Rocks of course must be mineral. It used to be a vegetable until it was compressed over time. This was clearly easy to define. All substances could be simply looked at, and determined whether it was a solid or not. Ice is a solid.
Water is a liquid.
Steam would be considered a gas. Benjamin Franklin concluded from his many experiments, that electricity had certain properties, such as pressure, current. He further concluded that it s movement was predictable, like the pouring of water from one glass to another. From his conclusions, it was conventionally assumed that while we might not be able to SEE electricity at work, we could see it s effects. And electricity, like unto water, acted like a fluid in every sense of the word.