. What I found out was – "The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases is always the same for equal volumes, or always proportional to the volumes. " The term used today is Avogadro's Hypothesis: Equal volumes of gas, under equal conditions of temperature and pressure, contain equal number of chemical entities. The entities are mostly molecules but, in the case of monoatomic gases such as Ne or Ar, the entity would be the atom. Less well-known is that Avogadro was also responsible for the concept, to use the modern term, of a diatomic gas. This is the discussion of it in his essay linked above: "On reviewing the various compound gases most mainly known, I only find examples of duplication of the volume relatively to the volume of that one of the constituents which combines with one or more volumes of the other. We have already seen this for water. In the same way, we know that the volume of ammonia gas is twice that of the nitrogen which enters into it. M. Gay-Lussac has also shown that the volume of nitrous oxide is equal to that of the nitrogen which forms part of it, and consequently is twice that of the oxygen. Finally, nitrous gas, which contains equal volumes of nitrogen and oxygen, has a volume equal to the sum of the two constituent gases, that is to say, double that of each of them. Thus in all these cases there must be a division of the molecule into two; but it is possible that in other cases the division might be into four, eight, &c. "