TRANSACTIONS. 
63 
Hydrogen gas was known before his time; but he greatly ex¬ 
tended our acquaintance with its properties. Nitrous gas, 
barely discovered by Dr. Hales, was first investigated by 
Priestley, and applied by him to eudiometry. To the chemical 
history of the acids derived from nitre, he contributed a vast 
accession of original and most valuable facts. He seems to 
have been quite aware that those acids are essentially gaseous 
substances, and that they might be exhibited as such, provided 
a fluid could be found that is incapable of absorbing or acting 
upon them*. He obtained, and distinctly described f, the 
curious crystalline compound of sulphuric acid with the vapour 
of nitrous acid, or, more correctly, of sulphuric and hypo-nitrous 
acids, which, being of rare occurrence, was forgotten, and has 
since been rediscovered, like many other neglected anticipa¬ 
tions of the same author. He greatly enlarged our knowledge 
of the important class of metals, and traced out many of their 
most interesting relations to oxygen and to acids. Pie un¬ 
folded, and illustrated by simple and beautiful experiments, 
distinct views of combustion; of the respiration of animals, 
both of the inferior and higher classes ; of the changes pro¬ 
duced in organized bodies by putrefaction, and of the causes 
that accelerate or retard that process; of the importance 
of azote as the characteristic ingredient of animal substances, 
obtainable by the action of dilute nitric acid on muscle and 
tendon; of the functions and ceconomy of living vegetables; and 
of the relations and subserviency which exist between the ani¬ 
mal and vegetable kingdoms. After trying, without effect, 
a variety of methods, by which he expected to purify air vitiated 
by the breathing of animals, he discovered that its purity was 
restored by the growth of living and healthy vegetables, freely 
exposed to the solar light. 
It is impossible to account for these, and a variety of other 
discoveries, of less importance singly, but forming altogether a 
tribute to science, greatly exceeding, in richness and extent, 
that of any contemporary, without pronouncing that their au¬ 
thor must have been furnished by nature with intellectual 
powers, far surpassing the common average of human endow¬ 
ments. If we examine, with which of its various faculties the 
mind of Dr. Priestley was most eminently gifted, it will, I be¬ 
lieve, be found that it was most remarkable for clearness and 
quickness of apprehension, and for rapidity and extent of asso¬ 
ciation. On these qualities were founded that apparently 
intuitive perception of analogies, and that happy facility of 
* 
Series I. Yol. ii. p. 175. 
f Series II. Yol. i. p. 26. 
