PROGEESS IN CHEMISTRY. 241 



double atom, of hydrogen splits; the same is the case with the.mole- 

 culo of chlorine; and each liberated atom of h3xlrogen unites with a 

 liberated atom of chlorine, forming a compound, hydrogen chkjride, 

 which equally consists of a molecule, or double atom. Thus 2 cubic 

 inches of hydrogen chloride consist-of a definite number of molecules, 

 equal in number to those contained in a cubic inch of hydrogen, plus 

 those contained in a cubic inch of chlorine. The case is precisely sim- 

 ilar if other compounds of gases be considered. 



Berzelius was at lirst inclined to adopt this theory, and indeed went 

 so far as to change many of his atomic weights to make them fit it. But 

 later he somewhat withdrew from his position, for it appeared to him 

 that it was hazardous to extend to liquids and solids a theory- which 

 could be held onl}' of gases. Avogadro's suggestion therefore rested 

 in abeyance until the puV)lication in 1858 b}^ Cannizzaro, now professor 

 of chemisti-y in Rome, of an essaj-^ in which all the arguments in favor 

 of the hypothesis were collected and stated in a masterly manner. It 

 will ]je advisable to revert to this hypothesis at a later point and to 

 consider other guides for the determination of atomic weights. 



EQUAL CAPACITY FOR HEAT. 



In 1819, Dulong (1785-1838), director of the Ecole Polytechnique at 

 Paris, and Petit (1791-1820), professor of physics there, made the dis- 

 covery that equal amounts of heat are required to raise equally the 

 temperature of solid and liquid elements, provided quantities are taken 

 proportional to their atomic weights. Thus, to raise the temperature 

 of 56 grams of iron through 1 degree requires approximately the same 

 amount of heat as is required to raise through 1 degree 32 grams of 

 sulphur, 63.5 grams of copper, and so on, these numbers representing 

 the atomic weights of the elements named. In other words, equal 

 numbers of atoms have equal capacity for heat. The number of heat 

 units or calories (one calory is the amount of heat required to raise 

 the temperature of 1 gram of water through 1^ C.) which is necessary 

 to raise the atomic weight expressed in grams of any solid or liquid 

 element through 1° C. is approximately 6.2. It varies between 5.7 

 and 6.6 in actual part. This affords a means of determining the true 

 value of the atomic weight of an element, as the following example 

 will show: The analysis of the only compound of zinc and chlorine 

 shows that it contains 17.19 per cent of zinc and 52.16 per cent of chlo- 

 rine. Now, 1 grain of hydrogen combines with 35.5 grains of chlorine 

 to form 36.5 grains of hydrogen chloride; and, as already remarked, 1 

 volume of hydrogen and 1 volume of chlorine combine, forming 2 vol- 

 umes of hydrogen chloride. Applying A\ogadro's hypothesis, 1 mole- 

 cule of hydrogen and 1 molecule of chlorine react to yield 2 molecules 

 of hvdrogen chloride, and as each molecule is supposed to consist in 



