LESSONS IN CHEMISTRY. 105 



may not be at once apprehended, but they underlie the whole 

 work of chemistry and will become clearer and more satisfactory 

 with every step of chemical investigation. 



Chemists have discovered and isolated some seventy or more 

 elements which make up the earth as we know it. The composi- 

 tion by weight, of the crust of the earth is as follows : Oxygen 

 45 per cent., silicon 25 per cent., aluminium 9 per cent., iron 

 8 per cent., calcium 5 per cent., magnesium 2 per cent., so- 

 dium 2 per cent., potassium 2 per cent. If we add to these 

 elements hydrogen, nitrogen, carbon, chlorine, iodine, phos- 

 phorus and sulphur we not only have those that make up the 

 solid crust of the earth, but the constituents of the air, water, 

 plants and animals as well, fifteen elements which those must 

 study who pursue chemistry as a branch of general education, 

 since a knowledge of their properties is essential to the expla- 

 nation of those chemical changes which are common about us 

 everywhere. Several other elements are interesting and will re- 

 ceive brief consideration. It is the study of the laws in accord- 

 ance with which the elements combine with one another, and of 

 the properties of the elements and of the compounds formed by 

 them, which constitutes the science of chemistry. 



The elements are divided into metallic elements, as iron, copper, 

 aluminium, sodium, mercury, etc., and non-metallic elements, as 

 oxygen, hydrogen, nitrogen, silicon, etc. The metals generally are 

 better reflectors of light and heat, better conductors of heat and 

 electricity, and as a rule have a higher specific gravity than the 

 non-metals. All these elements are designated by symbols, which 

 usually consist of the initial letter of the English or Latin name 

 of the element, as for oxygen, C for carbon, Si for silicon, Cl 

 for chlorine, Fe for iron, from its Latin name ferrum; Na for 

 sodium, from its Latin name natrium; K for potassium, from 

 its Latin name kalium ; Ca for lime, from calcium, its Latin 

 name. The symbols for other elements will be given as they are 

 described. The various sub-groups into which the elements seem 

 to be naturally divided can be more intelligently considered as 

 the elements are studied more in detail. 



