ABSORPTION OF FOOD MATERIALS 143 



If we examine the food-stuffs described as being essential, 

 we find that proteids contain carbon, hydrogen, oxygen, 

 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, calcium, 

 and iron. 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 cf 

 the substance of the plant entered 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 sodium very 

 generally present ; less frequently so, aluminium, copper, 

 zinc, manganese, silicon, bromine, iodine and others. All 

 of these are derived from compounds present in the soil, 



