▁is ▁sometimes ▁called ▁Dal ton ' s ▁Law , ▁named ▁after ▁John ▁Dal ton , ▁the ▁chem ist ▁who ▁first ▁expressed ▁it . ▁ ▁For ▁example , ▁Dal ton ▁knew ▁that ▁the ▁element ▁carbon ▁forms ▁two ▁ox ides ▁by ▁combining ▁with ▁o xygen ▁in ▁different ▁proport ions . ▁A ▁fixed ▁mass ▁of ▁carbon , ▁say ▁ 1 0 0   gram s , ▁may ▁react ▁with ▁ 1 3 3   gram s ▁of ▁o xygen ▁to ▁produce ▁one ▁ox ide , ▁or ▁with ▁ 2 6 6   gram s ▁of ▁o xygen ▁to ▁produce ▁the ▁other . ▁The ▁ratio ▁of ▁the ▁masses ▁of ▁o xygen ▁that ▁can ▁react ▁with ▁ 1 0 0   gram s ▁of ▁carbon ▁is ▁ 2 6 6 : 1 3 3 ▁= ▁ 2 : 1 , ▁a ▁ratio ▁of ▁small ▁whole ▁numbers . ▁Dal ton ▁interpreted ▁this ▁result ▁in ▁his ▁atomic ▁theory ▁by ▁propos ing ▁( correct ly ▁in ▁this ▁case ) ▁that ▁the ▁two ▁ox ides ▁have ▁one ▁and ▁two ▁o xygen ▁atoms ▁respectively ▁for ▁each ▁carbon ▁atom . ▁In ▁modern ▁notation ▁the ▁first ▁is ▁CO ▁( car bon ▁mon ox ide ) ▁and ▁the ▁second ▁is ▁CO 2 ▁( car bon ▁dio x ide ). ▁ ▁John ▁Dal ton ▁first ▁expressed ▁this ▁observation ▁in ▁ 1 8 0 4 . ▁A ▁few ▁years ▁previously , ▁the ▁French ▁chem ist ▁Joseph ▁Pr oust ▁had ▁proposed ▁the ▁law ▁of ▁definite ▁proport ions , ▁which ▁expressed ▁that ▁the ▁elements ▁combined ▁to ▁form ▁comp ounds ▁in