ABSOKPTION OF FOOD MATEKIALS 135 



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 pJiosphorus. 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 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 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 sodium 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 



