THE EFFECT OF HUMIDITY 



183 



very small convection currents causing them to touch the 

 hymenium, to which, owing to their adhesiveness, they would 

 become firmly attached. After liberation from the fruit-body 



PotlfyVKA 



3 o 78 T /O // 



Distance beloio gills in centimetres. 



f3 /If. 



FIG. 63. Curves showing the rate of fall of spores at various distances below the 

 gills in a long chamber. 



the spores fall much more slowly. This enables the wind to 

 carry them much further than would be possible if no decrease 

 in velocity were to take place. 



NOTE. The gradual decrease in the rate of fall of spores in a chamber 

 saturated with water-vapour finds its readiest explanation in the supposition 

 that the spores gradually become smaller owing to loss of water. The assumption 

 that the spores lose water in a saturated atmosphere is in harmony with the 

 well-known fact that the vapour pressure of a drop of liquid depends on the 

 amount of curvature of its surface. The greater the curvature, the greater is 

 the vapour pressure. This is illustrated by the following experiment. If a 

 small piece of sulphur is placed in a tube which is then evacuated and sealed, 

 and if the tube is gently heated near the sulphur, the latter condenses on the 

 cool part of the tube in the form of a great number of very small drops of 

 different sizes. In the course of a day or so, one finds that the larger drops 

 have become still larger, and that they have a clear space round them. The 

 clear space gradually grows bigger. The explanation of this phenomenon is 

 that the smaller drops with the greater vapour pressure distil over into the 

 larger drops with the smaller vapour pressure. Similarly we may suppose that 

 the large free drop of water in the compressor cell (Fig. 58, w, p. 167) grows at 

 the expense of water lost by the minute falling spores owing to the great differ- 

 ence in the curvature of the surfaces. 



