Definition of "cephalick"

I was at the Mathematical School, where the Maſter taught his Pupils after a Method ſcarce imaginable to us in Europe. The Propoſition and Demonſtration were fairly written on a thin Wafer, with Ink compoſed of a Cephalick Tincture. This the Student was to ſwallow upon a faſting Stomach, and for three days following eat nothing but Bread and Water. As the Wafer digeſted, the Tincture mounted to his Brain, bearing the Propoſition along with it.

1726 October 28, [Jonathan Swift], “The Author Permitted to See the Grand Academy of Lagado. […]”, in Travels into Several Remote Nations of the World. […] [Gulliver’s Travels], volume II, London: […] Benj[amin] Motte, […], part III (A Voyage to Laputa, Balnibarbi, Glubbdubdribb, Luggnagg, and Japan), page 78