The ash or mineral part of plants or animals has occupied a minor place in the discussions pertaining to the principles and problems of animal nutrition. Much is said and written about the carbon compounds of living organisms, but the compounds of the mineral world, in their relation to foods and to the processes of growth, are generally passed by with brief comment, much less than would be profitable. It is certainly desirable to gain a clear understanding of the combinations, distribution, and functions of these bodies. Their importance as necessary constituents of foods and animals is no less than pertains to the carbon compounds, although their scientific and commercial prominence as related to animal nutrition is much less.

As previously stated, the mineral portion of a plant or animal is measured by the ash or residue after combustion, the principal ingredients of which are the following:

Acids

Hydrochloric acid . . HC1

Sulfuric acid . . . H2SO4

Phosphoric acid . . H6P2O8

Silicic acid .... Si02

Carbonic acid . . . CO2

Potash...... K20

Soda...... Na2O

Lime...... CaO

Magnesia..... MgO

Iron oxid..... Fe2O3

Other mineral compounds are found in the ash from various forms of vegetable life, but those mentioned are all that we need to discuss at length.

33. Combination Of Ash Elements

The acids and bases do not exist in the ash as shown, but they are united to form salts, and so we have the chlorides, sulfates, phosphates, and carbonates of potassium, sodium, calcium, and magnesium. These are nearly all familiar objects in common life, as, for instance, sodium chloride (common salt), potassium chloride (the muriate of potash of the market), potassium sulfate (the sulfate of potash of the market), calcium sulfate (of which gypsum or land plaster is composed), calcium phosphate (burned bone is chiefly this compound), potassium phosphate (a compound of phosphoric acid and potash found chiefly at the druggist's), and calcium carbonate (limestone).

It should be remembered that the compounds in the ash are not necessarily those of the plant or animal. During the process of ignition, organic compounds are broken up, the acid and basic elements of which enter into other combinations in the salts of the ash. Much of the lime in the ash is in union with carbonic acid, which in the plant may have been associated with vegetable acids, such as oxalic and tartaric, and part of the sulfur and phosphorus of the ash comes from the nitrogen (protein) compounds.

These salts differ greatly in their properties. Some are soluble in water, others are not. To the former class belong all the chlorides, and the potassium and sodium sulfates and phosphates. The normal phosphates of calcium and magnesium are insoluble in water, but s&luble in various acids. These facts are important in showing what salts may be found in the plant and animal juices, and what effect leaching with water or other solvents might have upon the inorganic portion of human foods.