HYGROMETER. 
an alkaline or earthy base. The general 
properties of these substances are, that they 
are soluble in water, and are chrystalliza- 
ble; the solution is colourless, while the ac- 
tion of the air is excluded, but when that is 
admitted, a yellow colour is soon acquired, 
owing to the oxygen of the atmosphere 
combining with the hydrogen of a portion 
of the sulphuretted hydrogen, while the sul- 
phur combines with the remaining portion 
of it, forming a super-sulphuretted hydro- 
gen, in union with the base. Mr. Murray 
observes, that “ the knowledge which we 
have acquired of sulphuretted hydrogen, 
and of its combinations, has thrown light on 
the composition of the mineral sulphureous 
waters, and of the changes which they suf- 
fer. As sulphur is by itself insoluble in wa- 
ter, and, as frequently no traces of an al- 
kali, by which it might be rendered soluble, 
could be discovered in them, chemists 
found it difficult to conjecture by what 
means its solution was effected. The dis- 
covery of sulphuretted hydrogen, and of its 
solubility in water, solved the difficulty ; 
and the mutual action exerted between it 
and the oxygen elucidate the changes these 
waters suffer from exposure to the air.” 
HYGROMETER, a machine or instru- 
ment, to measure the degrees of dryness or 
moisture of the atmosphere. 
There are divers sorts of hygrometers ; 
for whatever body either swells or shrinks, 
by dryness or moisture, is capable ,of being 
formed into an hygrometer. Such are 
woods of most kinds, particularly ash, deal, 
poplar, Sec. Such also is catgut, the beard 
of a wild oat, &c. 
All bodies that are susceptible of imbib- 
ing water have a greater or less disposition 
to unite themselves with that fluid, by the 
effect of an attraction similar to chemical 
affinity. If we plunge into water several 
of these bodies, such as wood, a sponge, pa- 
per, &c., they will appropriate to them- 
selves a quantity of that liquid, which will 
vary with the bodies respectively ; and, as 
in proportion as they tend towards the 
point of saturation, their affinity for the wa- 
ter continues to diminish, when those which 
have most powerfully attracted the water, 
have arrived at the point, where their at- 
tractive force is found solely equal to that 
of the body, which acted most feebly upon 
the same liquid, there w ill be established a 
species of equilibrium between all those bo- 
dies, in such manner, that at this term the 
imbibing will be stopped. If there be 
brought ipto contact two wetted or soaked 
bodies, whose affinities for water are not in 
equilibria ; that whose affinity is the weak- 
est, will yield of its fluid to the other, until 
the equilibrium is established ; and it is in 
this disposition of a body to moisten ano- 
ther body that touches it, that what is call- 
ed humidity properly consists. Of all bo- 
dies, the air is that of which we are most 
interested to know the different degrees of 
humidity, and it is also towards the mean3 
of procuring this knowledge, that philoso- 
phers have principally directed their re- 
searches ; hence the various kinds of instru- 
ments that have been contrived to measure 
the humidity of the air. A multitude of 
bodies are known, in which the humidity, 
in proportion as it augments or diminishes, 
occasions divers degrees of dilatation or of 
contraction, according as the body is in- 
clined to one or other of these effects, by 
reason of its organization, of its texture, or 
of the disposition of the fibres of which it is 
the assemblage. For example, water, by 
introducing itself within cords, makes the 
fibres twist and become situated obliquely, 
produces between those fibres such a sepa- 
ration, as causes the cord to thicken or 
swell, and, by a necessary consequence, to 
shorten. The twisted threads, of which 
cloths are fabricated, may be considered as 
small cords, which experience, in like man- 
ner, a contraction by the action of hu- 
midity ; whence it happens, that cloths, es- 
pecially when wetted for the first time, 
contract in the two directions of their in- 
tersecting threads ; paper, on the contrary, 
which is only an assemblage of filaments 
very thin, very short, and disposed irregu- 
larly in all directions, lengthens in all the 
dimensions of its surface, in proportion as 
the water, by insinuating itself between the 
intervals of those same filaments, acts by 
placing them further asunder, proceeding 
from the middle towards the edges. Dif- 
ferent bodies have been employed succes- 
sively in the construction of hygrometers, 
chosen from among those in which humidity 
produces the most sensible motions. Phi- 
losophers have sought also to measure the 
humidity of the air by the augmentation of 
weight undergone by certain substances, 
such as a tuft of wool, or portions of salt, 
by absorbing the water contained in the 
air. But, besides that these methods were 
in themselves very imperfect, the bodies 
employed were subject to alterations which 
would make them lose their hygrometric 
quality more or less promptly; they had, 
therefore, the double inconvenience of be-. 
