GENERAL REMARKS REGARDING ELEMENTS. 57 



all the different hydrates, such as potassium hydrate, KHO, cal- 

 cium hydrate, Ca2HO, etc. 



According to the number of points of attraction left unpro- 

 vided for in a radical, we distinguish univalent, bivalent, tri- 

 valent, and quadrivalent radicals. 



Carbon is a quadrivalent element forming with the univalent 

 hydrogen the saturated compound CH 4 . By removal of one, 

 two, or three hydrogen atoms the radicals CH 3 ', CH 2 ", CH'", 

 are formed. 



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 the terms acid, basic, and neutral substances. By what 

 means can they be recognized ? 



77. Distinguish between mono-, bi-, 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. 



9. GENERAL REMARKS REGARDING ELEMENTS. 



Relative importance of different elements. Of the total number 

 of about sixty-six elements, but few comparatively (about one- 

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

 the phenomena taking place upon it. These important ele- 

 ments form the greater part of the mass of the solid portion of 

 the earth, of the water and atmosphere, and of all animal and 

 vegetable matter. 



Another number of elements are of less importance, either 

 because they are not found in any large quantity, or do not 

 take any active or essential part in the formation of organic 



