H YD 
does not appear to be so positively delete- 
rious as the other noxious gases. Scheele long 
ago observed, that he was able to breathe 
it for twenty inspirations. (Treatise on Air 
and Fire, p. 160). Fontana shewed, what 
Scheele indeed had observed, that if the 
lungs were previously emptied as much as 
possible of atmospheric air, by a forcible 
expiration, it cannot be breathed so long, 
though still it did not appear to him to be 
positively deleterious, like some of the un- 
respirable gases, (Opuscules Physiques, 
p. 2)., Rosier, even after expelling the air 
from the lungs, breathed hydrogen gas for 
several respirations ; and Mr. Davy, in his 
experiments on the respiration of the gases, 
remarked, that in one experiment, after a 
complete exhaustion of the lungs, he found 
great difficulty in breathing hydrogen for 
half a minute, though in a subsequent ex- 
periment, with the same preparation, lie 
breathed it for near a minute. The first 
six or seven inspirations produced no sensa- 
tions whatever ; in half a minute, a sense of 
oppression was felt at the breast, which in- 
creased until the pain of suffocation inter- 
rupted the experiment. (Chemical Re- 
searches, p. 400. 466). Hydrogen, there- 
fore, is incapable of supporting life ; the re- 
spiration of it can be continued only for a 
short time, and animals confined in it soon 
die. It appears only to prove fatal, not by 
a positively noxious quality, hut by exclud- 
ing atmospheric air, the due supply of 
which, by respiration, is indispensable to 
life. Blood exposed to it acquires a deep 
black colour, and the gas suffers a diminu- 
tion of volume. 
Hydrogen is not, as several of the other 
gases are, noxious to vegetable life ; at the 
same, time, it appears to contribute little to 
the nourishment of plants. Dr. Priestley 
having found, that it still continued inflam- 
mable after a growing vegetable had been 
confined in it v for several months. It can 
apparently supply, to a certain extent, the 
place of light, in supporting vegetation. 
Yon Humboldt observed, that some cryp- 
togamic plants in mines, and of course se- 
cluded from light, were not pale, but of a 
green colour, such as they would have had 
from growing under exposure to the light 
of day ; and he concluded, with sufficient 
probability, that the agency of light had, in 
this case, been supplied by the hydrogen 
gas, which is evolved in greater or less 
abundance, in such situations. 
Hydrogen gas is so sparingly soluble in 
water, that when agitated with it, it suffers 
I 
H YD 
no perceptible diminution of volume. When 
the water has been previously freed from 
atmospheric air, Mr. Henry found, that one 
hundred cubic inches take up 1.5 of the 
gas under a common atmospheric pressure ; 
under increased pressure, a larger quantity, 
equal to one-third of the volume of the wa- 
ter, is absorbed. 
The affinities of hydrogen seem princi- 
pally exerted towards inflammable bodies. 
It unites with sulphur, phosphorus, and car- 
bon, and forms gaseous compounds; it ap- 
pears to be capable of dissolving even some 
of the metals, particularly iron, zinc, and ar- 
senic. United with nitrogen, it forms one 
of the alkalies, ammonia ; with oxygen, wa- 
ter. It is also a constituent principle of 
the greater number of the vegetable and 
animal products. 
Hydrogen gas may be regarded as a pro- 
duct of some natural operations. It is 
found collected often in mines, derived pro- 
bably from the decomposition of water by 
metals ; it is known to the miners by the 
name of fire-damp, and is often the cause of 
accidents, from exploding on the approach 
of an ignited body. It is also extricated 
from stagnant water, and from marshy si- 
tuations, from tlie slow decomposition of 
vegetable and animal substances holding, 
dissolved in it, carbon, and perhaps also 
phosphorus and nitrogen, and forming, as 
has been supposed with some probability, 
gases which render the air of such places 
unhealthy. From its levity, it has been 
supposed that the quantity of it thus pro- 
duced at the surface of tiie earth, will rise 
through the atmosphere, and occupy the 
higher regions ; and on its presence, some 
of the phenomena of meteorology, particu- 
larly the sudden appearance of some fiery 
meteors, have been supposed to depend. 
Its affinities have not been ascertained with 
any precision, as to their relative force. 
HYDROGRAPHY, the art of measur- 
ing and describing the sea, rivers, lakes, and 
canals. With regard to the sea, it gives an 
account of its tides, counter- tides, sound- 
ings, bays, gulphs, creeks, &c. ; as also of 
the rocks, shelves, sands, shallows, promon- 
tories, harbours, the distance and bearing 
of one port from another, with every thing 
that is remarkable, whether out at sea, or 
on the coast. 
HYDROLEA, in botany, a genus of the 
Pentandria Digynia class and order. Na- 
tural order of Convolvuli, Jussieu. Essen- 
tial character : calyx five-leaved ; corolla 
wheel-shaped; filaments cordate at the 
