Upon investigation it was found that neither the fats nor carbohydrates were the chief constituents of the active tissues. It was found, in fact, that the carbohydrates occurred in very small quantities only in the muscles, and that frequently the quantity of fat was likewise limited. Other substances, containing nitrogen and sulphur in addition to carbon, oxygen, and hydrogen, which were invariably present, and which are essential constituents of all tissues and cells, both in animals and in plants, must be necessary to all known life. To these substances, believed at the time to be the fundamental constituents of all tissues, Mulder gave the name Protein, from the Greek, meaning "to take first place." Later investigations proved that, while the proteins were essential to the building and repairing of the tissues and cells in general, they were not the only factors concerned in the work; that certain mineral salts were necessary constituents of all tissues, and must be present in order for any normal growth and development to occur.[4] ~Composition of Proteins.~--The average nitrogen content of common proteins is about 16%; that is, in 100 grams of protein there will be approximately 16 grams of nitrogen, or in 6.25 grams of protein there will be 1 gram of nitrogen. To estimate the protein content of a food when the percentage of nitrogen is known, it is necessary simply to multiply the percentage of nitrogen present, by the nitrogen factor, 6.25; or, if the amount of nitrogen is desired, when the percentage of protein is given, to divide by same factor.