ATOMS AND MOLECULES 61 



you see how soon an alarm is given. Such an arrangement 

 might well be used where a dangerous or harmful gas is apt to 

 appear; it was really invented for coal mines, in which there 

 is a danger of the formation of explosive mixtures of lighter 

 combustible gases with air. 



In this way it is possible to distinguish, as you see, between 

 lighter and heavier molecules. This method may be of value 

 to the investigator, as we shall see. I have here a substance 

 whose molecules are fairly large; they are ordinarily in a solid 

 state, and they represent a substance which is neutral, as we 

 say in chemistry, a salt that has no effect upon vegetable 

 colors. I throw a little of it upon these pieces of red and blue 

 litmus paper, and you see that there is no change. Now a 

 piece of this salt (which we call chloride of ammonium) is 

 placed in this glass tube next to a porous partition wall; when 

 I warm the salt it breaks up into two substances called am- 

 monia and hydrochloric acid, which may be separated by the 

 method of osmosis just used to ring an electric bell. The 

 ammonia consists of molecules so light that it passes very 

 readily through this partition wall, while the hydrochloric 

 acid with heavier molecules passes less readily. You will be 

 able to notice this when I pump air through both parts of the 

 apparatus and let the two currents of air flow over these two 

 pieces of colored paper. You see how the red paper is turning 

 blue, and the blue paper is turning red; this is due to the fact 

 that the hydrochloric turns blue paper red and more of it re- 

 mains on the near side of the partition; the ammonia turns 

 red paper blue, and more of it has gone through the partition. 

 The experiment not only illustrates to you what I have shown 

 before in a somewhat different form, but it also shows us that 

 the big molecules of chloride of ammonia can be broken up 

 into two sets of smaller molecules, which themselves differ 

 from one another in size. We can even, although I am unable 



