Over ninety years ago, on November 8, 1845, Michael Faraday investigated the magnetic properties of dried blood and made a note" Must try recent fluid blood." If he had determined the magnetic susceptibilities of arterial and venous blood, he would have found them to differ by a large amount (as much as twenty per cent for completely oxygenated and completely deoxygenated blood); this discovery without doubt would have excited much interest and would have influenced appreciably the course ofresearch on blood and hemoglobin. 1 Continuing our investigations of the magnetic properties and structure of hemoglobin and related substances, 2 we have found oxyhemoglobin and carbonmonoxyhemoglobin to contain no unpaired electrons, and ferrohemoglobin (hemoglobin itself) to contain four unpaired electrons per heme. The description of our experiments and the interpretation and discussion of the results are given below.

Note on Nomenclature.-The current nomenclature of hemoglobin and related sub-stances was formulated at a time when, precise information about the chemical composition and structure of the substances was not available. Now that some progress has been made in gathering this information, especially in regard to chemical composition, it is possible to revise the nomenclature in such a way as to make the names ofsubstances more descriptive than the older names, without introducing any radical changes. In formulating the following set of names we haveprofited by thecontinued advice of Dr. Alfred E. Mirsky.