Philosophical Character of Dr. Priestleyi 35 



1 measure of fluor acid with 2 measures of do. 



1 measure of oxygen gas with 2 measures nitrous, very nearly ; 

 and that by the decomposition of 1 vol. of ammonia, 3 vols, of hy- 

 drogen are evolved. 



Let not, however, failures such as these, to reap all that was with- 

 in his compass, derogate more. than their due share from the merits 

 of Dr. Priestley : for they may be traced to that very ardor of tem- 

 perament, 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 re- 

 membered, that philosophers of the loftiest pretensions are charge- 

 able with similar oversights ; — that even Kepler and Newton over- 

 looked discoveries, upon the very confines of which they trod, but 

 which they left to confer glory on the names of iess 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 instan- 

 ces, 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 ob- 

 servations- Even in these few examples, his errors may be traced 

 to causes connected with the actual condition of science at the time ; 

 sometimes to the use of impure substances, or to the imperfection 

 of his instruments of research ; but never to carelessness of inquiry 

 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 coloring, a fact, to suit an hypothe- 

 sis. 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 colored medium, which sometimes steals imperceptibly over minds 

 of the greatest general probity. This security he owed to his free- 

 dom from all undue attachment to hypotheses, and to the facility, 

 with which he was accustomed to frame and abandon them ; — a fa- 

 cility resulting not from habit only, but from principle. " Hypothe- 

 ses" he pronounces, in one place, " to be a cheap commodity ;" 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 consid- 



