TRANSACTIONS. 
67 
and that by the decomposition of 1 volume of ammonia, 3 vo¬ 
lumes of hydrogen are evolved. 
Let not, however, failures such as these, to reap all that was 
within his compass, derogate more than their due share from 
the merits of Dr. Priestley; for they may be traced to that 
very ardour of temperament, which, though to a certain degree 
a disqualification for close and correct observation, was the 
vital and sustaining principle of his zealous devotion to the 
pursuit of scientific truth. Let it be remembered, that philo¬ 
sophers of the loftiest pretensions are chargeable with similar 
oversights;—that even Kepler and Newton overlooked dis¬ 
coveries, upon the very confines of which they trod, but which 
they left to confer glory on the names of less illustrious fol¬ 
lowers. 
Of the general correctness of Dr. Priestley’s experiments, it 
is but justice to him to speak with decided approbation. In 
some instances, it must be acknowledged, that his results have 
been rectified by subsequent inquirers, chiefly as respects 
quantities and proportions. But of the immense number of 
new facts originating with him, it is surprising how very few 
are at variance with recent and correct observations. Even in 
these few examples, his errors may be traced to causes con¬ 
nected with the actual condition of science at the time; some¬ 
times to the use of impure substances, or to the imperfection 
of his instruments of research; but never to carelessness of in¬ 
quiry or negligence of truth. Nor was he more remarkable for 
the zeal with which he sought satisfactory evidence, than for 
the fidelity with which he reported it. In no one instance is 
he chargeable with mis-stating, or even with straining or colour¬ 
ing, a fact to suit an hypothesis. And though this praise may, 
doubtless, be conceded to the great majority of experimental 
philosophers, yet Dr. Priestley was singularly exempt from that 
disposition to view phenomena through a coloured medium, 
which sometimes steals imperceptibly over minds of the greatest 
general probity. This security he owed to his freedom from 
all undue attachment to hypotheses, and to the facility with 
which he was accustomed to frame and abandon them;—a 
facility resulting not from habit only, but from principle. “ Hy¬ 
potheses,” he pronounces, in one place, “ to be a cheap com¬ 
modity;” in another to be “of no value except as the parents 
of facts;” and so far as he was himself concerned, he exhorts 
his readers “to consider new facts only as discoveries, and to 
draw conclusions for themselves.” The only exception to this 
general praise is to be found in the pertinacity with which he 
adhered, to the last, to the Stahlian hypothesis of phlogiston; 
e 2 
