CONSTITUTION OF MATTER. 29 



So, too, when a mixture of oxygen and hydrogen explodes 

 on the application of a light or electricity, the atoms of the 

 two gases merely unite in a very hurried manner, forming 

 water, and none of them experience the slightest change. 

 These ultimate atoms unite together to make molecules or 

 particles, which, unlike the atoms, can be both changed and 

 divided if the atoms composing them be of two or more 

 kinds, and can at least be divided if their atoms are of one 

 kind alone. 



26. Weight of Atoms. A most important attribute of 

 atoms has not yet been mentioned, viz., weight. Every ele- 

 mentary body is supposed to be made up of atoms of ex- 

 actly the same size and weight in the same body. The 

 weight of the atoms of different elements varies greatly ; 

 if we call the weight of the hydrogen atom l,then that of 

 oxygen is 16, while mercury is 200, gold 19 7, carbon 12, etc. 

 How it is that chemists are able to determine that the 

 atoms of the various elements differ in weight we can not ex- 

 plain to you in this work, but you must not imagine that sin- 

 gle atoms are ever weighed, only immense numbers of them 

 taken together. Nor is there any thing absolute with ref- 

 erence to their weight it is merely relative ; that is, the 

 figures for hydrogen and oxygen named, viz., 1 and 16, 

 do not stand for any particular quantity, say pounds or 

 grammes, but they signify that if the hydrogen atom weighs 

 1 gramme or 1 pound, then the oxygen atom, being sixteen 

 times as heavy, weighs 16 grammes or 16 pounds. Any 

 other unit than hydrogen might be taken; and actually 

 many years ago oxygen was placed equal to 100, and the 

 weights of the other atoms were proportionally heavier 

 hydrogen becoming 12.5, since 8 : 100 = 1 : 12.5. All the fig- 

 ures representing the relative weights of the atoms were 

 then 12.5 times heavier. Chemists now universally adopt 

 hydrogen as the standard, and make it unity. 



