Scientific discourse is characterized by multi-semiotic construction and the resultant semantic expansions. To date, there remains a lack of analytical methods to explicate the multiplicative nature of meaning. Drawing on the theories of systemic functional linguistics, this article examines the meaning-making processes across language and symbolism in scientific texts, and proposes a social semiotic approach to demonstrate how meaning is multiplied through the use of metaphor. This study also compares the semantic features of three closely related cross-modal mechanisms--inter-semiotic correspondence, inter-semiotic trans-categorization and inter-semiotic metaphor--and investigates their different functionalities in the historical evolution of modern chemistry. It is argued that the semiotic transition from language to symbolism expands the meaning potential of chemical discourse and re-construes everyday experience as scientific knowledge. (Contains 6 figures.)