74 Elementary Dynamical Principles.
and that we must not assume that after heat has entered a substance it exists in the form of heat within that substance. That we have no right to make such an assumption will be abundantly shown by the demonstration that heat may be transformed into and may be produced from something which is not heat.
Regnault's method of passing large quantities of the substance through the calorimeter will be described in treating of the properties of gases, and the Method of Cooling will be considered in the chapter on Radiation.
CHAPTER IV.
ELEMENTARY DYNAMICAL PRINCIPLES.
In the first part of this treatise we have confined ourselves to the explanation of the method of ascertaining the temperature of bodies, which we call thermometry, and the method of measuring the quantity of heat which enters or leaves a body, and this we call calorimetry. Both of these are required in order to study the effects of heat upon bodies; but we cannot complete this study without making measurements of a mechanical kind, because heat and mechanical force may act on the same body, and the actual result depends on both actions. I propose, therefore, to recall to the student's memory some of those dynamical principles which he ought to bring with him to the study of heat, and which are necessary when he passes from purely thermal phenomena, such as we have considered, to phenomena involving pressure, expansion, &c., and which will enable him afterwards to proceed to the study of thermodynamics proper, in which the relations of thermal phenomena among themselves are deduced from purely dynamical principles.
The most important step in the progress of every
