ON THE PHYSIOLOGY OP PLANTS. 101 



And now I shall proceed with such things as tend to give life and 

 health to plants ; namely, atmospheric air, carhonic acid gas, and 

 humic acid, not forgetting one principal support, water, which is 

 composed of two gases, oxygen and hydrogen, which it appears plants 

 have the power of decomposing. The more water is mixed with the 

 air when given to plants, the more beneficial it is to them, because it 

 is by that means enabled to obtain large portions of those gases 

 necessary to their life ; hence the smaller the holes in the rose of a 

 watering-pot are, the finer the water falls on the plants, and the more 

 atmospheric air it is enabled to obtain ; it is this alone which makes 

 river-water, when running a long course, better for plants than that 

 which has been motionless for a long time, such as ponds and lakes, 

 whose waters only contain a small portion of air; but this does not 

 apply so well to actually stagnant water found in ditches, &c, whose 

 deficiency of atmospheric air is made up by the greater portion of 

 carbonic acid derived from decaying animal and vegetable substances 

 found geneially in such places. 



If plants could live without a constant supply of water, then rain 

 would, even during the summer months, be a sufficient support to 

 them, as its passing through the air causes it to give more nourish- 

 ment than we can ever give by the use of the watering-pot or engine. 

 The air itself always contains more or less water in the shape of an 

 invisible vapour, which is always in proportion to the temperature; 

 the warmer the air, the more moisture it contains. 



Carbonic acid gas is another important thing to the life of plants, 

 and is found in great abundance in all animal and vegetable sub- 

 stances in a state of putrefaction, which, if mixed with the soil, will 

 be taken up by the spongelets of plants, and is passed into the main 

 body ; otherwise what would be the use of applying dung to plants, 

 which of itself contains a great deal of carbonic acid gas ? Carbonic 

 acid is heavier than air, and, consequently, when any rain falls, from 

 the carbon being close to the earth, a great quantity is washed into 

 the soil. 



To describe what any of the gases are would be beyond my pur- 

 pose, and the limits your work could give ; so that I shall not 

 attempt to enter more minutely upon them, and, therefore, proceed 

 to the only acid of itself beneficial to plants, viz., lnimic acid, which 

 may be found in great abundance in the water which drains from u 



