359] B. E. Livingston 161 



evaporating power of the air to include the first three of these, 

 since the gas phase is air in most studies and since these three 

 features are properties of this gas. The fourth feature de- 

 pends only indirectly, and to a comparatively slight degree, 

 upon the characteristics of the gas phase next to the evaporat- 

 ing surface. In climatological atmometry this is the effective 

 intensity of solar radiation, direct or indirect, which depends 

 upon the season, the time of day and the state of the sky, as 

 well as upon the nature of reflecting surfaces in the vicinity. 



Since evaporation is a process and not a state of matter, its 

 magnitude has to be expressed in terms of time rates. While 

 temperature, for instance (being a state of matter), may be 

 expressed in degrees on a thermometer scale, evaporation in- 

 tensities must be stated as the amount of water evaporated 

 in a unit of time. Atmospheric evaporating power refers 

 to the external surroundings of the evaporating surface (usu- 

 ally to the air space above it, about it, etc.) and it need not 

 specifically refer to the air itself, for if there were no air 

 present this space would still possess an evaporating power. 

 The evaporating power of the air over a surface is considered 

 as proportional to the reciprocal of the tendency of all the 

 conditions effective in the space over that surface to resist the 

 vaporization of water therefrom. 



There have been some who have objected to this expres- 

 sion, but they have not put forward another term. From a 

 long-continued attempt to acquire modes of expression by 

 which we may hope to deal with the dynamic aspect of plant 

 and animal environments an alternative expression has devel- 

 oped, which may be brought forward here. 



In all considerations of the dynamic relations between or- 

 ganisms and their surroundings we find it valuable to con- 

 sider the internal and the external complexes of influential 

 conditions separately, and each group of conditions may be 

 expressed, for any process we may have to deal with, as a 

 single value or index. We may thus speak of the index of 

 transpiring power, the index of environmental radiation, and 

 11 



