CHEMICAL CHANGE AND THE METHODS OF STUDYING IT 19 



A list of the elements appears on the inside of the cover, at the 

 end of this book, and contains about eighty names. Of these, a large 

 number are rare, and seldom encountered. More than 99 per cent 

 of terrestrial material is made up of eighteen or twenty elements and 

 their compounds. Only about twenty elements occur in nature 

 in their simple, uncombined condition. Three-fourths of the 

 whole number are found in combination exclusively, and must 

 be liberated by some chemical action. 



Taking the atmosphere, all terrestrial waters, and the earth's 

 crust, so far as it has been examined, F. W. Clarke has estimated 

 the plentifulness of the various elements. The first twelve, with 

 the quantity of each contained in one hundred parts of terrestrial 

 matter, and constituting together 99 per cent, are as follows: 



Oxygen 49 . 85 Calcium 3 . 18 Hydrogen 0. 97 



Silicon 26.03 Sodium 2.33 Titanium 0.41 



Aluminium 7.28 Potassium 2.33 Chlorine 0.20 



Iron 4.12 Magnesium 2.11 Carbon 0.19 



The significance of these figures is more clearly shown in Fig. 

 12. It will be seen that oxygen accounts for nearly one-half of the 

 whole mass. Silicon, the oxide of which when pure is quartz 

 and in less pure form constitutes ordinary sand, makes up half of 

 the remainder. Valuable and useful elements, like gold, silver, 

 sulphur, and mercury, are among the less plentiful which, all 

 taken together, furnish the remaining one per cent. 



Law of Definite Proportions. In the decomposition of 

 mercuric oxide (p. 16)* we find that, for every 100 parts of 



* References to previous pages are used in order to save needless repetition 

 in writing. The beginner requires endless repetition in his reading, however, 

 and must form the habit of examining, in conjunction with the current text, the 

 parts referred to. The passages cited are, by the reference, made part of the 

 current text, which will usually not be clear without them. The same remark 

 applies to topics referred to by name. Such topics must be sought in the 

 index. 



All terms, and especially those borrowed from physics, if not perfectly 

 familiar, must be looked up in a work on physics or in a dictionary. 



