68 VOLTAIRE. 



repandue dans 1'atmosphere : done dans toutes les autres 

 operations par lesquelles les matieres calcinees acqui- 

 rent du poids, cette augmentation de substance pourrait 

 aussi leur etre venue de la meme cause, et non de 

 la matiere ignee." About half a century later this 

 conjecture was verified, when the composition of the 

 atmosphere was discovered. Had Voltaire followed up 

 his felicitous conjecture by one or two experiments, he 

 would very probably have discovered both the nature 

 of oxygen and the process of oxydation, which last, in- 

 deed, he had in general terms described. 



Again, how near does he approach to the true 

 theory of fluidity, and even to the discovery of latent 

 heat, when, speaking of the effects on the thermometer 

 of mixing ammonia and vinegar, he says, " II y a cer- 

 tainement du feu dans ces deux liqueurs, sans quoi elles 

 ne seraient point fluides ;" and afterwards speaking of 

 the connection between heat and permanent or gaseous 

 elasticity, he says, " N'est-ce pas que 1'air n'a plus alors 

 la quantite de feu necessaire pour faire jouir toutes ses 

 parties, et pour le degager de 1'atmosphere engourdie 

 qui le renferme?" The experiments which he made 

 on the heat of fluids mixed together, of different tem- 

 peratures before their mixture, led him to remark the 

 difference of the temperature when mixed from what 

 might have been expected by combining the separate 

 temperatures before mixture. Need I add that this is 

 precisely the course of experiment and observation 

 which led Black to his celebrated discovery of latent 

 heat a quarter of a century later ? 



It was in these studies that the time passed at 



