THE CHEMICAL COMPOSITION OF LAKE WATERS 147 
to balance them, but may be supposed, for the sake of clearness, 
to go into solution as carbonates ; now carbonic is a very weak 
acid, so that these carbonates behave in solution in much the same 
way as uncombined bases or hydroxides, and give to the water a 
decided alkaline reaction, which would grow stronger (whereas alkalinity 
due to calcium carbonate would disappear) on evaporation. As a 
matter of fact, both calcium and the alkali metals are in solution as 
bicarbonate rather than normal carbonate, but it is less confusing for 
present purposes to think of only the latter as present. Alkaline 
waters, then, are those which may be regarded as containing a clear 
excess of sodium or potassium carbonate, and alkalinity can, in most 
cases, be detected from a statement of analysis if there is more carbonic 
acid reported than would be equivalent to the amount of lime present. 
In addition to these mineral derivatives, many organic substances, 
originating from the decay of animal and vegetable matter, find their 
way into inland waters. Little is known as to their nature, and a 
satisfactory quantitative statement of them is an impossibility ; more- 
over, they tend to be rapidly broken down, chemically and bacterially. 
Hence, generally speaking, solid organic solutes may be neglected by 
the physiographer. An exception, however, is to be made in the case 
of peaty waters. These contain humus, a degradation-product of 
vegetable matter, which is somewhat resistant to oxidative destruction, 
and is understood to impart an enhanced solubility to iron (in the 
ferrous state) and to silica. Dissolved in lake waters, humus has the 
property of inhibiting some forms of animal and plant life, and there 
is reason to believe that it aids greatly in the decomposition of 
minerals. It is a substance of high tinctorial power ; hence, notwith- 
standing the strikingly deep colour of many peaty waters, the organic 
matter dissolved in them is trifling in actual amount. Peaty waters 
are very common, indeed predominant, in the Scottish rivers and lakes. 
The matter held in solution by rivers varies both in quantity and 
quality with the geology and climate of the drainage area. In 
temperate climates the majority of river waters tend to a certain 
normal composition. They contain seldom more than 0*2 part per 
thousand of total solids, about one-half of which will consist of 
calcium carbonate held in solution by free carbonic acid ; sulphates 
come next in order of quantity, followed at some distance by chlorides, 
whilst magnesium and the alkali metals amount to only a few units 
per cent. This composition stands in glaring contrast to that of sea- 
salt, in which we have in descending order of percentages chlorine, 
sodium, sulphates, magnesium, calcium. Various abnormalities in 
the dissolved matter of river waters may be brought about by special 
local conditions of the drainage area : not only may the bulk of 
.solutes be considerably increased, but the proportion of certain single 
