81.  If it appears to the Registrar that an invention for which a patent has been granted formed part of the state of the art by virtue only of section 14(3), he may on his own initiative by order revoke the patent, but shall not do so without giving the proprietor of the patent an opportunity of making any observation and of amending the specification of the patent so as to exclude any matter which formed part of the state of the art as aforesaid without contravening section 84.