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weigh more than the other. In the last case, 
v, nich is most common, the quantity bv 
which the heavier body surpasses the lighter, 
;s called the specific weight or gravity. 
. Jf. a bo ' ;i y is heavier than the'fluid in which 
it is immersed, it is evident that it will sink to 
the bottom by its specific gravity. If a body 
is lighter than the same bulk of* the fluid into 
v.mich it is plunged, a part of it will swim, and 
the remaining part which is immersed dis- 
places a quantity of fluid which weighs ex- 
actly as much as the whole of the solid body. 
1 he instrument used for finding the sp’e- 
cinc gravities of bodies, is called the hydro- 
static balance (iig. 18.) See Specific 
Gravity. 
It differs very little from a common balance 
tiut is nicely made; only it has a hook at the 
bottom or one of the scales, on which different 
substances that are to be examined may be 
hung by horse-hairs, or silk threads, so as to 
be immersed in a vessel of water, without 
wetting the scale. 
It a body thus suspended under the scale, 
at one .end oi the balance, be first counter- 
poised in air by weights in the opposite scale, 
and then immersed in water, the equilibrium 
will be immediately destroyed; then, it as 
much weight be put into the scale from 
which the body hangs, as will restore the 
equilibrium, without altering the weights in 
til * opposite scale; that weight which re- 
stores the equilibrium, will be equal to the 
weight of a quantity ot water as large as the 
immersed body; and if the weight of the 
body in air be divided by what it loses in 
watei, i he quotient will shew how much that 
body is heavier than its bulk of water. Thus, 
if a guinea suspended in air, be counterba- 
lanced by 129 grains in the opposite scale of 
the balance, and then, upon its being im- 
mersed in water, it becomes so much lighter 
ao to lequire 7J grains put into the scale over 
it, to restore the equilibrium, it shews that a 
quantity of water of equal bulk with the gui- 
nea, weighs grains, or 7.25 ; by which di- 
vide 129 (the weight of the guinea in air), and 
the quotient will be 17.793; which shews 
that the guinea is 17.793 times as heavy as its 
bulk ot water. And thus, any piece of gold 
may be tried, by weighing it first in air, 'and 
finen in water; and if upon dividing the 
weight in air by the loss in water, the quotient 
comes out to be 17.793, the gold is good; if 
t he quotient be 18, or between 18 and 19, 
the gold is very fine ; but if it be less than 17* 
the gold is too much alloyed with, other 
metal. 
by this method, the specific gravities of all 
bodies*. that will sink in water, may be found ; 
first weighing the body in air, then in water’ 
and dividing the weight in air by the loss in 
water. 
But as to those which are lighter than wa- 
ter, as most sorts of wood are, the following 
method must be taken: A sort of pincers, 
or tongs, must be provided, to retain the 
substance to be examined underwater. First 
weigh l node in air; then having balanced 
the tongs in water,, fix to it the body to be 
weighed, which being lighter than water, 
wiii raise tn : tongs, and cause the other scale 
to preponderate. Observe the loss of weight 
of the body in water, and proceed as before. 
ibeie are some things that cannot be 
weighed in this manner; as quicksilver, frag- 
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incuts of diamonds, &c. which must be put 
into a glass- bucket hanging to the scale. 
II 'i J3ROSL LPHURETS, in chemistry, 
aulphureted, or sulphurated hydrogen -gas 
possesses die properties of.an acid-.--. It is ab- 
sorbed by water, in considerable quantities, 
and the solution reddens vegetable blues; it 
combines also with alkalies and earths, and 
with several metallic oxides. The combina- 
tions which su phureted hydrogen forms with 
bases have been called hydrosulphurets. 
Sulphurated hydrogen combines with alka- 
Iicm and earths, and forms with them com- 
pounds which may be distinguished by the 
following properties: 
1. they are all soluble in water, and the 
solution is colourless. 
2. When the solution is exposed to the air, 
it becomes green or greenish yellow, and de- 
posits sulphur on die sides of the vessel in 
the state of a fine black crust. 
3. After long exposure to the air, the solu- 
tion becomes limpid and colourless; and on 
examination is found to contain only the sul- 
phat ot the "base ot the original hydrosulphu- 
ret. 
-t. 1 ne solution cf the hydrosulphurets 
pi capitate all metallic solutions; iron and 
lead, black; antimony, orange; arsenic, yel- 
low. * J 
I he hydrosulphurets may be formed by 
dissolving or mixing the bases respectively 
with water, and causing sulphureted hy dro- 
gen gas to pass through them tiil they refuse 
to absorb any more. The excess of the gas is 
driven off by heating the solution. It is pro- 
per to cause the sulphureted hydrogen gas to 
pass through a small vessel of water before it 
reaches the base with which it is to combine, 
in order to separate any impurities with which 
it might be mixed. By this method solutions 
of the different hydrosulphurets in water may 
be obtained. J 
If these compounds be decomposed while 
they are colourless, by pouring upon them 
sulphuric acid, muriatic acid, or any other 
acid which does not act upon hydrogen, the 
sulphureted hydrogen gas exiiales°without 
tue deposition ot a single particle of sulphur ; 
out ii the hydrosulphurethas become yellow, 
some sulphur is always deposited during its 
decomposition, and the quantity of sulphur is 
proportioned to the deepness of the colour. 
The yellow colour, therefore, which hydro- 
sulphurets acquire by exposure to the atmos- 
pher - is, owing to a commencement of de- 
composition. Part of the hydrogen of the sul- 
phureted hydrogen abandons the sulphur, 
combines with the oxygen of the atmosphere, 
and forms water. By degrees, however, u por- 
tion of the sulphur is also converted into an 
acid; and when the proportion of sulphureted 
hydrogen is diminished, and that of the sulphur 
inci eased to a certain point, the sulphur and 
tne hydrogen combine equally with oxygen. 
If sulphuric or muriatic acids be poured 
upon a hydrosulpiuiret after it has been for 
some time exposed to the air, a quantity of 
sulphureted hydrogen gas exhales, sulphur is 
deposited, and after an interval of time sul- 
phureous acid is disengaged. It is therefore 
sulphurous, and not sulphuric acid, which is 
formed while the hydrosuiphuret spontane- 
ously absorbs oxygen. This acid, however, is 
not perceptible till after a certain interval of 
time, when separated from the hydrosulphu- 
ret by means of another acid; because as long 
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as it meets with sulphureted hydrogen a re- 
ciprocal decomposition takes place. The 
oxygen of the acid combines with the hydro- 
gen of the gas, and the sulphur of both is pre- 
cipitated. 
The affinities of the alkalies and earths for 
sulphureted hydrogen appear from the expe- 
riments of Bertholiet to be as follows : 
Barytes, Ammonia, 
Potass, Magnesia, 
Soda, Zireonia. 
Lime, 
HYGROMETER, a machine, or instru- 
ment whereby to measure the degrees of 
dryness, or moisture oi the atmosphere. 
Iheit ai e divas soits of hygrometers ; for 
whatever body either swells or shrinks’ by 
dryness or moisture, is capable oi being 
formed into an hygrometer. Such are woods 
of most kinds, particularly ash, deal, poplar, 
&c. Such also is catgut,* the beard of a wild 
oat, & c. Stretch an hempen cord or lute- 
string, as A B, (plate Hydrostatics, fig. 21.), 
along a wall, bringing it over a pulley, B ; and to 
the other end 13, fix a weight E ; into which fit 
an index F. On the same wall fit a .date of 
metal Ii I, divided into any number ofequaL 
parts, and the hygrometer is compleat. For 
it is known from experience that moisture 
sensibly shortens (he length of cords or fid- 
dle-string: ; and that as Uie moisture evapo- 
rates, they return to their former length. The 
weight, therefore, in the present case, upon an 
increase of the moisture of the air, will 
ascend ; and upon a diminution of the same, 
it will descend. 
Hence, as the index F will shew the spaces 
of ascent and descent ; and those spaces are 
equal to the increments and decrements of 
the length of cord, or gut, ADD;, the in- 
strument will discover whether the air is more 
or less humid now, than it was at another 
given time. 
But if a more sensible and accurate hy- 
grometer is required, strain a whipcord or 
fiddlestring over several pullies, C, D, E, F, 
and G, rig. 22, and proceed as in the former 
example. Nor does it matter whether the 
several parts of tne cord are parallel to the ho- 
rizon, as expressed in the figure or perpendi- 
cular to the same. 
The advantage of this above the former 
hygrometer, is, that'we have a greater length 
ot cord in the same compass; and conse- 
quently greater contraction or dilatation. 
T he following is much more lasting: Take 1 
a nice balance and place in it a sponge, or 
other body, which easily imbibes -moisture; 
and let it be in equilibrio, with a weight hunc 
at the outer end ot the beam. Now if the 
air become moist, the sponge becoming hea- 
vier, will preponderate ; if' dry, the sponge 
will be raised up. i his balance may be con- 
trived two ways; by either having the pin in 
the middle of the beam, with a slender tongue 
a root and a half long, pointing to the divi- 
sions on an arched plate fitted to it ; or, the 
other extremity of the beam may be made-so 
long as to describe a large arch on a board 
placed for the purpose, as is represented in. 
the figure, 
To prepare the sponge, it may be neces- 
sary to wash it m water; and when dry again, 
m water or vinegar, in which sal ammoniac, or 
salt of tartar, has been dissolved, and let it 
dry again, then it is fit to be used. 
