78 THE YEAR. 



artillery. It would be easy to multiply instances in 

 which the effect would be much greater than in the 

 one that has been stated ; but that which is at once 

 the most familiar and the most useful in practice, will 

 show that the most powerful agents in nature have 

 really nothing at all to do with weight, or with pon- 

 derable matter. 



Thus, although we speak about the constituent parts 

 of the atmosphere as being known, we are so far from 

 being warranted to conclude that the ponderable parts 

 the oxygen, the nitrogen, the carbonic acid, and 

 the vapour of water, make up the whole of it, that, 

 for aught that we yet know to the contrary, they may 

 be the mere media through which more active prin- 

 ciples are brought into operation. Respecting matters 

 that defy all our instruments to obtain them in a sepa- 

 rate state, we must speak with the greatest caution, 

 because we cannot, in such cases, distinguish between 

 substance and action ; cannot tell, for instance, whether 

 the heat, which produces so many, so wonderful, and 

 so important effects, be an actual part of the sun- 

 beam, or whatever else it appears in, or merely an 

 appearance attendant upon certain states of action. 

 When we find that a warm body heats a cold, or that 

 a cold cools a warm, we are very apt to suppose 

 that it can be nothing but the tranfusion of some sub- 

 stance from the one to the other. But, on the other 

 hand, when we hammer a piece of iron till it becomes 

 of a red heat, the heat is not abstracted from the anvil 

 and the hammer, for they also become hot. Sub- 

 stances also become cold or warm with very different 

 degrees of rapidity ; so that a difference in the effect 



