502 ORGANOLOGY. 



4. Fluids that have chemical affinities with certain gases attract 

 them when they come into contact with them, either free or through 

 a membrane saturated with the same : thus, for example, volatile 

 oils absorb oxygen in order to form resin, &c. 



5. Every solid body condenses vapour and gases upon its sur- 

 face. It does so more largely if it is pulverulent, and still more so 

 if it is porous. Recently-charred wood has this property in the 

 most striking degree. One vol. of box-wood charcoal absorbs 90 

 vol. of ammoniacal gas, 55 vol. of sulphuretted hydrogen gas, 35 

 vol. of carbonic acid gas, 9*25 vol. of oxygen gas, 7*5 vol. of nitro- 

 gen gas. Humus comes nearest to charcoal in this respect. Water 

 expels a part of the absorbed gases. The entire parenchyma of a 

 plant being permeable, by means of the stomates communicating 

 with the intercellular passages and the atmosphere, makes the 

 plant resemble such porous bodies. 



The labours of experimentalists on these physical relations are far 

 from complete. The experiments of Alton *, Theod. de Saussure f , 

 Graham J, and Mitscherlich , only embrace particular points of the 

 enquiry. 



When we pass from the simple experiments with one gas and one 

 fluid, or one gas with another, to a mixture of gases and fluids, which is 

 the case in plants, the question becomes more complicated than even our 

 comprehension or experimental art can embrace. 



The most simple, and perhaps also the most important relation to be 

 observed here, is the escape of gases from a fluid which, while subject 

 to a certain pressure, held them in solution. 



The importance of this fact is seen, as it must regulate the formation 

 of the nutrient fluid, and the absorption of gases through solid porous 

 bodies, and especially through charcoal, humus, and clay. 



The possible influence of these laws upon the life of the plant will 

 be seen in what follows. 



200. We have now to place certain phenomena of vegetable 

 life, which have been more or less well observed, in connection 

 with the following laws : 



1. The germinating seed takes up a certain quantity of oxygen, 

 and gives off a considerable quantity of carbonic acid. 



2. After the period of germination, the plant exhales, during the 

 day and in the sun-light, oxygen from its green surfaces, and takes 

 in carbonic acid through the same parts. By night the process is 

 reversed ; carbonic acid is exhaled, and oxygen gas is absorbed. ' 



3. All parts of the plant, not having a green colour, as the bark 

 and the root, absorb oxygen gas and exhale carbonic acid gas. 



4. The filaments in flowers absorb, in a very short time, an 

 immense quantity of oxygen gas, and give out in exchange carbonic 

 acid gas. 



* Gilbert's Annal. vol. xxviii. p. 390. J Elements of Chemistry. 



t Ibid. vol. xxxvii. p. 163. Lehrbuch der Chemie, vol. i. 



