Well, don't stick a lighted match into the top layer, or near it.
But my question really would be this...why on Earth would anyone be using Ether who did not know its detailed characteristics, especially its vapor characteristics.
I personally very keen on potential hazards being well known before students even associate with chemicals.
when two layers form during an ether/water extraction, what would be an easy, convenient way to tell which layer is which if the densities were not available?
2 answers
Bob Pursley brings up an important point. One solution is to add water, dropwise, and see if it floats or sinks to the bottom.