WHAT IS AN ELEMENT ? 151 



Name . . Lithium. Beryllium. Boron. Carbon. Nitrogen. Oxygen. Fluorine. 

 Atomic Weight .7 9'1 11 12 14 16 19 



Name . . Sodium. Magnesium. Aluminium. Silicon. Phosphorus. Sulphur. Chlorine. 



Atomic Weight . 23 24-4 2V1 28'4 31 32 35'5 



If one were to proceed further in the same manner, we 

 should find five elements in the first vertical column 

 namely lithium, sodium, potassium, rubidium, and 

 caesium. All of these are soft metals, easily cut with a 

 knife, white in colour like silver, rapidly tarnishing in air, 

 attacked violently by water so that they either catch fire 

 or run about on the surface of the water and rapidly dis- 

 appear. Their compounds with chlorine each consist of 

 one atom of each element : for example, using Na 

 (natrium) as the symbol for one atom of sodium, and Cl 

 for one atom of chlorine, the composition of the com- 

 pound of chlorine with sodium (common salt) is expressed 

 by the formula NaCI, implying that the compound is 

 formed of one atom of each element. So with the 

 others : the chloride of lithium is LiCl, of potassium 

 KC1, of rubidium RbCl, and of cesium CsCl. They all 

 resemble common salt ; the taste is similar in all cases, 

 the salts dissolve in water, they are all white in colour, 

 they all crystallise in cubes, and possess many other pro- 

 perties in common. The oxides, too, are all powders, 

 which dissolve in water and give liquids with a soapy 

 feel and a burning taste. For these and other similar 

 reasons, all these elements are believed to belong to the 

 one class. 



Let us take an example, too, from the other end of the 

 table. Fluorine, the first of the column, is a pale yellow 

 gas, with a suffocating odour. It combines instantly with 

 hydrogen, yielding a colourless gas, soluble in water, and 

 giving an acid liquid, which corrodes many metals. 

 Chlorine, the second member, is a greenish yellow gas, 

 very similar in properties to fluorine. The third member, 



