OF NATURAL PHILOSOPHY. 209 



observations purposely made under such a variety of 

 circumstances as fairly to embrace the whole range 

 of the phenomena which the theory is intended to 

 account for, we cannot refuse to admit them ; or 

 if we still hesitate to regard them as demonstrated 

 truths, we cannot, at least, object to receive them 

 as temporary substitutes for such truths, until the 

 latter shall become known. If they suffice to ex- 

 plain all the phenomena known, it becomes highly 

 improbable that they will not explain more ; and if 

 all their conclusions we have tried have proved 

 correct, it is probable that others yet untried will 

 be found so too ; so that in rejecting them altogether, 

 ye should reject all the discoveries to which they may 

 lead. 



(221.) In all theories which profess to give a true 

 account of the process of nature in the production 

 of any class of phenomena, by referring them to 

 general laws, or to the action of general causes, 

 through a train of modifying circumstances; before 

 we can apply those laws, or trace the action of those 

 causes in any assigned case, we require to know the 

 circumstances : we must have data whereon to ground 

 their application. Now, these can be learned only 

 from observation ; and it may seem to be arguing 

 in a vicious circle to have recourse to observation 

 for any part of those theoretical conclusions, by 

 whose comparison with fact the theory itself is to 

 be tried. The consideration of an example will 

 enable us to remove this difficulty. The most 

 general law which has yet been discovered in che- 

 mistry is this, that all the elementary substances in 

 nature are susceptible of entering into combination 

 p 



