4 INTRODUCTORY REMARKS. 



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 what- 

 ever 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 influence 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 ; 

 that 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 inflam- 

 mable 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 construction, and the crawling 

 of a fly upon the window. Yet we find that these two opera- 



