I thought the gram (a thousand of which make up a kilogram) is just the weight of a cubic centimeter of pure water.
The centimeter in turn was taken from a measure of the earth’s arc and then brought down to a usable dimension. This was done, I believe, at the behest of the French Revolution’s drive for objective standards.
Heck, that is what I learned in physics.
Yes, but water (with its various isotopes of hydrogen and oxygen) varies slightly in weight from sample to sample (as well as with temperature) and thus they wanted something more stable. Thus, the cylinder reference.
They could define it using the cubic decimeter of water again, using and H-1 as the references, 101325 Pa pressure (assuming said atmosphere is saturated with water) and 277.14 K (the point at which water is most dense) and I think you’d probably be able to get a stable enough standard for it. Of course, that’s just my speculation.