This book exhibits deep philosophical quandaries and intricacies of the historical development of science, namely the composition of water as H2O. Three main phases of development are critically re-examined, covering the historical period from the 1760s to the 1860s, the chemical revolution (through which water first became recognized as a compound, not an element), early electrochemistry (by which water?s compound nature was confirmed), and early atomic chemistry (in which water started out as HO and became H2O). This book offers a rare combination of philosophy, history and science in a bid to improve scientific knowledge through history and philosophy of science.