The constitution, properties, and derivatives of the so-called conjugated sulphurous and sulphuric acids have been made the subject of numerous searches, and have led, in the hands of Gerhardt and others, to very interesting results. I have examined at different times several members of the methylsulphuric, phenylsulphurous, nitrophenvlsulphous and other series, but have lately turned my attention to the analogous compounds of the phosphoric series. Some remarkable substances have been thus made, their constitution seeming to have a direct bearing upon the important question of the atomicity and equivalency of certain of the metallic elements. Several substances might have served as starting-points for these new inquiries. A curious compound, phenylphosphoric acid, C 6 H 5 H 2 PO 4 , was prepared; but its instability, and the oxidation to which it and its salts are liable, rendered it unsuited for the present purpose. I intend to describe in the present paper but one series of salts, formed from Pelouze’s ethylphosphoric acid, C 2 H 5 H 2 PO 4 . This compound, containing two atoms of easily replaceable hydrogen, appeared admirably adapted for the purpose in view. It is readily prepared by digesting (for 48 hours) finely crushed glacial phosphoric acid with alcohol of 90 percent.:— C 2 H 5 H } O + H PO 3 = C 2 H 3 H H } PO 4 .