ATOMS AND MOLECULES 57 



tions of atoms and molecules are, I now take up some interest- 

 ing experiments with substances that are elements; this is 

 because we know that when we are dealing with a single ele- 

 ment no change will occur which involves more than one kind 

 of atom. I have here a jar of the substance originally known as 

 oxygen, and the arrangement is such that I am able to pass 

 this oxygen through various pieces of apparatus, illustrating 

 properties of oxygen. By simply exposing the gas to a current 

 of electricity I give to it entirely different properties, as you 

 will see when I pass a little oxygen over this piece of paper, 

 and then expose a strip of the same piece to the action of the 

 electrified oxygen, or ozone, as it is called. You must not sup- 

 pose that there is any electricity in this ozone which makes it 

 different from the oxygen. I have here some ozone made by a 

 totally different process, and it shows the same effect. The 

 ozone can be converted back into the oxygen, as you will see 

 when I pass it through this heated tube; it now has lost its 

 power of affecting the impregnated paper; it has been con- 

 verted back into original oxygen. In fact, all our study shows 

 us that the sole difference between ozone and oxygen consists 

 in the arrangement of the atoms; one molecule of oxygen must 

 be assumed to contain two atoms, and one molecule of ozone 

 three atoms. 



If this were a single instance standing entirely by itself, the 

 opinion might be attacked; but we have many similar cases 

 in chemistry, although it is not always so easy to prove the 

 matter as in the case of the oxygen. I have here two specimens 

 of phosphorus, which to all appearances are entirely different 

 in their nature; the one is a coherent yellow mass, the other is 

 a red powder; and many of the properties are different like- 

 wise. This one is poisonous, that one not; this one catches 

 fire with great ease, the other will catch fire only after consider- 

 able heating. Once I have set the two a-burning, however, the 



