The practice of science as we know it developed incrementally, mostly from one natural philosopher building on the work of a predecessor and disproving their conclusions with further experiments (you may recognise this same drive in researchers today). And it paid dividends. Not only did the extent of scientific knowledge increase, but so did the rigour of experiments.
Before anyone really understood which materials were fundamental and how things could turn from one substance to another, there was alchemy. Alchemy, whilst meaning “the chemistry” is about as unlike chemistry today as it can be: chemistry starts from the elements of the periodic table and the basis of conservation of mass; the principle of alchemy is that materials are transmutable (which we can't really achieve without nuclear decay). And so hard water, when boiled, left behind limescale, so rock must be made from water. And until people started tinkering around with experiments, it seemed a pretty solid (ahem) theory.
Alchemist Van Helmont went further. He determined that plants were also made of pure water (although after he was arrested in 1634 for “violating God's law” by studying nature, he declined to publish anything whilst alive; strange choice, that). His conclusion is wrong. We know that carbon dioxide plays a part, that plants generate their own food via photosynthesis, but Van Helmont didn't know this, and whilst he was wrong, he was also very thorough.
In a five year experiment, Van Helmont carefully watered a 5lb willow tree stood in a pot containing 200lb of dried soil, before finally reweighing both tree and soil. The soil weight was unchanged, the willow had gained 164lb.
QED. Trees are made of water.
At least, willow trees.
Today, Van Helmont has been dubbed a chemistry pioneer and a founder of pneumatic chemistry. Even though he was wrong.
But the idea of gases evolved, and became of importance to fuel later investigations into the transmutability of water. In 1770, Lavoisier undertook an even more rigorous experiment, weighing a sealed glass flask of distilled water and boiling it off. Through this experiment he was able to show that the evolved “earth” came from the soft glass of the flask, not the water nor air.
Water was, in fact, neither soil nor tree. It was water.