He observed that when this reduced solution is shaken with air, the colour and the absorption spectrum return to their original states. He identified the red and purple forms of the blood pigment with its state in arterial and venous blood, which he called scarlet cruorine and purple cruorine respectively. At that time Germany was the center of the chemical world. Chemists like to work with pure substances.
Crystallization often indicates that a pure substance has been obtained.
Hemoglobin, however, was not the typical organic molecule or inorganic salt that crystallizes. Its size dwarfed anything previously known or suspected. It attracted attention. In 1871 Wilhelm Preyer published Die Blutkrystalle , a book which illustrated crystalline hemoglobins from more than 40 species. This normally implies different chemical compounds.