What outdated theory suggested that thermal energy was transferred through an invisible fluid?

The caloric theory is the outdated concept that proposed thermal energy was transmitted through an invisible fluid known as "caloric." According to this theory, caloric was thought to be a self-repelling fluid that could flow from hotter objects to cooler ones, thereby transferring heat. This idea was prevalent before the development of modern thermodynamics and the kinetic theory of heat.

The caloric theory was eventually displaced by the understanding that heat involves the motion of particles and is related to kinetic energy—a concept that better explains the behaviors of thermal energy and its transfer mechanisms. In contrast, thermodynamics presents more robust principles governing energy transitions, while kinetic theory focuses on the particle movement and how it relates to temperature and heat. Conduction theory explains heat transfer through direct contact between materials, but does not reference any fluid dynamics aspects of heat transfer.