240 
SECOND REPORT— 1832 . 
in so far as they consider the tension of vapour as totally inde¬ 
pendent of that of air, or the presence of air at all,—we must 
banish all confused notions from our minds about “ saturation 
of air with moisture,” “ solvent power of air,” &c., which are 
to be found even in the very writings in which Mr. Dalton’s 
principle is assumed as established. This should be guarded 
against with care, because it may insensibly lead in practice to 
the most inaccurate ideas regarding the influence of the 
presence of gaseous matter. We think nevertheless that in 
some cases the rage for purifying our scientific nomenclature 
has been carried too far, where even the results of reasoning are 
arraigned because they include the use of terms suggesting 
perhaps hypothetical views, but the adoption of which conven¬ 
tionally, need not be objected to. 
The easiest way of obtaining a distinct, simple, and accurate 
knowledge of the hygrometric state of the atmosphere at any 
moment, is to ascertain by some means the temperature at 
which the vapour then existing can no longer maintain its aeri¬ 
form state, or, in other words, to find the temperature of the 
dew-point. Then being furnished with a Table of the elastici¬ 
ties of aqueous vapour at different temperatures, the elasticity 
is of course equal to that of vapour which can just subsist at the 
temperature of the dew-point; whence the weight of grains in 
a cubic inch may be easily computed from the experiments of 
Gay-Lussac, and the expression of the sensible state of humi¬ 
dity of the atmosphere at its own proper temperature, must 
be obtained by the ratio of the vapour actually existing in a 
cubic inch, to wdiat might have existed without deposition, in 
the same space. 
Such, in few words, is the rationale of the dew-point experi¬ 
ment. Let us see now the means we have of arriving at this 
result. Regarding instruments, the simplest form of the experi¬ 
ment is that which Mr. Dalton employs. The dew deposited 
on the surface of a glass of cold water has been observed from 
the earliest times, and has been particularly alluded to by an¬ 
cient authors; let therefore the cold liquid be transferred from 
one glass to another till the deposition ceases,—the tempe¬ 
rature then measured will give the dew-point. Mr. Daniell’s 
elegant instrument is too well known to require minute descrip¬ 
tion : he has applied the principle of the Cryophorus of Wol¬ 
laston to obtain the requisite cold for the production of dew 
upon a ball of dark-coloured glass, the temperature of the 
aether inclosed, being measured by a delicate thermometer in¬ 
serted. This instrument has come into very general use, and 
notwithstanding some delicacy required in the management of 
