Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             chemical compounds
               Class hydrocarbons [CxHy] - compounds consisting of carbon and hydrogen only
                 Class acetylenes
                   Class alkynes [CnH2n-2; RC≡CR] - unsaturated acyclic hydrocarbons having one triple bond
                     acetylene [C2H2; HC≡CH]