The Cambridge Social Union is to have its opening tomorrow night. About fifty classes will be offered this year, fifty classes will be offered this year, comprising all the subjects given in public schools. With the exception of such subjects as short hand and telegraphy, for which paid teachers are supplied, the classes are taught by men in the University, who are willing to give up an hour one evening each week for this work. Men are needed in every subject, and those willing to teach should send their names and the subjects they prefer to F. G. Cheney, 27 Holyoke street.