# Definition of a calorie?

My copy of "Resnick and Halliday" states the following:

"Before scientists realized that heat is transferred energy, heat was measured in terms of its ability to raise the temperature of water. Thus, the calorie(cal) was defined as the amount of heat that would raise the temperature of 1g water from 14.5°C to 15.5°C."

This definition seems to account for the fact that heat really is energy in transit so why was this definition changed? Exactly what is so inherently wrong with defining heat in this manner? I'm afraid that I may have misunderstood the subtle distinction between heat and energy, if there is one.

The second reason is that the international system of units is nowadays used for units used in science. The idea is that there are a few (as few as possible) base units that are defined in terms of things occurring in the nature, and the rest are defined in terms of the base units. The unit of energy (=heat) is Joule whose definition is simply $\text{kg} \text{ m}^2/ \text{s}^2$, where kilogram, metre, and second are defined in a certain way.