MOTION. 157 



SECTION VII. 



Observation of Air and Water. 



As we can know and contemplate the powers with 

 which nature works only through the medium of those 

 substances in which they are manifested, a consider- 

 able portion of that which would, perhaps, with more 

 propriety, come in under this or some of the succeed- 

 ing parts of the book, has been already anticipated, 

 and what remains to be said may, in some instances, 

 Jiave the appearance of repetition. But that is 

 unavoidable ; for if we are to view nature as it exists 

 .living nature, we must view it in its connexion. 

 There is no dissecting till after death ; and then the 

 very finest anatomy that can be practised gives us 

 only disjointed members. But the observer wishes 

 to know nature in its activity and life ; and, there- 

 fore, there is no possibility of noticing any one thing 

 usefully to him without a glance at collateral things. 



In the case of the great agencies of gravitation 

 and cohesion, and light and heat, and motion and 

 resistance, that is especially necessary, inasmuch 

 as, apart from the subject in which their effects are 

 displayed, we have not the slightest conception or 

 means of knowing any one of these. There is no 

 weight, unless there is something that is heavy, and 

 has other properties besides weight ; there can be 

 no cohesion, unless there is matter to cohere ; light 

 never appears but *vhen it illuminates something ; 

 we know nothing of heat, unless there is something 

 that is warmed by resistance to it; and we can 

 know that there is motion, not merely when some- 

 thing moves, but when there is some other thing 

 O 



