108 PRINCIPLES OF CHEMISTRY. 



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. 



Tt 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. 



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 

 or on chemical changes generally. If, for instance, the analysis of a 

 substance shows of calcium 40 per cent., of carbon 12 per cent., and 

 of oxygen 48 per cent., we have a right to assume that the molecule is 

 made up of 1 atom of calcium, 1 atom of carbon, and 3 atoms of 

 oxygen, as the atomic weights of these elements are 40, 12, and 16 

 approximately. The molecular weight in this case is 100, and the com- 

 position is expressed by the formula CaCO 3 , but the molecular weight 

 might be 200 and the correct formula Ca 2 C 2 O 6 . There are actually 

 substances which contain such multiples of atoms, as, for instance, the 

 compounds C 2 H 2 and C 6 H 6 , and as their percentage composition is 

 identical, analytical methods are insufficient to indicate the number 

 of atoms contained in these molecules. 



The second method, based on Avogadro's law, is applicable to all 

 substances which are or can be converted into gases or vapors without 

 decomposition. Since equal volumes of all gases at the same temper- 

 ature and pressure contain the same number of molecules, the weights 

 of equal volumes of gases must bear the same ratio to one another as 

 the weights of the individual molecules. But the weights of mole- 

 cules are in the same ratio as the molecular weights. Hence we 

 deduce the following rule from Avogadro's Law : Densities of gases at 

 the same temperature and pressure are to each other as their molecular 

 weights. If we know the molecular weight of any gas and its 

 density, by comparing any other gas with it we can determine its molec- 

 ular weight. As we have seen, the molecule of hydrogen is known 

 to contain two atoms, that is, its molecular weight is 2, if we call the 



