Around 1640 a Flemish physician named Jan Baptist van Helmont planted a five-pound willow in two hundred pounds of oven-dried earth, covered the pot to keep the dust out, and gave it nothing but water for five years. The willow came out weighing about 169 pounds. He dried and weighed the earth again and found it had lost only about two ounces. So the tree had not been made of soil, which was a good result, and he concluded it had been made of water, which was mostly wrong. Most of a tree’s dry weight is carbon, and the carbon comes out of the air, taken in through the leaves as carbon dioxide. The answer was the one thing in his experiment he had no way to weigh. When a log burns, most of it goes back the way it came: up, as the gas it was built from, leaving a little ash, the part that really did come from the ground.