The Outline of Science 



energy and velocity of these infinitely minute molecules. He 

 tells us that molecules of oxygen, at the temperature of melting 

 ice, travel at the rate of about 500 yards a second more than a 

 quarter of a mile a second. Molecules of hydrogen travel at four 

 times that speed, or three times the speed with which a bullet 

 leaves a rifle. Each molecule of the air, which seems so still in 

 the house on a summer's day, is really travelling faster than a 

 rifle bullet does at the beginning of its journey. It collides with 

 another molecule every twenty-thousandth of an inch of its 

 journey. It is turned from its course 5,000,000,000 times in every 

 second by collisions. If we could stop the molecules of hydrogen 

 gas, and utilise their energy, as we utilise the energy of steam or 

 the energy of the water at Niagara, we should find enough in 

 every gramme of gas (about two-thousandths of a pound) to 

 raise a third of a ton to a height of forty inches. 



I have used for comparison the speed of a rifle bullet, and in 

 an earlier generation people would have thought it impossible 

 even to estimate this. It is, of course, easy. We put two screens 

 in the path of the bullet, one near the rifle and the other some 

 distance away. We connect them electrically and use a fine 

 time-recording machine, and the bullet itself registers the time it 

 takes to travel from the first to the second screen. 



Now this is very simple and superficial work in comparison 

 with the system of exact and minute measurements which the 

 physicist and chemist use. In one of his interesting works Mr. 

 Charles R. Gibson gives a photograph of two exactly equal pieces 

 of paper in the opposite pans of a fine balance. A single word has 

 been written in pencil on one of these papers, and that little 

 scraping of lead has been enough to bring down the scale ! The 

 spectroscope will detect a quantity of matter four million times 

 smaller even than this; and the electroscope is a million times still 

 more sensitive than the spectroscope. We have a heat-measuring 

 instrument, the bolometer, which makes the best thermometer 

 seem Early Victorian. It records the millionth of a degree of 



