CHAP, ii.] of Plants. Priestley. 493 



succeeded in setting himself quite free from the old notions, 

 and developed his antiphlogistic system into a connected 

 whole. It should be mentioned that he had discovered in 

 1777 that the respiration of animals is a process of oxidation 

 which produces their internal heat, heat being the product of 

 every form of combustion. This fact was equally important 

 for vegetable physiology, but it was some time before it was 

 used to explain the life of plants. 



The establishment of the fact, that parts of plants give 

 off oxygen under certain circumstances, did little or nothing to 

 further the theory of their nutrition * ; and that was all that 

 vegetable physiology owes to Priestley. Ingen-Houss on the 

 other hand determined the conditions under which oxygen is 

 given off, and further showed that all parts of plants are con- 

 stantly giving rise to carbon dioxide ; on these facts rests the 

 modern theory of the nutrition and respiration of plants, and 

 we must therefore consider that Ingen-Houss was the founder 

 of that theory. But since we are dealing here with a discovery 

 of more than ordinary importance, it seems necessary to go 

 more closely into the details. 



A work of Priestley's appeared in 1779, which was translated 

 into German in the following year under the title, ' Versuche 

 und Beobachtungen iiber verschiedene Theile der Naturlehre,' 

 and contained among other things the writer's experiments on 

 plants. His way of managing them was eminently unsuitable, 

 nor did he arrive at any definite and important result, though 

 he expressed the idea which had led him to make them clearly 

 enough, where he says, ' If the air exhaled by the plant is of 

 better character (richer in oxygen) than atmospheric air, it 

 follows that the phlogiston of the air is retained in the plant 



1 Still less was gained from an observation made by Bonnet, that leaves 

 exposed to sunlight in water containing air show bubbles of gas on their 

 upper surface. Bonnet expressly denied the active participation of the 

 leaves in the phenomenon, since the same thing happens with dead leaves 

 in water containing air. 



