 Dalton’s contemporaries raised other objections as well. Jacob Berzelius (1779–1848) argued that Daltonian atomism provided no explanation of chemical combination, how elements hold together to form compounds (Berzelius, 1815). Since his atoms are intrinsically unchanging, they can suffer no modification of the kind Aristotle thought necessary for combination to occur. Lacking anything like the modern idea of a molecule, Dalton was forced to explain chemical combination in terms of atomic packing. He endowed his atoms with atmospheres of caloric whose mutual repulsion was supposed to explain how atoms pack together efficiently. But few were persuaded by this idea, and what came later to be known as Daltonian atomism abandoned the idea of caloric shells altogether.