SUBSTANCES PRODUCED BY PLANTS. 71 



forming cellular fibre, and the various compounds that render 

 plants " the sustenance and the banquet of animated nature!" 

 The agent by which these alterations are effected is light. 



103. When some fresh leaves of a healthy plant are placed 

 in a tumbler filled with water, and inverted in a saucer also 

 filled with water, and exposed to the sunshine, in a short 

 time bubbles of air are observed to ascend to the bottom of 

 the tumbler. This air when examined is found to possess 

 the properties which belong to pure oxygen gas (7), and is 

 believed to be produced by the decomposition of carbonic 

 acid dissolved in the water (27)^ for when boiled water is 

 employed in the experiment no oxygen is separated from 

 the leaves of the plant. It is only, however, in the pre- 

 sence of light that the leaves of plants possess the power of 

 decomposing carbonic acid (97). During the night season 

 their roots continue to absorb the moisture from the soil and 

 to evaporate it from their leaves, but the carbonic acid which 

 they receive escapes without undergoing decomposition ; in- 

 stead of giving off oxygen they condense it in their structure, 

 though numerous experiments prove that the amount of that 

 gas which vegetables liberate during the day considerably 

 exceeds that which they abstract from the air at night. The 

 following beautiful experiment, tried by the illustrious Davy, 

 will show you the effect which plants exercise upon the air 

 that smTOunds thenu A piece of turf four inches square, 

 clothed with grass, was placed in a porcelain dish which swam 

 on the surface of a larger basin containing water in which 

 carbonic acid was dissolved; over the dish containing the 

 grass a glass vessel of the capacity of 230 cubic inches was 

 inverted, so as to cut off all communication with the external 

 air. The apparatus was exposed in an open place and so 

 arranged that fresh water containing carbonic acid could be 

 supplied occasionally to the water in the larger dish. After 

 eight days it was found that the air in the glass vessel had 

 increased by at least 30 cubic inches of gas which, when 

 examined, was found to contain four per cent more oxygen 

 than the air of the atmosphere. 



105. It will be recollected that it was stated that the 

 poisonous gas, carbonic acid, is from innumerable sources 

 continually escaping into the atmosphere (26). The air that 

 wc exhale from our lungs contains a quantity of that gas, 

 which, if existing in the same proportion in the air that we 



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