359] B. E. Invingston 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 
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