Question

Briefly discuss the weakness with the caloric theory

Answer 1

Modern theories reproduce caloric in the effective sense (similar to effective "particles" like phonons) for most phenomena it was used to explain. Mass was not really an issue because the fluid was assumed to be very "subtle" to easily permeate ordinary matter, and therefore to have negligible mass (from modern perspective, heating does add some minuscule mass due to mass-energy equivalence).