CHAPTER IV. 



THE DIFFUSION OF GASES THROUGH SOLUTIONS AND OTHER LIQUIDS. 



36. Purpose. — In the earlier investigation (Chapters I and II) certain 

 questions were left outstanding. The first of these refers to the diameter 

 of the column of liquid through which diffusion takes place. It must be 

 decided, if possible, whether variations in the area of the column exceeding 

 its minimum area have as small an effect as was assumed. Again, when a 

 gas other than air is examined, the Cartesian diver must be charged in an 

 artificial atmosphere of the gas in question, so that there may be no access 

 of air. The artificial atmosphere must therefore be constantly renewed. 

 Even the partial exhaustions should be made so far as possible in the 

 absence of air. Again, endeavors are to be made to secure adequately 

 equable temperature conditions, but the facilities of the laboratory for 

 this purpose are meager. 



Finally, the effect of the solution of solids and liquids on the rate of 

 diffusion of the gas through water is an interesting question. What is to 

 be determined is the degree to which the physical pores of the pure liquid 

 are stopped up with different quantities and different kinds of solute. 

 Different solvents may also be taken in question. 



37. Apparatus. — Hence the apparatus with which the present experi- 

 ments are to be undertaken must, at least in part, 

 be of the type shown in fig. 4, Chapter II. They 

 were constructed in some variety, but the form 

 shown in the annexed fig. 16 was finally preferred. 

 Here vd is the Cartesian diver capable of rising in 

 the tube cmm, open below, closed above by the 

 stoppered thermometer / and kept full of water. 

 Thus the bulb of this thermometer serves addi 

 tionally as a stop for the diver on flotation. The 

 diver must fit very loosely in the tube, so that 

 there may be a minimum of viscous resistance to 

 its vertical motion. There should be at least 0.5 

 cm. clear space all around the diver. To prevent 

 it from sinking completely, a vertical sheet of 

 mica, e, slightly flexed so as to hold its position in 

 an axial plane by reason of its elasticity, was 

 eventually adopted in preference to wire gauze. 

 The wider and outer tube, A, is so chosen that the area r of the mouth of 

 the diver may be equal to that of the annular space without, as nearly as 

 possible, throughout. The gas in the space H above the free surface / of 

 the liquid may be changed and kept at any pressure by aid of the tubes 

 a and b. 



55 



Fig. 16. — Improved Car- 

 tesian diver with double 

 tube and influx pipe. 



