In previous work, I have attempted to explore historically some of the cognitive techniques that a particular community of nineteenth-century European chemists learned to use, with unparalleled success, to explore a realm of nature that was in principle inaccessible to the direct reach of their senses, namely the world of atoms and molecules. I concluded that the routine use of mental imagery was an essential element of their intellectual toolkit. This paper seeks to extend that study in certain directions, in order better to understand the “middle ground” between sensory perceptions of material substances, and cognitive strategies of how best to understand them. In particular, the paper offers some ways to delineate, in the context of techniques characteristic of European chemists in the nineteenth century, the heuristic interactivity between the ephemeral world of mental images, and the more concrete entities that chemists created to represent those images, namely atomistic paper formulas and three-dimensional molecular models.