Much like the others in the series. Asimov does the historical approach well, and even has an instant of dry humor. A timeline would have been nice.

Detailed review:

1. Atoms and Pressure
Hero points out that air has volume. Democritus has his atomic theory, much earlier. Robert Boyle does some experiments on the compressibility of air with an enormous J-shaped tube of mercury. Boyle also defines what it means to be an element. A little earlier than Boyle, Torricelli invents the first barometer. Blaise Pascal sends his brother-in-law up a mountain with a barometer, and his brother in law determines that the air pressure drops.

2. Gases
From the German word for chaos. Evaporation. Van Helmont, who predates Torricelli, defines gases by their behavior in a closed container, that is, the gas fills the whole container. How could he tell this? Van Helmont's "gas sylvestre", i.e., gas from wood, is just CO2, as is the gas given off by the transformation of heated limestone to lime. Because lime turns into limestone when it contact with air, Joseph Black, an 18th century British chemist, decided that air must contain some of this same gas. Conclusion: air is not just one element, but many. (Carbon dioxide is not a significant constituent, typically, just 0.035 percent). Black tried breathing on lime, and found that it turned into limestone faster that way, than if it was just left out in the ordinary air. Burning candles had the same effect as breathing people. A candle can not burn in Black's gas, and if a candle is left to burn in an enclosed space full of air, it will eventually go out. If lime is then added to the container, turning into limestone and presumably removing all of Black's gas, the candle still will not burn. This makes the composition of air seem more puzzling. But Daniel Rutherford takes up the problem, with phlogiston on his brain. The gas which can not be removed by lime, but does not allow a candle to burn he calls "phlogisticated air", but it is subsequently called nitrogen. Joseph Priestley comes up with another way to isolate a gas using a bath of mercury. Priestley worked with rusty mercury, and found out that it gave off a gas, when heated, in which stuff burned very easily. Still in phlogiston mode, he called it dephlogisticated air. It gets renamed, oxygen, meaning acid-producer, which is quite misleading, but these were chemistry's early days. Cavendish, who predates Priestley a bit, applies acids to metals and draws off a gas. Things don't so much burn in this gas as explode, and it is very light. Cavendish calls it "fire air", but later it is called hydrogen, because when things burn in it, water is produced. Again, a bad name. Around the same time as Priestley, Lavoisier gets the hang of conservation of mass. He also notices that sometimes the residue from burning is heavier than the initially burned stuff. He does a cool rusting experiment:

* Invert jar in water with a rustable thing at the top and a lot of air between the rustable thing and the water. The pressure of the air keeps the water from rising up in the container.
* Watch the stuff rust. Make sure that the level of water outside the container stays the same.
* Gradually, the water inside the container will rise. Why? Because something in the air is combining with the rusting thing to form the rust.
* Eventually, the water will stop rising. Why? Because rusting has stopped, because the thing in the air that combines with the rustable has all been used up.
* How much has the water risen/ the gas been reduced in volume? About 1/5th. So, there is something in air that takes up about 1/5th of its volume that attaches to metal when it rusts. Oxygen!

Lavoisier comes up with a whole new theory of combustion and kicks phlogiston out. Of course, things that have a lot of carbon in them, give off a lot of carbon dioxide when they burn. Wood turns to ash, which is much lighter than the original, because combustion has converted most of it to gas.

3. Molecules and Heights
Going back to Cavendish, he takes air and gets rid of all the oxygen and nitrogen cleverly, and finds, to his surprise, that there is a substantial amount of stuff left over, which he can't get to react w/ anything. Avogadro comes up w/ a law about the number of molecules in a given volume of a gas.

In 1783 the first hot air balloon goes up. Next year, a hydrogen balloon. Same year, the first scientific exploration by balloon. In 1804 an ascent of 4+ miles, and it seems that the composition of air is much the same their as at the surface, but not dense enough to breathe comfortably. In 1875, three men go up six miles, but two die. In 1892, unmanned balloons are sent up, with instruments. The troposphere and the stratosphere get their names.

4. Noble Gases and Ions
Lord Rayleigh tries to isolate nitrogen from various things, but it always seems heavier when isolated from air. Sir William Ramsay isolates argon just as Cavendish had, and uses spectroscopic methods to identify it as a new element. Then he goes looking for the other noble gases and finds helium (already discovered in the sun but not known to exist on earth). Then he gets lots of air and finds neon, krypton, and xenon. This work is much assisted by the newfound ability to liquefy air. August Picard constructs enclosed balloons and ascends eleven miles in a helium balloon wearing a funny hat. The air temperature rises as one ascends in the stratosphere. Then comes the upper atmosphere, which causes the pretty meteor showers and the aurora. Even at 1200 miles above the earth's surface, there is not a complete vacuum. Marconi bounces radio waves off the Heaviside layer, and the ionosphere is named. The ozone layer is sort of between the troposphere and the stratosphere, about 15 miles up.

5. Other Worlds
The moon has insufficient gravity at its surface to hold its gas molecules, so it has no atmosphere. Why? It's kind of cold out there, so they can't have that much energy. Mercury has no atmosphere, but Mars has some because it is colder. (In the illustration, Pluto is way too big, showing that estimates in the 80s far exceeded its actual size). The gas giants have atmospheres that are mostly hydrogen and helium. They are big enough, and it is cold enough out there, that these light gases can be retained. The inner planets have atmospheres of CO2 and nitrogen, where they have any, except for weirdo Earth. Titan's is mostly nitrogen and methane.

THE END ( )