Qualities themselves cannot be measured. Measurements are abstractions; qualities are by nature concrete and experiential. Whereas measurements and quantitative analyses, if done carefully, are always exact and can be expressed in exact mathematical terms, qualities can only be directly experienced and expressed in terms that somehow convey, or point to, this experience. This is a major reason why, wherever possible, qualities have been eliminated from the exact sciences.
In materials science, the specific weight, hardness, tensile strength, malleability, melting point, electrical conductivity, etc., of physical substances can be measured. All such measurements, however, are indications of the qualities of a substance. Chemical reactions reveal aspects of the innate qualities of substances. A distinctive attribute of iron or steel, for example, is that it oxidizes, i.e., undergoes a burning process, when it comes in contact with oxygen. This happens when a thin layer of iron oxide, or rust, forms on its surface. When an axe is sharpened on a grinding wheel, however, tiny fragments of steel fly into the air and immediately ignite as sparks. The iron in our red blood cells is essential for absorbing oxygen in our lungs and transferring it to our muscles. When the chemical reactions involved in these processes are reduced to an abstract quantitative formula—Fe2 + O2 → Fe2O3 + heat—our understanding of the processes gains a certain accuracy but loses its concrete, experiential, qualitative nature. Rust formation, sparks, and the activity of iron in our blood are qualitatively different manifestations of iron oxidation.