220 CONSERVATIVE ORGANS. [LECT. V. 



ring which the thermometer had varied from 59 

 to 70, the root was cut from the plant, and re- 

 moved from the jar: scarcely any diminution 

 of the air was perceptible ; but lime-water became 

 instantly milky on agitating- it in the air of the 

 jar, thereby demonstrating the presence of car- 

 bonic acid gas. The quantity of the gas was not 

 ascertained in this instance. 



The effect of oxygen gas, as a healthy sti- 

 mulus to roots, was ascertained by Mr. Daniel 

 Hill. He applied it to the roots of a plant of the 

 Horseshoe Geranium, Pelargonium zonale, in a 

 sickly state, and found that the plant soon revived, 

 and grew with great vigour ; and phials of oxygen 

 gas inverted in water glasses, in which Hyacinth 

 bulbs were placed, rapidly advanced the growth 

 , of the plants. 



Soils contain CARBONIC ACID GAS also, inde- 

 pendent of that which is formed by the vitiation of 

 the atmospherical air. This is produced from the 

 fermentation or decomposition of vegetable sub- 

 stances ; and when it is not too abundant, it is 

 useful by loosening the soil, by its expansion when 

 first liberated: too much of it, however, checks 

 germination and impedes the growth of plants. 



When the vegetable matter which the soil con- 

 tains putrefies, carbonated hydrogen gas is given 

 out ; and this is always present to a certain extent. 

 It can be useful, like the former air, by the me- 



