290 PRINCIPLES OF BOTANY, ETC. 
very rapidly after a thunder storm with rain, as this 
last contains much carbonic acid gas. On this point, 
however, the opinions of philosophers are very con- 
tradictory, for many of them deny that a greater 
quantity of carbonic acid gas exists in rain during a 
thunder storm. That vegetables imbibe oxygen 
from the ground, seems to be confirmed by the ob- 
servation communicated to me by my often men- 
tioned friend Mr Humboldt, in a letter dated May ist, 
1798. ‘The following are his words: ‘If I took 
‘¢ 400 parts of atmospheric air, of a known quality, 
‘*¢ for instance, 28 parts of oxygen, and brought 
“‘ it im contact with mould, (4umus), or loam; 
‘¢ from 50 to 70 parts disappeared, but scareely 3 
«¢ or 5 parts of carbonic acid were given out, and 
«¢ the rest of the inclosed air contained hardly 12 
‘¢ or 14 parts of oxygen. ‘The ground therefore 
‘¢ imbibes oxygen in a solid form from the atmo- 
<¢ sphere. ‘The oxygen combines, I believe, with 
«© the hydrogen and carbon of the humus, and the 
“¢ product is an oxyd of hydrogen and carbon, 
*¢ which has not yet formed water or carbonic acid. 
«¢ This light compound is easily taken up by the 
‘¢ vegetable fibre.””? From this we might be able to 
explain, why oxygen, as we shall find presently, is 
indispensably necessary for the vegetable fibre and 
stimulates it to growth, (§ 251). Hence plants 
erow better in newly dug garden earth; and trees 
planted in holes, which were during the whole win- 
ter exposed to the influence of the open air, thrive 
better than when planted in long used earth, or in 
eround covered with turf, 
n 
La) 
The 
