98 Mr. Burnett on the Development 



taper, did not in any instance again support combustion, 

 although exposed for the usual time to the sun's rays ; and 

 furthermore those which were placed in ordinary air depraved 

 it and formed abundance of carbonic acid even in the sun- 

 shine. In subsequent experiments healthy plants exposed to 

 brilliant gas light for four or five hours purified depraved air, 

 though less decidedly than daylight. 



2dly. Are these changes produced in atmospheric air by 

 plants dependent on their vegetation, i. e. are they effected by 

 the influence of vegetable life, or are they the effects of mere 

 mechanical structure, such as their filamentary covering and the 

 numerous points and angles that plants afford, by which the 

 solar light may be assisted in decomposing and separating the 

 constituents of the air or water ? To ascertain these points, 

 which have frequently been mooted, numerous experiments 

 were instituted, in which water with sand, hanks of cotton, 

 wool, &c. &c., were inclosed as in the former experiments, the 

 oxygen being converted into carbonic acid by combustion, and 

 in some the carbonic acid being thrown down by lime water, 

 but in no instance, whether with or without the presence of this 

 gas, was oxygen reproduced, so as to re-allow the combustion 

 of a taper. Similar experiments were tried with dead plants 

 and with similar results. Thus it is evident, that light, assisted 

 only by the filamentary forms of lifeless matter, is unable to 

 effect those changes which the living plant so quickly and so 

 certainly induces ; nay, further experiments shew that the de- 

 caying leaves of plants and newly turned up mould deprave 

 the air in which they may be confined. 



3dly. Are these changes, which living plants effect, the varied 

 results of the same, or the unvarying results of different 

 functions ? 



Experiments on unhealthy plants shew that they deprave 

 the air, both in the sunshine and the shade ; if the leaves of 

 healthy plants be crushed so as to interfere with the due per- 

 formance of their functions, they deteriorate the atmosphere 

 likewise. Healthy plants inclosed in vessels of carbonic acid 

 are speedily destroyed, whether kept in the light or not. 

 Plants made to grow in the dark, although they increase in 

 size, do not augment the absolute volume or weight of their 



