144 MEMOIR OF PRIESTLEY 



indefatigable perseverance of his inquiries and the number and variety 

 of his expedients. While others sedulously conceal how much they 

 owe to hazard, Priestley seemed inclined to credit it with everything. 

 He tells us, with unexampled candor, how often he was indebted to it 

 unconsciously, how often new substances presented themselves to him 

 without being recognized ; nor does he ever dissemble the erroneous 

 views which sometimes governed him and from which he was only 

 detached by further experience. 



These avowals did honor to his modesty without disarming jealousy. 

 Those, whose own views and methods had proved abortive in discovery, 

 called him a mere experimenter without method or design ; no wonder, 

 they observed, if, among so many trials and combinations, some should 

 be lucky. 



But the true physicist is not to be duped by such interested criti- 

 cisms. He is well aware how many efforts it costs to arrive at those 

 happy ideas which are the source and regulating principle of all others ; 

 while the very men who have thought fit to augment our admiration 

 of their own great achievements in science by the luminous order in 

 which they propound them, will be far from taking it amiss, that 

 others, like Priestley, have chosen to give us their discoveries just as 

 they made them, and have ingenuously retraced all the windings of 

 the path over which they have travelled. 



This peculiarity results from his manner of composition. We have 

 here no finished construction, no digest of theorems rigidly deduced 

 the one from the other, as they may be supposed to present them- 

 selves in the conceptions of pure reason. What he has given us is 

 the journal of his thoughts in all the disorder of their succession, 

 wherein we see him at first feeling his way in profound obscurity, 

 spying out the faintest rays of light that he may collect and reflect 

 them, sometimes misled by a treacherous and transient glimmer, but 

 sure to arrive in the end at some region of vast extent and fertility. 



Should we be displeased if the great masters of the human race, 

 an Archimedes or a Newton, for instance, had thus admitted us to the 

 confidence of their genius? Newton, when asked how he arrived at 

 his great discoveries, replied, " By long thinking about them." With 

 what pleasure should we not have learned that long series of thoughts 

 by which Newton was conducted to that master thought which has 

 inspired the meditations of all his successors. If his works at present 

 teach us to appreciate the forces of nature, we should, by thus knowing 

 the actual processes of his mind, have been enabled thoroughly to 

 comprehend that noblest of nature's works — the genius of a great 

 man. 



We must not suppose, however, that the discoveries of Priestley were 

 all perceived by himself, or that they were set forth in his writings as 

 clearly or in the same manner as we should now state them. At 

 the time of making them he knew no chemical theory but that of 

 Stahl, and this, being founded on experiments in which air was taken 

 no account of, was of course incapable of providing for its phenomena. 

 Hence there results a degree of hesitation as to principles, and of em- 

 barrassment and uncertainty as to results. Seeking everywhere for 

 phlogiston, Priestley is obliged to suppose it differently constituted 



