Volatile solids are those solids lost on ignition (heating to 500 degrees C.) They are useful because they give a rough approximation of the amount of organic matter present in the water sample. ‘‘Fixed solids’’ is the term applied to the residue of total, suspended, or dissolved solids after heating to dryness for a specified time at a specified temperature. The weight loss on ignition is called ‘‘volatile solids.’’ Source: American Public Health Association