"Faraday[']s Gold Given to me himself after his lecture at the RI [Royal Institution.]"
This inscription is scratched directly onto the slide; the occasion was probably the lecture given on June 12th 1858, entitled 'On the relation of Gold to Light'. Faraday was famous for his captivating lectures featuring superbly choreographed demonstrations, and would have used a projecting microscope to show the slide.
"Faraday's Gold" is in fact a colloid, a material that is a mixture of two or more solids, liquids, or gases together. The particular type of colloid that Faraday was interested in was a dispersion of very fine gold particles suspended in a liquid. This type of preparation is known as a colloidal suspension or, as Faraday named it, a gold "sol". Other more familiar examples of sols include paint, mud, and toothpaste. The particles of gold present in the sol are on the nanometre scale - each is one billionth of a metre in length.