280 SIR ISAAC NEWTON. 



But all this only shows that the discoveries of Newton, 

 great and rapid as were the steps by which they advanced our 

 knowledge, yet obeyed the law of continuity, or rather of 

 gradual progress, which governs all human appi'oaches 

 towards perfection. The limited nature of man's faculties 

 precludes the possibility of his ever reaching at once the 

 utmost excellence of which they are capable. Survey the 

 whole circle of the sciences, and trace the history of our pro- 

 gress in each, you find this to be the universal rule. In 

 chemical philosophy the dreams of the alchemists prepared 

 the way for the more rational though erroneous theory of 

 Stahl : and it was by repeated improvements that his errors, 

 so long prevalent, were at length exploded, giving place to 

 the sound doctrine which is now established. The great 

 discoveries of Black and Priestley on heat and aeriform fluids, 

 had been preceded by the happy conjectures of Kewton, and 

 the experiments of others. Xay Voltaire* had well nigh 



colours by refraction, is remarkable ; but far less so than Marcus's in 1648 

 on the ' Iris Trigonia,' as he calls the spectrum, and his observation of the 

 colours not changing by a second refraction, so nearly approaching 

 Newton's ' Experimentum Crucis.' It is best to mention this, because 

 writers on the history of science have so often stated that nothing like a 

 trace of the Newtonian doctrine of light can be found in the works of 

 former observers. There is no appearance whatever of Newton having 

 known Marcus's work. 



* In his Prize Memoir we find (among many great errors chiefly arising 

 from fanciful hypotheses) such passages as this, being an observation on 

 one of his own experiments, ' II y a certainement du feu dans ces deux 

 liqueurs, sans quoi elles ne seraient point fluides ;' and again, in speaking 

 of the connexion between heat and permanent or gaseous elasticity, ' N'est 

 ce pas que 1'air n'a plus alors la quantite de feu necessaire pour faire jouer 

 toutes ses parties, et pour le de'gager de I'atmosphere engourdie qui le 

 renferme.' The experiments which he made on the temperature of liquids 

 mixed together, led him to remark the temperature of the mixture as 

 different from what might have been expected, regard being had to that 

 of the separate liquids. Again, speaking of his experiments on the calci- 

 nation of metals, ' II est tres possible que 1'augmcntation du poids soit 

 venue de la mattere re'pandue dans I'atmosphere ; done dans toutes lea 

 autres operations par lesquelles les matieres calcinees acquierent du poids 



