When the scene takes place in the past at the chemistry laboratory, the solution that Mr. Shatner says has to be heated is aqua blue. A common high school level solution that is of the same color is Copper(II) sulfate. Copper(II) sulfate cannot be used to find the melting point of any compound and it will most likely react before it acts as a solvent. Moreover, everything on the lab bench in in liquid phase. You cannot take a melting point of anything that is already in liquid phase.