II. THE PHYSICAL DOCTRINE OF LIFE- 

 CHEMISTRY AND MECHANICS. 



"Neither formerly nor at this time have men endeavoured to deter- 

 mine or discover the differences in the effects of the vital force and those 

 of the inorganic forces, and their likeness or unlikeness." LIEBIG, 1846. 



SCIENTIFIC progress is advanced by the temperate but free 

 and open discussion of scientific questions upon which 

 different observers may have been led to entertain the 

 most diverse and perhaps conflicting opinions. The very 

 essence of science is the repeated testing of conclusions 

 already arrived at. By this course alone can errors be 

 corrected, and it is mainly by going over scientific ground 

 which is by no means new, and repeating experiments which 

 have been, perhaps, performed many times before, that new 

 facts are demonstrated and new principles discovered. 

 Nevertheless it is certain that in these days some scientific 

 men dislike discussion, and seem to be offended if any one 

 ventures to criticise their observations or to express any 

 opinions opposed to their own. Sometimes the view of an 

 opponent is spoken of with a sneer, and not unfrequently 

 those who differ are affectedly pitied or despised. But 

 every real student of science, so far from attempting to 

 suppress discussion, will encourage it in every way in his 

 power, for he must know that it is almost impossible that 

 the truth of many complex scientific problems can be arrived 

 at without long and patient discussion. The analysis of 



