208 ELEMENTARY GENERAL SCIENCE (HAP. 



the flash. Such changes as these are called chemical changes. 

 It is with changes of this second kind that Chemistry is con- 

 cerned, and we may define this science thus : Chemistry is that 

 branch of knowledge which deals with chemical changes ; those, 

 namely, which result in the formation of new substances with new 

 properties. 



Chemical Elements. The result of a large number of ex- 

 periments made from time to time by different chemists has been 

 to show that there are upwards of seventy different forms of 

 matter which can by no known methods be broken up into 

 anything simpler. By this is meant that if any one of these be 

 selected and treated in any way with which chemists are familiar 

 for example, if it were subjected to a very high temperature 

 we should find it impossible to get anything having properties 

 different from those of the substance itself ; bodies of this 

 simple kind are called elements. 



But it must be carefully borne in mind that, as the methods 

 which chemists adopt become more and more refined, it is quite 

 likely that some of these may be found to be wrongly regarded 

 as elements. Up to the time of Davy (1807) the substances 

 soda, potash, and lime were regarded as elements. He found, 

 however, that they could be split up into simpler constituents. 

 From soda he obtained a soft metal, sodium, and two gases, 

 oxygen and hydrogen, and from that time, of course, soda could 

 not be regarded as an element. Similarly, if at any future 

 time it should be found that any of the forms of matter which 

 we call elements can be split up into simpler bodies with different 

 properties, the element which is thus decomposed will have to 

 be struck off the list. 



A list of the substances now regarded as elements is given at 

 the end of Chap. XIX. 



Metals and Non-Metals. A good many of these substances 

 are possessed of certain distinctive characters in which they 

 resemble one another. They have a bright lustre, a high specific 

 gravity (see p. 96), are good conductors of heat and electricity, 

 and are known to chemists as metals. There is no difficulty in 

 deciding in a large number of instances that the substances 

 possess the characters of a metal, and the student will imme- 

 diately think of gold, silver, copper, iron, &c. Other bodies, 

 however, are quite as plainly not of this class ; they have no 

 lustre-, they are not heavy, nor do they conduct heat and elec- 

 tricity well. These are spoken of as non-metals, and phosphorus, 



