APPENDIX 335 



able to maintain respiration. His discovery of oxygen which 

 soon followed, and the determination of the composition of 

 carbonic acid, provided the explanation of the nature of this 

 connexion between the two organic kingdoms. The animal 

 inhales oxygen and exhales carbonic acid ; the plant inhales 

 carbonic acid and exhales oxygen, retaining (precipitating) 

 carbon. Plants and animals present a chemical antithesis. 

 A series of further investigations showed that this process, which 

 decomposes carbonic acid and restores good air, has yet another 

 more important significance, which is that it provides the plant 

 with food. Carbon remains inside the plant, forms its organic 

 matter, and serves to build up its body. It follows that the 

 carbonic acid of the atmosphere must be considered the main 

 food of a plant. Although this function was long attributed to 

 the black particles of the soil, i.e. to humus, the inadequacy 

 of this view was established by exact experiments. 



Priestley, however, had to experience one of the greatest dis- 

 appointments that ever befell a scientist. He failed some time 

 after, when he tried to repeat the experiment which made him 

 so famous : he could not obtain his former results ; the plants 

 persisted in refusing to decompose carbonic acid and set free 

 oxygen. Although these disappointments did not shake his 

 confidence in his earlier experiments, it became evident, never- 

 theless, that some condition important for the experiment had 

 been overlooked, owing to which fact the experiment could 

 not be repeated. This condition neglected by Priestley was 

 soon after discovered by Ingenhousz. In order to fuUy appre- 

 ciate this discovery we shall dwell a little longer on the nature 

 of the phenomenon itself. 



Let us for the last time turn to our balls. We have been 

 comparing chemical combination or combustion with the con- 

 cussion of two balls against each other ; heat and light liberated 

 during the process serve as a measure of the affinity or tension, 

 i.e. of the mutual attraction of these bodies (represented in our 

 illustration by the tension of the coils). In order to separate 

 them again, to break up the connexion between them, in order 

 to bring the balls into the former free position, we must on the 

 contrary expend energy, and expend as much of it as is liberated 

 at the moment of collision. Thus it becomes evident that a 

 phenomenon contrary to that of combustion has to be accom- 



