50 PRINCIPLES OF CHEMISTRY. 



heats, and subsequent determinations may cause a more absolute 

 agreement. 



However, the agreement is sufficiently close to justify the 

 deduction of a law which says : The atoms of all elements have 

 exactly the same capacity for heat. This law was first recognized 

 by Dulong and Petit in 1819, and is simply a generalization of 

 the facts stated. 



To show more clearly what is meant by saying that all atoms 

 have the same capacity for heat, we will select three elements 

 to illustrate this law. 



If we take of lithium 7 grams, of sulphur 32 grams, of silver 

 108 grams, we have of course in these quantities equal numbers 

 of atoms, because 7, 32, and 108 represent the atomic weights 

 of these elements. If we expose these stated quantities of the 

 three elements to the same action of heat, we shall find that the 

 temperature increases equally for all three substances; that is to 

 say : the same time will be required to raise 7 grams of lithium 

 1, which is necessary to raise either 32 grams of sulphur or 108 

 grams of silver 1. 



The quantity of heat necessary to raise the atom of any ele- 

 ment a certain number of degrees is, consequently, the same. 

 As heat is the consequence of motion, the result of the facts 

 stated may also be expressed by saying : It requires the same 

 energy to cause different atoms to vibrate with such a velocity 

 as to acquire the same temperature, no matter whether these 

 atoms be light or heavy. 



It is evident that these facts give us new means of deter- 

 mining atomic weights by simply dividing 6.5 by the specific 

 heat of the element. The specific heat of sulphur, for instance, 

 has been found to be 0.2026. 6.5 divided by this number is 

 31.6, or nearly 32. Originally the atomic weight of sulphur had 

 been determined by chemical methods to be 16, but its specific 

 heat, as well as other properties, has shown this number to be 

 but one-half of the weight, 32, now adopted. 



It may be mentioned that elements possess essentially the 

 same specific heat, whether they exist in a free state or are in 

 combination; this fact will, in many cases, be of use in the 

 determination of atomic weights. 



