490 D'ALEMBERT. 



object of all philosophy ; and experimental science, in parti- 

 cular, is occupied with such investigations, giving us general 

 views, and enabling us to explain the appearances of nature, 

 that is, to show how one appearance is connected with another. 

 But we are now considering only the gratification derived 

 from learning these things. 



It is surely a satisfaction, for instance, to know that the 

 same thing, or motion, or whatever it is, which causes the 

 sensation of heat, causes also fluidity, and expands bodies in 

 all directions ; that electricity, the light which is seen on the 

 back of a cat when slightly rubbed on a frosty evening, is the 

 very same matter with the lightning of the clouds; that 

 plants breathe like ourselves, but differently by day and by 

 night; that the air which burns in our lamps enables a 

 balloon to mount, and causes the globules of the dust of 

 plants to rise, float through the air, and continue their race ; 

 in a word, is the immediate cause of vegetation. Nothing can 

 at first view appear less like, or less likely to be caused by the 

 same thing, than the processes of burning and of breathing, 

 the rust of metals and burning, an acid and rust, the influ- 

 ence of a plant on the air it grows in by night, and of an 

 animal on the same air at any time, nay, and of a body 

 burning in that air ; and yet all these are the same operation. 

 It is an undeniable fact, that the very same thing which makes 

 the fire burn, makes metals rust, forms acids, and enables 

 plants and animals to breathe; but these operations, so 

 unlike to common eyes, when examined by the light of 

 science, are the same, the rusting of metals, the formation 

 of acids, the burning of inflammable bodies, the breathing of 

 animals, and the growth of plants by night. To know this is 

 a positive gratification. Is it not pleasing to find the same 

 substance in various situations extremely unlike each other ; 

 to meet with fixed air as the produce of burning, of breathing, 

 and of vegetation ; to find that it is the choke-damp of mines, 

 the bad air in the grotto at Naples, the cause of death in 

 neglected brewers' vats, and of the brisk and acid flavour of 

 Seltzer and other mineral springs ? Nothing can be less like 

 than the working of a vast steam-engine, of the old construc- 

 tion, and the crawling of a fly upon the window. Yet we 



