THE MOLECULAR FORCES IN PLANTS. 157 



fluence on vegetation, it is certainly not without interest to make 

 the following experiment : We procure two cubical zinc boxes, 

 6 to 8 cm. in diameter. In one we put air-dry soil ; the other 

 we fill with an equal quantity of soil which has been more or 

 less completely saturated with water. We now expose the two 

 boxes to direct sunlight for a few hours, preferably in a wooden 

 box and surrounded with a bad conductor of heat (e.g. cotton 

 wool), so that the sun-rays will act on the soil almost exclusively 

 from above. If we introduce a thermometer to a depth of 1 cm. 

 into the soil in both boxes, we shall readily make out that the 

 dry soil becomes far warmer than the moist, and I found in 

 observations conducted, however, in rather a different manner 

 that a dry peat soil exposed to direct sunlight for an hour and 

 a half had acquired a temperature of 34*3 C. at the surface, 

 while the temperature of the wet soil was only 29'5 C. The 

 lower temperature of the soil soaked with water, as compared 

 with that of the dry soil, is due to the higher specific heat of 

 water, and to the fact that heat is rendered latent by the evapora- 

 tion of water. Very watery soils we may well term cold a. mode 

 of expression which indeed agrees with the facts. We cannot 

 here investigate further the thermal properties of the soil. 2 



1 See Askenasy, Botan. Zeitung, 1875. 



2 For further information see Detmer, Lehrbuch d. Bodenkunde, 1876, p. 256. 



63. Differences of Electric Potential in Plants. 

 To detect differences of electric potential in plant structures 

 we require various delicate instruments, and especially an electro- 

 meter (frequently Lippmann's capillary electrometer is used), or 

 a galvanometer. I have satisfied myself that the latter instru- 

 ment is suitable for the purpose. For a list of firms supplying 

 mirror galvanometers of excellent quality, see the Appendix. 

 The apparatus must of course be set up vibration-free. We 

 further require an electric key and two unpolarisable electrodes. 

 The stands for these electrodes are so arranged that we can easily 

 put the electrodes in any position (see Fig. 53). Each electrode 

 consists of a glass tube a few cm. in length, into the lower end 

 of which prepared clay is kneaded. From the clay project the 

 ends (about J cm. long) of well- washed white cotton threads. 

 These threads, which must be washed by means of a vigorous 

 stream of water before the electrodes are used, and in place of 



