EARLY HISTORY. 323 



anj' active cells containing chlorophyll granules are subjected 

 to conditions favorable to assimilation, hypochlorin is formed 

 in considerable amount ; but when the conditions for assimi- 

 lation are not present, only traces of it are produced. Prings- 

 heira used an entirely novel method of experimenting ; namely, 

 that of subjecting the chlorophyll granules to the action of 

 intense light from which the heat rays had been extracted as 

 perfectly as possible ; and under these conditions he failed to 

 detect an}- hypochlorin, but observed a marked increase in the 

 amount of CO 2 given off as in ordinary respiration (see Chapter 

 XI.). Hence he arrived at the conclusion that assimilation 

 proper is the characteristic office of chlorophyll granules solely 

 on account of their pigment, which tempers the light reaching 

 them. According to him, the pigment, by its absorption of the 

 so-called chemical rays, serves as a regulatory screen gov- 

 erning the amount of light, and so controlling the amount of 

 respiration and assimilation proper. 



851. Outline of the early history of assimilation. The follow- 

 ing extracts from the works of early experimenters upon the 

 relations of green leaves to the atmosphere show the manner in 

 which the problem of assimilation was first attacked. 



852. Priestle}' 1 discovered in 1771 2 that air in which candles 

 can no longer burn, and which is irrespirable, can be restored to 

 its original condition by the presence in it, for a time, of vig- 

 orous plants. The account below is given in his own words : 



" Finding that candles would burn very well in air in which plants 

 had grown a long time, and having had some reason to think, that 

 there was something attending vegetation which restored air that had 

 been injured by respiration, I thought it was possible that the same 



1 Experiments and Observations on Different Kinds of Air (3d edition, 

 1781), p. 51. 



2 In 1754 Bonnet published his observations upon the behavior of leaves 

 in water. It is well known that when green leaves are immersed in water and 

 exposed to sunlight for a time, bubbles of air appear on their surface. Bonnet 

 believed that the leaves drew common air from the water and this swelled into 

 conspicuous bubbles under the heat of the sun. He was confirmed in this 

 belief upon ascertaining that bubbles did not appear on green leaves ex- 

 posed in water which has been boiled to expel the air (Recherches sur 1'usage 

 des Feuilles dans les Plantes, p. 26). If we consider the state of chemical 

 science at the time of Bonnet's researches, his error is in no wise surprising. 

 It is now known that the bubbles which Bonnet took to be air are nearly 

 pure oxygen which escapes as a by-product of assimilation. But from water 

 which has been boiled, all the carbonic acid essential to assimilation has been 

 expelled. 



