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useful in inflammatory fevers ami in nervous 
complaints, but it is injurious in dropsy. 
Hydrogen gas dissolves sulphur, phospho- 
rus, and carbon. The compounds are called 
sulphureted, phosphuretad, and carbureted 
hydrogen gas. Sec ArR, Chemistry, See. 
HYDROGENIZED sulphurels, certai 
bases combined with sulphureted hydrogen. 
HYDROGRAPHY, the art of measuring 
and describing the sea, rivers, lakes, and 
canals. 
With regard to the sea, it gives an account 
of the tides, counter-tides, soundings, bays, 
gulphs, creeks, See. as also of the rocks, 
shelves, sands, shallows, promontories, har- 
bours, the distance and bearing of one port 
from another, with every thing that is re- 
markable, whether out at sea or on the coast. 
IIYDROLEA, a genus of the digynia 
order, in the pentandria class of plants, and in 
the natural method ranking with those of 
which the order is doubtful. The calyx is 
pentaphyllous; the corolla rotaceous; the 
filaments at the base are cordate; the capsule 
is bilocular and bivalved. There are four 
species, herbs of the East and West Indies. 
HYDROMETER, an instrument much 
used for determining the specific gravities of 
liquids. It usually consists of a hollow sphere 
of glass or metal B, and balanced by another b 
below, containing quicksilver, or a metallic 
weight. Fig. 19'. The larger ball has a shorter 
neck at C, into which is screwed the gradu- 
ated stem AC, which by a small weight at A 
causes the instrument to descend in the fluid 
with part of the stem. When this instrument 
is swimming in the jar ILMlv, the part of the 
fluid displaced by it will be equal in bulk to 
the part of the instrument under w r ater, and 
equal in weight to the whole instrument. 
This instrument, therefore, marks the differ- 
ences in the density of fluids by the place at 
which the surface cuts the stem as it floats, 
or by the proportionate addition or diminu- 
tion of weight which is required to make it 
float at the same level in each. 
Beaume’s areometer is often referred to in 
the works of French chemists. The inventor 
seems to have been led to a method of mak- 
ing instruments of this kind, which, should 
agree with each other by reflecting on the 
way in which thermometers are graduated, 
that is, by means of an interval between two 
stationary points of temperature. Thus M. 
BeaiunG adopted two determinate densities, 
in order to mark an interval in the stem of his 
hydrometer. These were that of pure water, 
and that of water containing I L parts of its 
weight of pure common salt at lb 0 Reaumur 
or 57° Fahrenheit. The instrument (repre- 
sented by fig. 20.) was so balanced as to 
sink in the water to 0 near the top of the 
stem. In the saline solution the latter emerg- 
ed more, and a certain additional portion ap- 
peared above, which was assumed to contain 
15 degrees. By means of a scale ayd com- 
passes, a scale of equal degrees was thus 
.formed, adapted to the length of the stem, 
and being fixed within it, at a proper height, 
the latter was sealed at the blow-pipe without 
altering the weight. It is usually constructed 
in two pieces, to avoid an inconvenient 
length; one of which serves for the lower, the 
other for the higher, degrees of density. The 
glasses may be procured from the makers of 
thermometers, and it is convenient to have a 
passage from the top quite down to the lower 
bulb for introducing the quicksilver, which 
being adjusted, may be secured there by a 
small morsel of cork. It is obvious that per- 
sons of ingenuity may adapt such instruments 
to their own purposes, by fixing on two liquids 
of known specific gravities to serve as the ex- 
tremes of their scale, and may thus apply 
them to the trial of all liquids, from ether to 
sulphuric acid, in respect to gross differences, 
or for the purpose of having them of uniform 
strength. But the instrument is not to be de- 
pended on for those accurate reports on the 
specific gravity of liquids which are expected 
in detailing chemical experiments. 
Fahrenheit contrived a more accurate me- 
tallic hydrometer, having a very slender 
stem, terminating in a small dish. The middle, 
or half-length of the stem, is distinguished by 
a fine line across. In this instrument every 
division of the stem is rejected, and it is al- 
ways immersed to the line by means of 
weights put into the dish. Thus, as the part 
immersed is constantly of the same magni- 
tude, and the whole weight of the hydrome- 
ter is known; this last weight, added to the 
weight in the dish, will be equal to that of the 
fluid, displaced by the instrument. Conse- 
quently, as the whole weight of the hydro- 
meter and its load, when adjusted in distilled 
water, is to 1.000, so is the whole weight when 
adjusted in any other fluid, to the number 
expressing the specific gravity of that fluid. 
M ere the instrument constructed so that 
it should displace exactly 1.000 grains of dis- 
tilled water at a mean temperature, the grains 
to be added or taken away lor any other fluid 
would express the difference of the specific 
gravity at once. But as this would require 
much nicety, the weight of the instrument 
and its load adjusted in distilled water may 
be ascertained, and a set of decimal weights 
provided, eacli of which should bear the same 
proportion to its nominal value in grains that 
the whole adjusted in distilled water did to 
1.000. By this, or by the former expedient, the 
trouble of calculation maybe avoided; but 
those who use the instrument often will find 
it expedient to construct a set of tables 
adopted to the Stfme purpose. See Spirits. 
IiT DRO-OXIDES, metallic oxides com- 
bined with water. 
HYDROPHOBIA. See Medicine. 
IiT DROPHILUS, a genus of insects of 
the coleoptera order. The generic character 
is, antennas clavate-perfoliate ; hind legs vil- 
lose, formed for swimming. The principal 
European species of this genus, which is not 
an uncommon insect in our own country, is 
the hydrophilus piceus, perhaps the largest of 
the British coleoptera, if we except the luca- 
nuscervus; measuring nearly an inch and 
half in length. It is entirely black, and of a 
smooth surface, and is particularly distin- 
guished by the form of its thorax, which is 
produced beneath into a very long and sharp- 
pointed spine, stretching to a considerable 
distance down the abdomen ; the hind legs are 
furnished on each side with strong, but" very 
fine, hairs. It is a native of stagnant waters, 
where its larva is principally observed to prev 
on the smaller kind of water snails, and is dis- 
tinguished by a particularity in the highest 
degree remarkable; this consists in the ap- 
parently anomalous situation of the legs, 
which seem, unless very accurately consider- 
ed, to be placed, not beneath the thorax, as 
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in other insects, but on the upper part, ami 
thence to be deflected towards the sides. 
This uncommon appearance, however, is not 
owing to a real dorsal insertion of. the 
but principally to the peculiar shape and po- 
sition of the head; and the deception .is so 
much heightened by the inverted posture in 
which the insect generally swims and rests, 
that it is by no means easy, even for the most 
scientific observer, to divest himself of the er- 
roneous idea beforementioned. 
i he larvui of the hydrophils are supposed 
to remain about two years before they change 
into pupai or chrysalides. When the larva is 
arrived at its full growth, it secretes itself in 
the bank of the water it inhabits, and having 
formed a convenient cavity or cell, lies dom 
nnmt lor some time, after which it divests 
itself ol its skin, and appears in the form of 
a chrysalis, in -which state having continued 
lor some lime longer, it again delivers itself 
from its exuviap, and appears in its complete 
oj beetle form. W hen first disengaged front 
tiie skin of the chrysalis, it is of a pale colour, 
and very tender; but in the space of a few 
hours the elytra, or wing-cases, acquire a 
degree of strength and colour, which gradu- 
ally grows more and more intense, till the 
animal, finding itself sufficiently strong, comes 
f orth from its retreat, and commits itself in its 
new form to the waters. 
The male is distinguished from the female 
by the structure of the fore legs, which are 
furnished, near the setting on of the feet, 
with a sort of horny, concave flap or shield; 
tne legs ot the females being destitute of this 
part. 
r I he female of the hydrophilus piceus af- 
loids an example ol a faculty which seems to 
be exercised by no other coleopterous insect, 
viz. that of spinning a kind of web or flatfish 
circular case of silk, which it leaves floating 
on the water, and in which it deposits its eg»s. 
inis case is terminated on its upper surface 
a lengthened conical process resembling a 
horn, ot a brown colour, and of a much 
stronger or denser nature than the case itself, 
which is white. The young larva-, as soon 
as patched, make their escape from the en- 
v elopement of the case, and commit tliem- 
sehes to the water. 
The hydrophilus caraboides is a species 
measuring about three quarters of an inch in 
length, and is of a polished black colour, 
and of an oval shape. 
1 he genus hy drophilus has been greatly 
increased by the persevering researches of 
modern entomologists. Mr. Marsham enu- 
merates 28 British species. 
IiT DROPIIT LLAX, agenus of the mo- 
nogynia order, in the tetrandria class of 
plants 1 he calyx is tetrapartite ; the corolla 
fuimeMiaped; the fruit two.-edged and one- 
seeded. M here is one species, an aquatic of 
the East Indies. 
HYDROPIIYLLUM, water-leaf, a ge- 
nus or tne nionogynia order, in (lie pentan- 
1 , , class °/ plants, and in the natural method 
ranking with those of which the order is 
doubtful. 1 he corolla is campanulated, with 
uve meniferous longitudinal stria ou the in- 
side; the stigma is bifid; the capsule globose 
and luvalved. J here are two species, the 
vnginianum, or water-leaf of Morinus, grows 
naturally in Canada and many other parts of 
America on moist spongy ground. 1 he root 
