LECTURE VII. 



WHETHER the words of our motto, which the poet has 

 placed in the mouth of the Evil Spirit, are true ; whether 

 the saying of common life, as of the sacred poetry, that 

 Man springs from dust, and to dust and ashes returns, is 

 more than a poetic figure only natural science only 

 physiology can tell us. 



I took leave, in a former Lecture, to defend the natu- 

 ralist when he asserted that Man lives on air alone, springs 

 from it, and again returns into the same. Decomposition 

 dissolves all animal bodies into ammonia, carbonic acid and 

 water, and these exhale as gas and watery vapour into the 

 air. Mankind extract their food, mediately or immediately, 

 wholly from the vegetable kingdom, and this lives essentially 

 at the cost of the carbonic acid, ammonia, and water of the 

 atmosphere. 



These views, for which we have to thank the researches 

 which the most distinguished inquirers of the last hundred 

 years have occupied themselves in following out and com- 

 pleting, have, in the present times, however, been first so 

 expressed by Liebig, as to attract universal attention. 

 Loud cries have arisen against him, in very various quarters, 

 but the reasons and objections which have been made good, 



