ATMOMETRY AND THE ATMOMETER 65 



Alteration of Porous Evaporating Surfaces by Operation 



All evaporating surfaces are apt to alter more or less with the 

 progress of evaooration and with changes in the surroundings. 

 As has been said, the secondary wind effect is completely removed 

 in the case of non-flexible porous clay surfaces, while the removal 

 of water by animals is practically serious only with free water. 

 The accumulation of dust, etc., upon the saturated solid is per- 

 haps more serious in its effects than the same sort of accumu- 

 lation upon open water. Dust settling upon a pan of water 

 decreases the rate of water loss, but usually (in its primary effect, 

 at any rate) increases that rate from a paper or porous clay 

 atmometer. Practically to remove this trouble, however, in case 

 of porous porcelain, requires only that the surface be thoroughly 

 cleaned occasionally. The papers, not being readily cleaned, 

 should be replaced by new ones from time to time. 



Besides the primary effect of the accumulation of dust upon 

 saturated solid surfaces, a secondary and more serious one lies 

 in the incrustation of the latter by soluble salts, which may be 

 derived from dust. Such soluble salts, crystallizing out at points 

 of most intense evaporation, sometimes decrease the rate of water 

 loss and at other times appear to increase it. With paper, this 

 trouble can be removed only by frequent replacement; with por- 

 celain, cleansing of the exposed surface at short intervals usually 

 obviates the difficulty. Passing pure water through the porous 

 porcelain, in the direction of the usual movement of water, 

 largely removes soluble material, beyond the limit of its adsorp- 

 tion. 



It is absolutely essential, with all imbibed solids that only 

 very good distilled water be employed, to avoid accumulation of 

 salts as the water is evaporated. Impure water, especially with 

 calcium salts, clogs the pores and eventually almost glazes the 

 surface. This accumulation seems impossible of correction by 

 frequent surface cleansing, as has been shown by actual tests. 

 It can be removed, if not too. pronounced, by forcing water or 

 acid through the porcelain. 



