ABSORPTION OF FOOD, AND EXCRETION. 501 



now let us allow to each of these plants 4 Q' space of soil ; then upon the 

 old Hessian acre there would stand 10,000 plants, which in 120 days would 

 exhale 1,500,000 Ibs. of water. 



A cabbage * exhaled in twelve hours of the day 1 Ib. 6 oz. of water : 

 now if, according to Block, each plant occupies 5 D' of soil, and if we 

 reckon an inferior expenditure for the night, yet the plants on an acre 

 would exhale 1,200,000 Ibs. of water in 120 days. 



A dwarf pear-tree, according to Hales, exhaled in 10 hours of the day f 

 15 Ibs. of water. Allowing for each such tree 20 D' of soil, the trees of 

 an acre would exhale 3,600,000 Ibs. of water, and probably another third of 

 the quantity might be added for the grass between the trees, which would 

 make for the acre almost 5,000,000 Ibs. of water. 



An acre of 40,000 square feet, planted with hops, exhaled in 120 days 

 4,250,000 Ibs. of water through the hops alone. $ 



A square foot of soil covered with Ppa annua exhaled, according 

 to Schiibler , daily, on an average, during the summer, 33*12 cubic inches 

 of water : thus an acre of meadow land about 6,000,000 Ibs. 



199. The relation of plants to the gases of the atmosphere has 

 been least of all investigated or understood. Under this head the 

 following processes claim attention. 



1. A fluid that comes into contact with a gas, or that is only 

 separated from it by a membrane saturated with the same fluid, 

 absorbs, according to the specific nature of the gas and the fluids, a 

 definite measure of the gas, corresponding to the density of the 

 volume and the pressure under which the gas stands. Thus 100 vol. 

 water at 28" barom. and 15 C, will absorb 6*5 vol. oxygen, 4*2 vol. 

 nitrogen, 106*0 vol. carbonic acid; 100 vol. of sugar and water of 

 1,104 sp. gr. will absorb 72 vol. carbonic acid; 100 vol. gum water, 

 1,092 sp. gr., 75 vol. carbonic acid. 



2. When a fluid contains more of a gas than, according to the 

 nature of the gas, and the pressure under which it stands, the fluid 

 can hold in solution, the superfluous gas will escape. It will be 

 the same whether the fluid is free or is covered with a membrane 

 saturated with its own moisture. Since water can only hold in 

 solution 6*5 vol. per cent, of oxygen, and a solution of gum or sugar, 

 similar to the contents of the cells, can only hold 4 '6 vol. per cent, 

 of oxygen in solution, it follows that oxygen must escape from the 

 surface of the plant when the juices in the cells contain more than 

 their due proportion of this gas. 



3. When a fluid which is free, or covered merely by a membrane 

 saturated with itself, and is already mixed with as much of some 

 gas as it may be able to take up, comes into contact with another 

 gas, an interchange takes place between the two gases ; a part of 

 that which was first absorbed escapes, and a portion of the free gas 

 is taken up in its place, and this in proportion to the solubility of 

 the two gases. 



* Vegetable Statics, p. 7. \ Ibid. p. 19. 



f Ibid. p. 1 7. Meteorologie, 74. 



