DETERMINATION OF ATOMIC WEIGHTS. 5| 



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 : T/ie 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 

 108 grammes, 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 heat will be 

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

 either 32 grammes of sulphur or 108 grammes of silver 1. 



(The quantity of heat necessary to raise the atom of any element aj 

 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. 



It is evident that these facts give us another means of determining 

 atomic weights, by simply dividing 6.5 by the specific heat of the ele- 

 ment. 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. 

 f It may be mentioned that elements possess essentially the samg 

 \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. 



Determination of molecular weights. From the statements 

 made regarding the determination of atomic weights, it is evident 

 that we may use a number of methods for determining molecular 

 weights, these methods being to some extent analogous to the former. 



Thus we have methods which are based entirely on chemical analysis 



