492 POPULAR SCIENCE MONTHLY. 



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 tem- 

 perature recorded and samples reserved for chemical analysis. Solid 

 and liquid excreta were likewise weighed and analyzed. The observa- 

 tions, 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 physi- 

 cal 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 re- 

 mains 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. Con- 

 versely, withdrawing heat will condense the vapor to water, when a fur- 

 ther 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 mat- 

 ter 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 



