1999 : WHAT IS THE MOST IMPORTANT INVENTION IN THE PAST TWO THOUSAND YEARS? [1]

In his work on the foundations of chemistry, it occurred to Antoine Lavoisier (and also, I suppose to Joseph Priestly) that the smell of a chemical was not necessarily a 'property' of that chemical, but a property of some related chemical that had the form of a gas, which therefore could reach the nose of the observer. Thus solid sulfur itself has no smell, but its gaseous relatives, sulfur dioxide and hydrogen sulfide have plenty of it. Perhaps this tiny insight was the key to the transformation of chemistry from a formerly incoherent field into the great science of the 19th and 20th centuries.