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.
Again, wood disappears when burned and seems to be destroyed. And yet we know that the weight of the resulting smoke and ashes is exactly equal to that of the wood. The matter has been changed in form and composition, but its mass cannot be altered. It is not so easy to bring the smoke and ashes into combination again and so restore the matter to its original form as in the case of ice and steam. But this is done by nature. Ashes go to the soil, smoke into the atmosphere. The forces of nature bring these elements together again in plant and tree, and so it comes about that the materials resulting from the burning of wood again become wood, and over and over again the cycle is repeated as time rolls on. Many other examples might be cited to show what is meant by the indestructibility of matter, or the conservation of matter; but these will suffice to show that the one essential fact is that the matter or stuff of a body cannot be destroyed.
Although matter is protean and its transformations limitless, there are certain changes which cannot be made. Iron cannot be turned into silver, nor silver into gold, nor oxygen into nitrogen. There appear to be indeed about seventy or eighty distinct kinds of matter, and so far as