PHYSICO-CHEMICAL INTRODUCTION 19 



length of the diffusion column. The same holds good for the diffusion 

 of water vapour into strong sulphuric acid. If a diaphragm be placed 

 at the free end of the diffusion column, it is found that the amount of 

 gas which diffuses is proportional to the diameter of the aperture, and 

 not to its area. Another remarkable fact is that, if in the diffusion 

 tube a diaphragm be placed containing many minute perforations, 

 the diffusion flow is checked but little or nil, each aperture in a multi- 

 perforate diaphragm acting independently of the others. 



It has been found that the assimilation of CO 2 by the leaf of a plant 

 is due to a similar process of diffusion. If the stomata or pores of the 

 leaf be blocked, no assimilation of CO 2 takes place. The amount of 

 C0 2 entering the leaf depends on the concentration of CO 2 inside the 

 leaf and the linear dimensions of the stomata. It is found experi- 

 mentally with leaves of Helianthus that the amount of C0 2 taken in is 

 but a fraction of the amount calculated for the leaf as a multiperforate 

 septum. This is because the gas, having entered the stomata, has 

 to pass into solution in the leaf fluid a relatively slow process as 

 compared with gaseous diffusion. 



This brings us to the next point the passage of gases into solution 

 in water. The power of water to dissolve a gas varies markedly with 

 the nature of the gas ; the solubility of the same gas in the water also 

 varies with the temperature and the pressure at which the absorption 

 is taking place, and with the concentration of other substances in 

 solution in the water. In regard to temperature, the solubility di- 

 minishes as the temperature rises. The solution of the gases of the 

 atmosphere in fat, on the other hand , is independent of the temperature. 

 In regard to pressure, it is found that the quantity of gas dissolved, 

 either by weight or volume, at normal temperature and pressure 

 (N.T.P.) in a given volume of water at a given temperature is directly 

 proportional to the pressure; thus, by doubling the pressure twice as 

 much gas passes into solution (Henry's law). 



The definite relationship between the gas and the absorbing fluid 

 is sometimes termed the " absorption coefficient " that is, the volume 

 of gas reduced to N.T.P. which is taken up by that volume of the 

 fluid under the normal pressure of one atmosphere. In the following 

 table the absorption coefficients are given for oxygen, nitrogen, and 

 C0 2 , in water. 



Temp. Oxygen. Nitrogen. Carbon Dioxide. 



0-0489 0-0239 1-713 



10 0-0380 0-0190 1-194 



20 0-0310 0-0164 0-878 



30 0-0262 0-0138 0-665 



40 0-0231 0-0118 0-520 



The table shows that the absorption coefficient of nitrogen in water 

 at 30 is 0138 that is to say, one c.c. of water at 30 C. absorbs at 

 atmospheric pressure 0-0138 c.c. of nitrogen measured at N.T.P. 

 It also shows that oxygen is more soluble than nitrogen, and C0 2 more 

 soluble than either. Fat dissolves between five and six times as much 

 of these gases as water. 



