Often the concentration of the component in the agar gel should be substantially less concentrated ( perhaps an order of magnitude or more ) than the one placed on top of the gel.
12.
Also I'm not sure this would even work as StuRat describes as the bacteria would have nothing to eat ( unless they eat the actual gelatin ) Most bacteria are grown on agar gel that has had some sort of nutrient added to it.
13.
If a hot solution of agar gel, also containing a dilute solution of potassium dichromate is poured in a test-tube, and after the gel solidifies, a more concentrated solution of silver nitrate is poured on top of the gel, the silver nitrate will begin to diffuse into the gel.