Percy Julian and Josef Pikl used the fact that melting point-the temperature at which a substance changes from a solid to a liquid-is a characteristic property of a substance to prove that the British chemist Robert Robinson could not have succeeded in synthesizing physostigmine. What do we mean by a " characteristic property"? Basically it means that every substance has a very specific melting point. For example, the melting point of ice (solid water) is 0 degreesC. It's not -1 degreesC or +1 degreesC, but always 0 degreesC. If you have a substance that looks like ice but melts at -4 degreesC, it's not ice.