Libra/ry, 



J. HE greater the progress, wliicli Natural History makes in 

 the knowledge of facts and their inner coherence, the more 

 easy it is to understand those causes which lay the founda- 

 tion of the facts. Whilst formerly, as an explanation for al- 

 most every phenominal appearance, the existence of its own 

 peculiar substance was admitted (for example "Caloric", which 

 was considered as imponderous, or, still earHer, that so-caUed 

 "Phlogiston", which is supposed to contain such a negative 

 weight, that through its existence the other bodies which are 

 connected with it, become much lighter) ; we now attain, more 

 and more, the knowledge, that every phenomenon of Nature 

 is to be traced back to the motions of the smallest particles 

 of which all matter is composed. 



The phenomena on which I intend to discourse, are very 

 remote from this notion. However minutely they may have 

 been investigated separately, and whatever great results they 

 may have led to in a practical sense, in order to explain them 

 fully, it is necessary for us to make an assumption ("hypo- 

 thesis" as it is called by men of science), for the accuracy of 

 which we have no proof, and it is only devised for the pur- 

 pose of concentrating the whole series of phenomena into a 

 general point of view. The value of such suppositions con- 



1872. IL 1. (125) A3 



