lO 



JOHN JOHNSTON AND L. H. ADAMS 



directly for the experiment; this insured that evaporation occurred, 

 not at the sides, but at the upper surface only. Likewise, evapora- 

 tion of water at the lateral surface of the sandstones was prevented 

 by coating them with wax. The observations are brought together 

 in Table I, to which we append a few results obtained by Bigelow 

 and Bartell/ who determined the air pressure required to just force 

 water out of the pores of the material. 



TABLE I 



Results of Atmometer Experiments with Various Materials 



Material 



Kind 



Thick- 

 ness of 

 Layer 

 Cm. 



Rela- 

 tive 

 Rate of 

 Rise 



Equivalent 



Height of 



Mercury 



Column 



Cm. 



Virtual 



Height of 



Water 



Column 



Supported 



Cm. 



Equivalent 



Pressure 

 Difference 



Atm. 



Calculated 

 Diameter 

 OF Pores 



"Alundum" 



Refractory clay 



Pressed magnesia . . . . 



Porcelain 



Portland cement .... 



Plaster of Paris 



Marble 



Diorite 



Sandstone (Daubree) . 



0.6 



5° 

 75 

 35 



IOC 



20 

 20 



IO-5 

 16.3 



18.5 

 59-6 



143 

 222 



810 



o. 14 



o. 21 



o. 24 



79 



Did not come to equilibrium; 

 pressure difference at 

 equilibrium presumably 

 greater than i atm. 



0.8 



Porcelain . 

 Porcelain . 



2. St 



14 



12 



3-8 



<3 



2.7t 



1 . 2 

 0.19 



* I M =0.001 mm. The wave-length of the D line is o. s m- 



j Direct observations of Bigelow and Bartell. 



% This result is calculated from Daubree's data as follows: The pressure of i .8 atm. recorded by him 

 corresponds, as he himself points out, to a temperature of 113° at the lower surface of the sandstone. At 

 'this temperature the surface tension of water is 54.3; the pressure exerted by the capillary curved surface 

 is 0.8 atm., corresponding to a column of water 830 cm. in height. Consequently from equation (4), 

 r = .000134 cm. oT D = 2 7 fj.. 



These results are not especially characteristic of the material; 

 they pertain merely to the particular samples which we happened 

 to use, and correspond to the widest pores in those samples. More- 

 over we have observed that the differences for layers of the same 

 material of different thickness are no greater than one would expect 

 from the probable variation in size of the widest pores. 



From what has gone before, it is obvious that the Daubree 

 experiment is in principle identical with the experiments just 



'Bigelow and Bartell, Jour. Am. Chem. Soc, XXXI (1909), 1194. Analogous 

 experiments have also been made by others. 



