318 BROWN PAEASITES. [BOOK n. 



Olefiant gas, in the quantity of four cubic inches and a half, 

 and in the proportion of a hundredth part of the air, had no 

 effect whatever in twenty-four hours. (Turner and Christison.) 



The protoxide of nitrogen, or intoxicating gas, the last we 

 shall mention, is the least injurious of all those we have 

 tried ; indeed it appears hardly to injure vegetation at all. 

 Seventy-two cubic inches were placed with a mignonette 

 plant, in a jar of the capacity of 500 cubic inches, for forty- 

 eight hours ; but no perceptible change had taken place at 

 the end of that time. (Turner and Christison.) 



Goppert has also found that hydrocyanic acid in a gaseous 

 state is fatal to vegetation. Numerous experiments upon the 

 action of this and other substances deadly to plants are to be 

 found in this author's dissertation, De Acidi Hydrocyanici Vi 

 in Plant as: Vratislav. 1827. It is the custom of gardeners 

 to strew the crushed leaves of the Laurocerasus over the 

 floor of hothouses, in order to kill insects ; but they find that 

 any overdose of it destroys their plants as well. 



All the foregoing observations apply to plants whose natural 

 colour is green in consequence of the formation of chlorophyll 

 within their tissue. In those species, however, which are 

 called BROWN PARASITES, including FUNGI, the phenomena 

 of respiration are very different. Marcet has shown, from care- 

 fully conducted experiments, " that mushrooms, vegetating in 

 atmospheric air, produce on that air very different modifications 

 from those of green plants in analogous situations ; in fact, 

 that they vitiate the air promptly, either by absorbing its 

 oxygen to form carbonic acid at the expense of the carbon of 

 the vegetable, or by disengaging carbonic acid formed in 

 various ways. That the modifications which the atmosphere 

 experiences when in contact with growing mushrooms are 

 the same day and night. That if fresh mushrooms are placed 

 in an atmosphere of pure oxygen, a great part of that gas 

 disappears at the end of a few hours. One portion of the 

 oxygen which is absorbed combines with the carbon of the 

 plant to form carbonic acid ; whilst another part appears to 



