234 M. Marcet on the Changes which the Atmosphere 



recollection of which has escaped me, since his unpublished me- 

 moir was read some years ago to the Society. 



Desirous to learn, if I could, by a direct experiment, and 

 without removing the mushroom from the spot where it grew, 

 what were the changes which it produced upon the atmosphere 

 during its growth, I at first attempted to subject it to experi- 

 ment without removing it from the soil. With this intention, 

 having covered with a large glass vessel, a plant which was just 

 projecting from the earth, I surrounded its base with a kind of 

 fatty lute, to which I made the bell-glass adhere, taking at the 

 same time all possible precautions, to prevent all communication 

 between the air in the receiver and the external atmosphere. 

 After the lapse of two or three days, when the mushroom had 

 greatly increased in bulk, the air under the bell-glass was sub- 

 jected to analysis. This experiment was repeated many times, 

 and always with the same result, viz. that in no instance had 

 the confined air appeared to have undergone any sensible change, 

 with the exception of the presence, from time to time, of an ex- 

 tremely minute quantity of carbonic acid gas. 



The negative result of these experiments giving ground to 

 fear that the apparatus was imperfect^ and that it would be dif- 

 ficult, if not perhaps impossible, to prevent all communication, 

 even that which might take place through the soil, between the 

 external and included air, I found myself compelled to resort to 

 another method, which is perhaps less direct, viz. to confine my 

 examination to the atmospherical changes produced by mush- 

 rooms which have been raised from the soil, and which, of course, 

 were not wholly in their natural condition. This plan, which 

 has been successfully employed in the examination of the change 

 in the atmosphere produced by green plants, is not quite free 

 from objection, especially when we are working with vegetables 

 so subject to decomposition, as are a great number of the mush- 

 room tribe, and still more so, as it is often difficult to perceive the 

 exact moment when they cease to live, and when the process of 

 fermentation or spontaneous decomposition begins. To free 

 myself as much as possible from this source of error, I took the 

 following precautions. 1st, I took care to choose for my expe- 

 riments those mushrooms only which were most consistent and 

 tough, and the very nature of which must therefore have pre- 



