276 ELEMENTARY GENERAL SCIENCE CHAP. 



of manganese and chlorine. As, however, it is improbable that 

 the compound MnCl 4 exists, it is preferable to write the reaction 

 in one equation : 



Mn0 2 + 4HC1 - MnCl 2 + 2H 2 O + Cl,. 



The student should endeavour to represent other actions, as 

 the burning of sulphuretted hydrogen, &c., by equations, taking 

 great care that the same number of atoms of each element occur 

 on the two sides of the equation, and paying attention to the 

 known formulae and the valencies given. It must be remembered, 

 however, that an equation is a representation of a fact, and the 

 student must be careful to avoid attempting to make his version 

 of chemical facts agree with guessed equations which though 

 apparently possible do not really represent actual reactions. 



CHIEF POINTS OF CHAPTER XIX. 



Atoms and Molecules. All matter is supposed to be built up of 

 minute particles termed atoms. These usually exist combined with 

 other atoms to form molecules. The molecules of a compound are 

 composed of different kinds of atoms united together. In an 

 element the atoms are all of the same kind. 



Chemical Symbols. Chemists represent the atoms of various 

 elements by letters which are in general the first letter, or the 

 characteristic letters, of the English or Latin name of an element. 

 Thus H represents the atom of hydrogen and Cl the atom of 

 chlorine. 



Avogadro's Law. Equal volumes of all gases under similar con- 

 ditions of temperature and pressure contain equal numbers of 

 molecules. 



Chemical Equations enable the chemist to represent chemical 

 reactions by means of sjnnbols. The formulae of the reacting bodies 

 are placed on the left-hand side of the equality sign, and those of 

 the products on the right. 



The Atomic Weight of an element is the ratio of the weight of its 

 atom to the weight of an atom of hydrogen. Thus, when we say the 

 atomic weight of chlorine is 35 '5, we mean that the atom of chlorine 

 is 35 '5 times as heavy as the atom of hvdrogen. 



The Equivalent Weight of an element is that weight of it which 

 can just replace 1 gram of hydrogen in a compound. 



The Valency of an element means the number of atoms of hydrogen 

 one atom of it is able to replace. 



Atomic Weight - Equivalent Weight x Valency. 



