 By the first decades of the 19th century, the nascent sub-discipline of organic chemistry began identifying and synthesizing ever increasing numbers of compounds (Klein 2003). As indicated in section 2.2, it was during this period that the phenomenon of isomerism was recognized, and structural formulas were introduced to distinguish substances with the same compositional formula that differ in their macroscopic properties. Although some chemists thought structural formulas could be understood on a macroscopic basis, others sought to interpret them as representations of microscopic entities called molecules, corresponding to the smallest unit of a compound as an atom was held to be the smallest unit of an element.