180 Analysis of Scientific Books. 
Section 3d relates to the Hygrometer, in which, after some of 
the usual remarks on the various bodies, animal, vegetable, and 
mineral, called hygroscopes, because absorbent of moisture from 
the atmosphere, Mr. Leslie proceeds to describe his own hygro- 
metric contrivances; the account of these is spread over about 
twenty columns of typography, besides two pages of table. The 
first instrument is his ivory hygroscope, consisting of a ther- 
mometer-formed vessel, filled with mercury, in which the de- 
scent and ascent of this liquid is determined by the swelling 
and shrinking of the ivory bulb, with the variations of its 
moisture and dryness. The tardiness of the indications of such 
an instrument, disqualifies it, even in Mr. Leslie’s eyes, for every 
sort of delicate observation. 
We agree with our author in thinking that philosophers en- 
tertained, till very lately, crude notions respecting the as- 
sociation of moisture with air; and of the different circum- 
stances which regulate or influence the process of evaporation. 
But we are greatly surprised at the silence observed with 
regard to Mr. Dalton, to whom we are indebted for exposing 
those crude notions, and establishing a correct theory on this 
important subject; the more so, as Mr. Dalton’s merit is uni- 
versally recognised by all those French writers, with whose 
works Mr. Leslie appears perfectly conversant. The following 
statement seems unaccountable in a British author: But experi- 
ments on the influence which an increase of temperature exerts on the 
quantity of evaporation, have been prosecuted with various success by 
Lampadius, Saussure, and Kirwan. The results thus obtained, unfortunately 
differ very widely ; and though the researches of the celebrated naturalist 
of Geneva were those conducted with the most care and address, yet 
they seem, from the vagueness of their elements, not entitled to much con- 
fidence*. Here we have not one word of Mr. Dalton’s decisive 
investigations, published long ago in the Manchester Memoirs, 
and thence transferred into almost every elementary book. 
The Professor next presents us, at great length, we would 
say superfluous prolixity, with his analysis of the operation 
by which cold is produced by evaporation, on which is 
founded the action of his own hygroscope. We have no desire 
to transcribe any part of this tedious developement, especially 
as we are satisfied that his instrument is far inferior to the 
simple process of Le Roy or Dalton, and still more so to the 
refined contrivance of Mr. Daniell. Mr. Leslie even adopts 
the exploded notion of a gas holding its hygrometric water by 
a peculiar solvent power. 
The energy of hydrogen gas (says he) is therefore scarcely less remark- 
able in dissolving moisture, than in containing heat. Contined witha pow- 
erful absorbent substance, while common air marks eighty degrees of dry- 
ness, hydrogen gas willindicate seventy. This gas must in similar circum- 
stances, therefore, hold in solution seven times as much moisture as the at- 
mospheric medium +. And the marginal title of his next paragraph 
* P. 337. + P. 341, 
