ABSORPTION OF FOOD MATERIALS 129 
If we examine the food-stuffs described as being essential, 
we find that proteins contain carbon, hydrogen, orygen, 
nitrogen, sulphur, and perhaps phosphorus. Carbohydrates 
and fats contain only the first three of these elements. To 
make a destructive analysis of the plant, it must be dried 
at 110°-120° C. to drive off the water it contains, and it 
must then be carefully burnt, and the residue of the com- 
bustion collected. The volatile products given off can also 
be absorbed by appropriate methods, and their nature and 
amount ascertained. 
The incombustible residue, which is known as the ash, 
is composed of ‘several metals and some other elements, 
which vary in nature and amount in different, cases. An 
analysis of this ash will reveal the nature of its constituents, 
but it will not tell us in what condition or combination 
they existed in the living plant, on account of the various 
chemical changes which go on during.the combustion. 
The ash of plants when analysed is always found to 
contain the four metals potassium, magnesium, calcvum, 
and tron. These are not present in the metallic condition, 
but are in combination with various acids, forming nitrates, 
sulphates, chlorides, carbonates, phosphates, &c. 
The presence of these nitrates, sulphates, &c. must not 
lead us to infer that they have all been absorbed as such from 
the soil and retained unaltered in the plant. Part no doubt 
may be accounted for in this way, but much of the 
nitrogen, sulphur, and phosphorus which formed part of 
the substance of the plant enters into combination with 
the different metals and with oxygen during the combustion. 
Some of the carbon of the carbonates found may have had 
a similar origin. 
Besides the four metals mentioned, various plants may 
individually contain larger or smaller quantities of many 
other elements variously combined. We find sodiwm very 
generally present ; less frequently so, aluminium, copper, 
zine, manganese, silicon, bromine, 1odine and others. All 
of these are derived from compounds present in the soil, 
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