In these salts X = NO 2 and M = one atomic proportion of a monovalent metal, or the equivalent quantity of a divalent metal.
If an element or radical combined with one atom of hydrogen, it was termed monovalent; if with two (or with one atom of oxygen, which is equivalent to two atoms of hydrogen) it was divalent, and so on.
Equally well we may derive it from methane by replacing a hydrogen atom by the monovalent group CH 2 CH 31 named ethyl; hence propane may be considered as " ethylmethane."
Now suppose two of the attached atoms are replaced by one atom, then this atom must have two valencies directed to the central atom; and consequently, in the same unit of time, the central atom will collide once with each of the two monovalent atoms and twice with the divalent.
Infants under 12 months of age should receive a dose of monovalent (single antigen) measles vaccine before departure.