48 PRINCIPLES OF CHEMISTRY. 



tincombined state, but combine with each other to form molecules. 

 Atoms having the tendency of combining with, or attaching them- 

 selves to other atoms, are bound to exert that attraction, and if they 

 are not combined with atoms of other elements, they combine with 

 each other. For instance: Oxygen gas is not a mass of oxygen 

 atoms, but of oxygen molecules, each molecule being formed by the 

 union of two atoms. 



7. DETEEMINATION OF ATOMIC AND MOLECULAR WEIGHTS. 1 



Determination of atomic weights by chemical decomposition. 

 The great difficulties originally encountered in the determination of 

 atomic weights cannot well be described here. Consideration will be 

 given alone to the three principal methods at present in use. These 

 methods depend either on chemical action or on physical properties. 



One of the chemical methods used for the determination of atomic 

 weights has been stated before in describing the decomposition of the 

 red oxide of mercury by heat. The principle of this method is the 

 determination of the proportions by weight in which the element, the 

 atomic weight of which is unknown, combines with an element the 

 atomic weight of which is known. For instance: If in decomposing 

 a substance we find it to contain in 72 parts by weight, 16 parts by 

 weight of oxygen and 56 parts by weight of another element, we 

 have a right to assume the atomic weight of this second element to be 

 56, provided, however, that the compound is actually formed by the 

 union of one atom of oxygen and one atom of the other element. 

 These 56 parts by weight might, however, represent 2 or 3 or more 



QUESTIONS. 51. State the law of the constancy of composition. 52. What 

 is the difference between a mixture and a chemical compound ? 53. Mention 

 some instances of the production of molecular mixtures. 54. State the law of 

 multiple proportions. 55. What considerations led Dalton to the adoption of 

 the atomic theory? 56. What regularity regarding volume is noticed when 

 gases combine chemically? 57. To what was the term equivalent quantities 

 applied formerly, and what is to be understood by it to-day? 58. Explain the 

 term quantivalence or atomicity. 59. Mention some univalent, bivalent, tri- 

 valent, and quadrivalent elements. 60. Suppose a certain volume of hydrogen 

 to weigh 20 grains, how much will an equal volume of oxygen and how much 

 will an equal volume of hydrochloric acid gas weigh, provided pressure and 

 temperature be the same ? 



i The consideration of Chapter 7 should be postponed until the student has become familiar 

 with chemical phenomena generally. 



