20 Professor Macaire on the 



" I composed,'* lie says, '* with carbonic acid gas and com- 

 mon air, in which the phosphoric eudiometer indicated jVo o^ 

 oxygen gas, an artificial atmosphere, which occupied 290 

 cubic centimetres. Lime water added to it indicated 7i hun- 

 dredths of carbonic acid gas. This aeriform mixture was en- 

 closed in a receiver shut up with mercury, moistened or 

 covered with a very thin layer of water to prevent the con- 

 tact of this metal with the air which surrounded the plants ; 

 for I have ascertained that this contact, as the Dutch che- 

 mists had intimated, is hurtful to vegetation in prolonged ex- 

 periments. 



*' I introduced into this receiver seven plants of the peri- 

 winkle {pervenche), each two decimetres in height ; taken to- 

 gether, they displaced ten cubic centimetres : their roots were 

 plunged into a separate vase containing 15 cubic cent, of 

 water. The quantity of this liquid under the receiver was 

 insufficient to absorb a sensible quantity of acid gas, particu- 

 larly at the temperature of the place, which was never less than 

 17° E. (70° F.) 



" This apparatus was exposed for six consecutive days, 

 from five o'clock in the morning to eleven, to the direct rays 

 of the sun, weakened, however, when they became too intense. 

 On the seventh day I withdrew the plants, which had not 

 undergone the least alteration. Their atmosphere (every cor- 

 rection being made) had not changed in volume, at least as far 

 as can be judged in a receiver of 1-3 decimetre in diameter, 

 when a difference of 20 cubic cent, is almost inappreciable ; 

 but the error could not be beyond that. 



" Lime water no longer indicated in it carbonic acid gas ; 

 the eudiometer announced 24J^ hundredths of oxygen gas. I 

 prepared a similar apparatus with pure atmospheric air, and 

 the same number of plants, and submitted them to the same 

 exposure : it underwent no change, either in purity or volume. 



" It appeared from the eudiometrical observations stated be- 

 low, that the mixture of common air and acid gas contained, 

 before the experiment, 



4199 cubic cent, of azotic gas. 



1116 of oxygen gas. 



431 of carbonic acid gas. 



5740, or 290 cubic inches. 



