ADDRESS. 23 



of mecbanical energy, and must be taken into account ; and this factor we 

 are as yet unable to estimate in our usual terms. It concerns the action 

 of the mind upon the body, and, although incapable of exact expression, 

 exerts none the less an important influence on the physics and chemistry 

 of the body, so that a connection undoubtedly exists between intellectual 

 activity or mental work and bodily nutrition. In proof that there is a 

 marked difference between voluntary and involuntary work, we need only 

 compare the mechanical action of the heart, which never causes fatigue, 

 with that of the voluntary muscles, which become fatigued by continued 

 exertion. So, too, we know well that an amount of drill which is fatiguing 

 to the recruit is not felt by the old soldier, who goes through the evolutions 

 automatically. What is the expenditure of mechanical energy which accom- 

 panies mental effort, is a question which science is probably far removed 

 from answering. But that the body experiences exhaustion as the result 

 of mental activity is a well-recognised fact. Indeed, whilst the second law 

 of thermodynamics teaches that in none of the mechanical contrivances 

 for the conversion of heat into actual energy can such a conversion be 

 complete, it is perhaps possible, as Helmholtz has suggested, that such a 

 complete conversion may take place in the subtle mechanism of the 

 animal organism. 



The phenomena of vegetation, no less than those of the animal world, 

 have, however, during the last fifty years been placed by the chemist on 

 an entirely new basis. Although before the publication of Liebig's cele- 

 brated report on chemistry and its application to agriculture, presented 

 to the British Association in 1840, much had been done, many funda- 

 mental facts had been established, still Liebig's report marks an era in 

 the progress of this branch of our science. He not only gathered up in a 

 masterly fashion the results of previous workers, but put forward his own 

 original views with a boldness and frequently with a sagacity which gave 

 a vast stimulus and interest to the questions at issue. As a proof of this 

 I may remind you of the attack which he made on, and the complete 

 victory which he gained over, the hnmus theory. Although Saussure and 

 others had already done much to destroy the basis of this theory, yet the 

 fact remained that vegetable physiologists up to 1840 continued to hold 

 to the opinion that humus, or decayed vegetable matter, was the only 

 source of the carbon of vegetation. Liebig, giving due consideration to 

 the labours of Saussure, came to the conclusion that it was absolutely im- 

 possible that the carbon deposited as vegetable tissue over a given area, 

 as for instance over an area of forest land, could be derived from humus, 

 which is itself the result of the decay of vegetable matter. He asserted 

 that the whole of the carbon of vegetation is obtained from the atmospheric 

 carbonic acid, which, though only present in the small relative proportion 

 of 4 parts in 10,000 of air, is contained in such absolutely large quantity 

 that if all the vegetation on the earth's surface were burnt, the proportion 

 of carbonic acid which would thus be thrown into the air would not be 

 sufficient to double the present amount. 



