Thermal physics is the study of objects warm and cold, and how they interact with each other. It is difficult because most of the vocabulary dates from the time before scientists understood what makes an object hot. Terms like “heat,” “heat capacity,” and “latent heat” suggest that warm objects contain some material that depends on their temperature. As was discussed in the chapter on Energy, it wasn’t until the early 1800s that our present understanding was developed. Yet, almost 200 years later, our common usage is based on earlier ideas.