LESSONS IN CHEMISTKY 9 



elements are constantly used in science, and it has been 

 found convenient to refer to these substances by symbols, 

 the first letter or letters of the name being used in most 

 cases. Thus we mean carbon when we use the capital 

 letter C; stands for oxygen; S for sulphur; and P for 

 phosphorus. I stands for iodine, and so Ee (Latin ferrum = 

 iron) is used for iron. An element may be defined as a sub- 

 stance that cannot be decomposed into simpler substances. 



Compounds. We have seen that a certain amount of heat 

 causes the element phosphorus to unite with some of the 

 element oxygen, and that thus a new substance is formed 

 called oxid of phosphorus ; more heat will produce oxid of 

 sulphur by a chemical union between sulphur and oxygen, 

 while at a still higher temperature carbon unites with 

 oxygen, forming oxid of carbon or carbon dioxid. All of 

 these oxids are chemical compounds. Hence a chemical com- 

 pound may be defined as a substance that can be decomposed 

 into simpler substances. A compound, therefore, is formed by 

 the chemical union of two or more elements. 



We will now consider the composition of two of the most 

 important of the compounds that we have been studying. 

 Eepeated experiments have proved that one part of carbon 

 combines with two parts of oxygen. Chemists, therefore, 

 give to the gas thus formed the name carbon dioxid and 

 represent its composition in the symbol C0 2 ; both name and 

 symbol tell us that it is composed of one part carbon and 

 two parts oxygen. Water, too, is a compound, a fact that 

 can be proved in the following way. If an electric current is 

 passed through a dish of water, the liquid becomes separated 

 into two gases, one of which is oxygen, and the other a gas 

 called hy'dro-gen (Greek, hu'dor= water + gen = maker). We 

 find, on collecting the two gases, that we get twice as great 

 a volume of hydrogen as of oxygen. Hence we represent the 

 composition of 'water by the symbol H 2 0, which means that 

 two parts of hydrogen are combined with one part of oxygen. 

 That such a combination really takes place can be demon- 



