4 CHEMISTRY OF FARM PRACTICE 



is allowed to unite with an excess of chlorine, it is found 

 that it will, in every case, combine with 35.46 times its 

 own weight of the chlorine. Hydrogen chloride therefore 

 contains hydrogen and chlorine in the proportion of 1 

 to 35.46. There are reasons for believing that the mole- 

 cule of hydrogen chloride consists of one atom of hydrogen 

 combined with one atom of chlorine. If this is true, then, 

 taking the hydrogen atom as a unit of weight, the chlorine 

 atom weighs 35.46 and the molecule of hydrogen chloride, 

 HC1, weighs 36.46. It has been found that the elements 

 which compose a compound always combine in propor- 

 tion to their atomic weights or to some multiple of their 

 atomic weights. It will be seen that this must be the 

 case if an atom is indivisible. 



The atomic weight of an element is fixed by deter- 

 mining with great care the weight of the element that will 

 unite with another element whose atomic weight has pre- 

 viously been determined. As nearly all the elements form 

 compounds with oxygen, while comparatively few unite 

 with hydrogen, the atom of oxygen with an assigned weight 

 of 16 is really used as the standard instead of the atom 

 of hydrogen, which it will be observed in the table on page 

 6, weighs 1.008. 



5. Molecules of Elements. The common elementary 

 gasss, such as oxygen, hydrogen, nitrogen, and chlorine, 

 have molecules consisting of two atoms of each element. 

 The molecules of some of the elements, such as phosphorus, 

 arsenic, and antimony, have four atoms in each molecule, 

 when they are at a temperature slightly above vaporiza- 

 tion. In most cases the number of atoms in a molecule 

 of an element depends upon its temperature. Thus sul- 

 phur vapor at 468 C. has eight atoms to the molecule; at 

 830 C. its molecule has but two atoms. 



Some elements, such as sodium, potassium, mercury, 

 and zinc, when in a vaporous state, have but one atom 

 to each molecule. Such molecules differ from atoms in 



