6 Dr Henry'^s Estimate of 



observation ; — an enlarged sphere of chemical phenomena ;— an 

 acquaintance with a far greater number of individual bodies, 

 than were then known ; from the properties of which, and from 

 those of their combinations, tentative approximations to general 

 principles might at first be deduced ; to be confirmed or correct- 

 ed, enlarged or circumscribed, by future experience. It would 

 have retarded the progress of science, and put off, to a far dis- 

 tant day, that affluence of new facts which Priestley so rapidly 

 accumulated, if he had stopped to investigate, with painful and 

 rigid precision, all the minute circumstances of temperature, of 

 specific gravity, of absolute and relative weights, and of crystal- 

 line structure, on which the more exact science of our own times 

 is firmly based, and from which its evidences must henceforward 

 be derived. Nor could such refined investigations have then 

 been carried on with any success, on account of the imperfec- 

 tion of philosophical instruments. It would have been fruitless, 

 also, at that time, to have indulged in speculations respecting the 

 ultimate constitution of bodies ; — ^speculations that have no solid 

 ground-work, except in a class of facts developed within the last 

 thirty-five years, all tending to establish the laws of combination 

 in definite and in multiple proportions, and to support the still 

 more extensive generalization, which has been reared by the 

 genius of Dalton. 



It was, indeed, by the activity of his intellectual faculties, 

 rather than by their reach or vigour, that Dr Priestley was 

 enabled to render such important services to natural science. 

 We should look, in vain, in any thing that he has achieved, for 

 demonstrations of that powerful and sustained attention, which 

 enables the mind to institute close and accurate comparisons ; — > 

 to trace resemblances that are far from obvious ; — and to dis- 

 criminate differences that are recondite and obscure. The ana- 

 logies which caught his observation lay near the surface, and 

 were eagerly and hastily pursued ; often, indeed, beyond the 

 boundaries within which they ought to have been circumscrib- 

 ed. Quick as his mind was in the perception of resemblances, 

 it appears (probably for that reason) to have been little adapt- 

 ed for those profound and cautious abstractions, which supply 

 the only solid foundations of general laws. In sober, patient, 

 and successful induction, Priestley must yield the palm to many 



