The chamber was so constructed and fitted with electrical and other devices as to afford the means of measuring the quantity of heat which the subject of the experiment gave off from his body. And in order to keep the temperature of the room constant this heat was absorbed and carried away by a stream of cold water, the latter flowing through a series of copper pipes within the chamber, and coming out considerably warmer than it entered. So delicate were the regulating devices that the temperature could be maintained constant, hour after hour, to within one or two hundredths of a degree. In some cases the man under investigation worked regularly eight hours a day, the work done being measured by apparatus designed for the purpose. Food and drink were passed into the chamber three times a day through an air-tight trap. Both were accurately weighed, their temperature recorded and samples reserved for chemical analysis. Solid and liquid excreta were likewise weighed and analyzed. The observations, analyses and computations of a single experiment thus involved a vast amount of labor and expense, which was only justified by the importance of the question under investigation. In order to be able to understand just what this question is, let us see what is meant by the conservation of matter and energy in the physical world. The impossibility of creating or destroying matter is very generally recognized. Its forms or properties may be altered, chemical and physical changes may be effected, it may, indeed, vanish from sight, but its quantity remains unchanged. Thus ice may turn to water and water to invisible steam, but the total quantity or mass of the substance remains constant; and if by refrigeration the steam be brought to the condition of ice again, there will be precisely the same amount as before. These are physical changes and are easily effected. We simply apply heat to melt the ice and then more heat to vaporize the water. Conversely, withdrawing heat will condense the vapor to water, when a further subtraction of heat will change the water into ice.