; : L The Outline of Science 



killed. The danger of this poisonous gas escaping from a leaking 

 or badly burning stove is well known. White mice used to be 

 carried by submarines in order to detect the dangerous CO gas; 

 but there are finer methods now in use. 



When a tumbler is inverted into a basin of water, we cannot 

 press it quite down, and we know that the rise of the water is being 

 resisted by the invisible air in the tumbler. Sometimes we can 

 ourselves almost lean up against the rapidly moving invisible air ! 

 Still more convincing is the test by which Lavoisier founded mod- 

 ern chemistry the test of the balance. For every invisible mate- 

 rial has its weight, even the hydrogen, which is fourteen and a half 

 times lighter than air, and therefore lifts the balloon so easily. 

 We do not need to go any further in indicating how chemistry 

 demonstrates the invisible, but it would be dull indeed not to refer 

 to the modern demonstration of invisible gases by turning them 

 into liquids or solids. 



3 



Liquefaction of Gases 



Modern science has given us a vivid picture of that condition 

 of matter which we call gaseous. Professor Clerk Maxwell com- 

 pared the molecules in calm air to a swarm of bees, when every 

 individual bee is flying furiously, first in one direction and then in 

 another, while the swarm, as a whole, either remains at rest, or sails 

 slowly through the air. But we have to add to the picture the 

 collision of one bee with another bee, for a molecule can travel only 

 a short distance (its mean path) without striking another. In- 

 deed, as Clerk Maxwell calculated, the number of collisions which 

 a molecule must undergo in a second must be reckoned by thou- 

 sands of millions. In his famous discourse on molecules he spoke 

 of the time it took for the smell of an opened bottle of ammonia to 

 pervade the room. The molecules of the ammonia have a velocity 

 of six hundred metres per second, but they are not able to spread 

 at that rate through the room. They strike against the molecules 

 of air and are delayed. "Each molecule of ammonia is so jostled 



