fluid body, 
water, is amazing ; that a body fo very rare, and 
which has a vaft over-proportion of pores, or inter- 
fperfed vacuity, to folid matter, fhould yet be per- 
fectly incompreffibie by the greateft force ; ^nd yet 
this fluid is eafily reducible into that firm, tranfparent, 
friable body which we call ice, by being only expofed 
to a certain degree of cold. 
One would think, that though the particles of Wa- 
ter cannot come near enough to attract each other, 
yet the intervening frigorific matter doth, by being 
mingled per minima, ftrongly attract them, and is it- 
felf likewife ftrongly attracted by them, and wedges 
or fixes all the . mafs into a firm body ; which folid 
body lofes its folidity again, when by'heat the vincu- 
lum is folved, and thefe frigorific particles are dis- 
joined from thofe of the water, and are forced to 
fly out of it ; and, perhaps, juft thus may the fumes 
of lead fix quickfilver. 
When a firm folid body, fuch as a metal, is by heat 
reduced into a fluid, doth not the fire disjoin and Te- 
5 Q, parate 
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