Atomism and the Reasoning by a Non-Classical Logic

Often, in the original scientific writings, a double negated statement (DNS) is not equivalent to his corresponding positive one; that means the inferring law 'non non A -> A' does not apply. Recent studies recognized in the failure of this logical law the borderline between classical and non-classical logics. Original writings by classical chemists dealing with the problem of atomism are particularly characterized by the occurrences of DNSs. An historical case, Avogadro's contribution to atomism (i.e. the well-known hypothesis about the constitution of gases), is here analyzed in such terms. It turns out that, in order to support his ideas, Avogadro suggested several ad absurdum proofs, indeed a way of reasoning typically linked to the use of DNSs.