8 Dr Henry^s Estimate of 



and repeatedly presented itself to his observation, without his 

 being aware of its true distinctions from other kinds of inflam- 

 mable air ; and it was reserved for Mr Cruickshank of Woolwich 

 to unfold its real nature and characters. It is remarkable also, 

 that in various parts of his works, Dr Priestley has stated facts* 

 that might have given him a hint of the law, since unfolded by 

 the sagacity of M. Gay-Lussac, " that gaseous substances com- 

 bine in definite volumes." 

 He shows that 



1 measure of fixed air unites with 1^ measure of alkaline air, 



1 measure of sulphurous acid with 2 measures of do. 



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 

 hydrogen are evolved. 



Let not, however, failures such as these to reap all that wa& 

 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 philosophers of 

 the loftiest pretensions are chargeable with similar oversights ;-^ 

 that even Kepler and Newton overlooked discoveries, upon the 

 very confines of which they trod, but which they left to confer 

 glory on the names of less illustrious followers. 



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 vari- 

 ance with recent Ihd correct observations. 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 



