Based on the law of definite composition, Dalton proposed that one atom of carbon combines with two atoms of oxygen to always produce CO2.

Question

Similarly,he proposed that two atoms of hydrogen combine with one atom of oxygen to give a molecule of H2O.

Finally,Dalton reasons that two atoms of hydrogen in water could substitue for each of the oxygen atoms in carbon dioxide. This would result in a molecule with one carbon and four hydrogen atoms, that is CH4.

Question:Please help me to understand how he arrives at 'H4'in the formula CH4 (4 hydrogen atoms)when he's substituting two atoms of hydrogen from H20.

Thank You.