Complex sentence: He also put the silver nitrate in the oven which had no effect on it's color, proving definitively that heat wasn't what facilitated the transformation as people had long suspected, rather, it was the light.
Split into simpler sentences: He also put the silver nitrate in an oven which had no effect on it's color. This proved definitively that heat wasn't what facilitated the transformation as people had long suspected, rather it was the light.