1] INTRODUCTION 37 
It will be seen that, whilst undertaken with a different 
object, the results of the observations on the upper atmosphere 
by Professor Schuster and his staff have several important 
bearings on vegetation, though the significance of these cannot 
be correctly appraised until the observations have been con- 
tinued for a considerable number of years. Though admittedly 
incomplete, they still help to furnish clues as to the different 
conditions under which vegetation lives at the different altitudes. 
NOTE ON THE USE OF THE WORDS “ACIDIC” AND “Basic.” Geologists 
have long distinguished between “acidic” and “basic” igneous rocks. 
The former are poor, the latter rich in soluble mineral salts. From 
the standpoint of the distribution of vegetation, the amount of soluble 
mineral salts in the soil is a fundamental matter. Acidic rocks, soils, 
peats, and waters are those which contain only a small amount of soluble 
mineral salts ; and basic rocks, soils, peats, and waters are those which 
contain a large amount. In time, when more analyses have been made, 
it may be possible to express this relationship in quantitative terms ; 
but, in the present state of knowledge, the matter can only be expressed 
in a general way. Acidic waters are neutral to acid in reaction, basie 
waters neutral to alkaline. 
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