ABSORPTION OF FOOD MATERIALS 185 
dried at 110°-120° C. to drive off the water it contains, and 
it must then be carefully burnt, and the residue of the 
combustion 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 com- 
bination they existed in the living plant, on account of the 
various chemical changes which go on during the combustion. 
If we examine the food-stuffs described as being essential, 
we find that proteins contain carbon, hydrogen, oxygen, 
nitrogen, sulphur, and perhaps phosphorus. Carbohydrates 
and fats contain only the first three of these elements. 
The ash of plants when analysed is always found to 
contain the four metals, potassium, magnesium, calcium, 
and won. 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, zinc, 
manganese, silicon, bromine, and iodine ; others occur only 
exceptionally and in small traces. All of these are derived 
from compounds present in the soil, or the water with 
which they are in contact ; indeed, the composition 
