. . . "ps, and X1 and X2 both represent --O(CH2)n SO3 M in which M is a hydrogen atom, an alkali metal ion or NH4 +, and n is an integer of 2-4, or either of X1 and X2 represents --O(CH2)n SO3 M in which M and n are the same meanings as given above and the other represents a hydrogen atom." . .