. . . "These include, in particular, thermal polymerization inhibitors, e.g. hydroquinone, hydroquinone derivatives, 2,6-di-tert.-butyl-p-cresol, nitrophenols, N-nitrosamines, such as N-nitrosodiphenylamine, or the salts, in particular the alkali metal or aluminum salts, of N-nitrosocyclohexylhydroxylamine." . .