. . . . "To exemplify the compound of the general formula R1 m Si(OR2)n X4-m-n used in the invention, wherein R1 and R2 are each a hydrocarbon radical having 1 to 24, preferably 1 to 18, carbon atoms, a hydrocarbon radical containing oxygen, sulfur or nitrogen, or hydrogen, X is a halogen atom, m is 0???m<4 and n is 0???n???4, provided 0???m+n???4, mention may be made of the following: silicon tetrachlorid" . . .