60 PRINCIPLES OF CHEMISTRY. 



Double salts are salts formed by replacement of hydrogen in an 

 ( acid by more than one metal. For instance : Potassium-sodium sul- 

 phate, KNaSO 4 . 



Residue, radical, or compound radical, are expressions for un- 

 saturated groups of atoms known to enter as a whole into different 

 compounds, but having no separate existence. For instance: The 

 bivalent oxygen combines with two atoms of the univalent hydrogen, 

 forming the saturated compound H 2 O, water. If we take from this 

 H 2 O one atom of H, there is left the group of atoms HO (now gener- 

 ally written OH), consisting of an atom of oxygen in which but one 

 point of attraction is actually saturated, the second one not being 

 provided for. 



This group, OH, is a residue or radical, and is known to enter into 

 many compounds ; it is, for instance, a constituent of all the different 

 hydroxides (formerly called hydrates), such as potassium hydroxide, 

 KOH, calcium hydroxide, ca(OH)-2, etc. 



According to the number of points of attraction left unprovided 

 for in a radical, we distinguish univalent, bivalent, trivalent, and 

 quadrivalent radicals. 



Carbon is a quadrivalent element forming with the univalent hy- 

 drogen the saturated compound CH 4 . By removal of one, two, or 

 three hydrogen atoms the radicals CH 3 ', CH 2 ", CH //r , are formed. 



ENEEAL EEMAEKS EEGAEDING ELEMENTS. 



Relative importance of different elements. Of the total 

 jr number of about sixty-nine elements, comparatively but few (about 

 one-fourth) are of great and general importance for the earth, and the 

 phenomena taking place upon it. These important elements form 

 the greater part of the mass of the solid portion of the earth, and of 

 the water and atmosphere, and of all animal and vegetable matter. 



QUESTIONS. 71. What physical actions have a tendency to decompose com- 

 pound substances ? 72. Explain the terms reaction and reagent. 73. Mention 

 some instances of decomposition produced by the action of one substance upon 

 another substance. 74. Why can no general rules be established in regard to 

 the amount of attraction which different elements have for each other ? 75^ 

 What is the difference between analytical and synthetical methods ? 76. Define 

 an acid, and state the general properties of basic and neutral substances. By 

 what means can they be recognized? 77. Distinguish between mono-, di-, and 

 tri-basic acids. 78. What are salts and how are they formed? 79. Define 

 neutral, acid, and double salts. 80. Explain the term radical or residue. 



