GENERAL REMARKS REGARDING ELEMENTS. 59 



Metals are all elements which have that peculiar lustre known 

 as metallic lustre ; which are good conductors of heat and elec- 

 tricity; which, in combination with oxygen, form compounds 

 generally showing basic properties ; and which are capable of 

 replacing hydrogen in acids, thus forming salts. 



Non-metals or metalloids are all elements not having the above- 

 mentioned properties. Their oxides in combination with water 

 generally have acid properties. In all other respects, the chemi- 

 cal and physical properties of non-metals differ widely. Their 

 number amounts to 14, the other 52 elements being metals. 



Natural groups of elements. Besides classifying all elements 

 into metals and non-metals, certain members of both classes 

 exhibit so much resemblance in their properties, that many ot 

 them have been arranged into natural groups. The members 

 of such a natural group frequently show some connection be- 

 tween atomic weights and properties. 



Chlorine, 35.5 Sulphur, 32 Lithium, 7 Calcium, 40 



Iodine, 127 Tellurium, 128 Potassium, 39 Barium, 137 



Bromine, 80 Selenium, 79.5 Sodium, 23 Strontium, 87 



Each three elements mentioned in the above four columns 

 resemble each other in many respects, forming a natural group. 

 The relation between the atomic weights will hardly be sus- 

 pected by looking at the figures, but will be noticed at once by 

 adding together the atomic weights of the first and last ele- 

 ments and dividing this sum by 2, when the atomic weights 

 (very nearly, at least) of the middle members of the series are 

 obtained. Thus : 



**6-l?Z = 81 . 25 . 



- . _ 



2 2 



~ 39 40 -f 137 _ 



This relationship between atomic weights and properties has 

 been used for arranging all elements systematically in such a 

 manner that the existing connection is clearly pointed out. For 

 several reasons, however, these systems are not altogether per- 

 fect, and need therefore not be further considered in this book. 



