Complex sentence: Gray's caveat describes a liquid transmitter that entails two electrodes that are nearly, but not quite touching, both electrodes submerged in the liquid, which had to be contained in an insulated vessel such as that made of glass, as stated in the caveat.
Split into simpler sentences: Gray's caveat describes a liquid transmitter that entails two electrodes that are nearly, but not quite, touching. Both electrodes are submerged in the liquid, which had to be contained in an insulated vessel such as one of glass, as stated in the caveat.