DETERMINATION OF ATOMIC WEIGHTS. 107 



In comparing atomic weights with the numbers expressing the spe- 

 cific heats, it is found that the higher the atomic weight the lower tHe 

 specific heat, and the lower the atomic weight the higher the specific 

 heat. This simple relation may be thus expressed : Atomic weights 

 are inversely proportional to the specific heats ; or the product of the 

 atomic weight multiplied by the specific heat is a constant quantity 

 for the elements examined. 



Elements. Specific heat*. Atomic weights. Product of specific heal 



(Water = 1 .) X atomic weight. 



Lithium, 09408 7 6.59 



Sodium, 0.2934 23 6.75 



Sulphur, 2026 32 6 48 



Zinc, 00956 65 6.22 



Bromine (solid), 0.0843 79 6.66 



Silver, 0.0570 107 6.10 



Bismuth, 0.0308 209 644 



An examination of this table will show this relation between 

 atomic weight and specific heat, and also that the product of atomic 

 weight multiplied by specific heat is equal to about 6.5. The varia- 

 tions noticed in this constant quantity of about 6.5 may be due to 

 errors made in the determinations of the specific heats, and subse- 

 quent 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 grammes, of sulphur 32 grammes, of silver 

 107 grammes, we have of course in these quantities equal numbers of 

 atoms, because 7, 32, and 107 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 heat will be 

 required to raise 7 grammes of lithium 1, which is necessary to raise 

 either 32 grammes of sulphur or 107 grammes of silver 1. 



The quantity of heat necessary to raise the atom of any element 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 ex- 

 pressed by saying : It requires the same energy to cause different 

 atoms to vibrate with such a velocity as to acquire the same tempera- 

 ture, no matter whether these atoms be light or heavy. 



