It's been a while since I wrote a post in this series. And these days it's more like "58 Years of Science" but I an going to continue to stick with the original theme anyhow. This is the tenth post in the series. You can find an index to all the posts in the series at http://sigma5.blogspot.com/2017/04/50-years-of-science-links.html. I update that post every time I add a new entry to the series.
I take Isaac Asimov's book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was when he wrote the book (1959 - 60). In these posts I am reviewing what he reported and what's changed since. For this post I am starting with the chapter Asimov titled "The Origin of Air". I will then move on to the next section called "The Elements" and discuss the chapter he called "The Periodic Table".
There is nothing static about the composition of air if we look across the entire history of the Earth. There are processes that add to it and processes that subtract from it. Asimov starts his discussion with the latter. What's on top of the air? Nothing! So why hasn't it all rushed away? Asimov's answer is "escape velocity" and it's the correct one. Particles need a certain amount of speed to escape the pull of Earth's gravity. It turns out that only a tiny fraction of air molecules have the required speed (6.98 miles per second, if you ignore air friction, etc.).
Asimov launches into a very sophisticated discussion of all this. And he makes a critical observation. It is far easier for light molecules to escape than heavy ones. Oxygen and Nitrogen, the primary constituents of air, are heavy. Hydrogen (you can make it by splitting water molecules) is light. So the tiny amount of air that leaks away every year is made up mostly of Hydrogen. And that means there is essentially no Hydrogen left in Air. (Since a water molecule includes a heavy Oxygen atom little water vapor leaks away so the Hydrogen that is bound up in the water in the air is still with us.)
If we contrast Earth to Jupiter and Saturn we see a big difference. Both of these far more massive planets have much stronger gravitational fields. As a result they have very high escape velocities and thus have been able to hang on to their Hydrogen (and Helium, the second lightest element). Hydrogen is the most common element in the universe. (Helium is the second most common.) So it makes sense that the atmospheres of Jupiter and Saturn have lots of both. And the early Earth likely did too. But Earth's mush weaker gravitational field has let it all leak away over Earth's lifetime.
Asimov correctly observes that hotter molecules move faster than colder ones. So a hot atmosphere should leak away much faster than a cool one. This feeds into a discussion of how the solar system was created. A theory of the time posited that some catastrophe like two stars passing close by might be how it happened. Asimov shoots this down by observing that the Earth has an atmosphere and proceeding from there. Good enough so far. But then he launches into the then prevailing theory of the origin of the solar system.
The Sun throws off a lot of heat. This makes it hard, the theory goes, for Hydrogen and Helium to condense into a planet that is close to the Sun. So "gas giant" planets would form in the outer solar system and rocky planets composed of "refectory materials" (stuff with a high melting point) would form close to the Sun. This handily explains why Mercury, Venus, Earth, and Mars (all refectory planets) formed close to the Sun while Jupiter, Saturn, Uranus, and Neptune (all gas giants) formed further out. (Pluto is an exception that we will just ignore.)
This all worked just fine until the Kepler satellite (launched well after Asimov's book was written) and other exo-planet finders came along and found lots of gas giants in orbits that were extremely close to their respective suns. In many cases the orbits of these gas giants are closer to their respective suns than Mercury is to our sun. All these close in gas giants orbiting other stars means that the standard model for the formation of solar systems is wrong. New models have recently come to the fore but it's early days so things will likely change as more is learned and more study, modeling, and theorizing is done.
The planetary formation model of the day is still holding together. It seemed pretty solid to scientists of the time. It is viewed as more wobbly by contemporary scientists due to the way a planetary formation model interacts with a solar system formation model. In any case, here it is, compliments of Mr. Asimov.
Material would clump together eventually growing large enough for gravity to kick in. At that point two things would start to happen. The attraction of material to the clump would accelerate so the planet would start growing quickly once it reached a critical mass. The other thing that would happen is that heavy things would start sinking toward the center and light things would float to the top. So Iron, for instance, would collect at the core while gasses would rise to the surface. Interestingly enough, the material that makes up the "crust", the Earth's surface that we can see and touch, is, for the most part, made up of light materials. So the distribution of heavy material toward the center and light material toward the surface that we see with Earth aligns with this idea.
As noted above the lightest gases would escape. We still have a considerable amount of Hydrogen around because it is locked up in water molecules and other chemicals. And this sort of thing complicates the situation. Asimov estimates that only one part in seventy million of the Earth's original reservoir of Neon is left because Neon doesn't combine to make molecules. Oxygen likes to combine into molecules and is relatively heavy so one in six Oxygen atoms is still around. Nitrogen falls somewhere in the middle so one part in 700,000 of Nitrogen remains. The larger point is that the original composition of Earth and its current composition are quite a bit different. Scientists figure their job is not done until they can account for all of this. And one particular puzzle is water.
How much water has accumulated on Earth over its lifetime and how did the current amount come to be? These are still very active subjects of investigation. Asimov briefly mentions two then popular theories. In the first water was squeezed out of rocks early in Earth's life. It then was turned to atmospheric vapor since things were hot at the time. As things cooled it condensed and formed the oceans much as they are now early in the life of the Earth. Another theory goes for gradualism. The water was squeezed out of rocks slowly over time. (BTW, modern rocks actually contain a lot of water.) So according to this theory the oceans grew to their current size slowly over a long period of time.
There is a third potential source of water that Asimov doesn't mention. Stuff continuously rains down onto Earth from space and it contains a decent amount of water. This process was unknown in Asimov's time because the instruments necessary to study this sort of thing didn't exist back then. Was this process the source of a lot of the water we now see? We don't know. The basic "water" problem is still far from being solved. We now have a lot of data on the level of oceans going back at least hundreds of thousands of years. We know their total volume changed little over that period. So if some process is gradually adding or subtracting water to the oceans it is a very slow process.
We do know that the Earth's atmosphere had a quite different composition when the planet first formed. As I have noted elsewhere (see http://sigma5.blogspot.com/2018/07/deep-genetics.html, for instance) the atmosphere of the early Earth contained lots of Carbon Dioxide. (Venus contains lots of Carbon Dioxide to this day. The result is a surface temperature of 800 degrees.) A study of the geologic record indicates that vast amounts (billions of tons) of Sulfur precipitated out of the atmosphere at one point. Prior to this it is likely that a significant component of air was Sulphuric Acid.
Later a vast amount of Iron (again billions of tons) precipitated out. The effect of that much Iron being in or adjacent to the air and the oceans (rather than being locked up in rocks as it now is) is not as obvious as that of Sulphur but it is important. Scientists have figured out various tricks for determining the amount of Oxygen in the air. Early in the life of the Earth the amount was effectively none. Now it makes up about 20% of what's in Air. (Almost all of the rest is Nitrogen.) Scientists now have a better idea of the history of the composition of air but I don't know any more than I have noted above.
One final note before moving on, Asimov speculated that the air pressure on Mars was about 10% that of Earth. We now know it is far lower. Even so, Mars has weather in the form of dust storms, mini-tornadoes, and other phenomena. Now on to "The Periodic Table".
The idea that there are four elements: Earth, Air, Fire, and Water, goes back to the ancient Greeks and may go back even farther. To this the Greeks added a fifth element. The heavens were composed of Ether (often spelled Aether with the "a" and the "e" smashed together). Asimov correctly characterizes the Greek approach as "theoretical and speculative". As such, they felt no need to subject their theories to experimental verification. The first group to actually subject this kind of thinking to experimental verification was, of all people, the medieval alchemists.
They started adding elements to the list. Mercury was responsible for metallic properties. Sulfur imparted flammability. Salt imparted resistance to heat, according to one of the best of the medieval alchemists, Paracelsus. With this theoretical framework it made perfect sense to believe that a "philosopher's stone" existed that would turn "base metal" (lead) into precious metal (gold). Success would produce vast wealth so quackery eventually became rampant. Kings could be induced to provide the medieval equivalent of "research grants" in the reasonable expectation of a substantial return. This quackery eventually destroyed the reputation of alchemists and alchemy but there were many honest and intelligent practitioners. One of them was Sir Isaac Newton.
Eventually the ethical alchemists started calling what they were doing chemistry and, in an effort to distance themselves from unethical alchemists, disavowed any attempt to find the philosopher's stone. Boyle wrote "The Skeptical Chymist" as part of this distancing process. He is now considered a serious scientist and is credited with discovering a modern scientific tenet, "Boyle's Law". It states that if the temperature of a fixed amount of gas is held constant then an increase in pressure will result in a decrease in volume and vice versa.
He also proposed a very modern definition of the word "element". It is "a basic substance which can be combined with other elements to form 'compounds' and which, conversely cannot be broken down to any simpler substance". We can now be more precise due to our subatomic view of the proceedings. But as a practical matter the definition still works well.
This definition now seems obvious. The problem then was that most compounds were not very pure and these impurities confused things massively. But over time scientists got better at creating pure samples and getting predictable, repeatable, results. That slowly led to progress in classifying elements and compounds. Cavendish demonstrated that water was a compound consisting of Hydrogen and Oxygen. Lavoisier showed that air consisted of Oxygen and Nitrogen (the rest of air's constituents are present in such low concentrations that they could then be effectively be ignored). The list of actual elements grew slowly as "compounds" like Tin were added to the list of elements and "elements" like Salt were added to the list of compounds.
And technical advances were necessary. Electrolysis was necessary to break down compounds like line and magnesia (Oxygen plus a new element Magnesium). On the other hand Chlorine was initially thought to be a compound composed of Hydrochloric Acid (assumed incorrectly to be an element) and Oxygen. And an old concept that dated back to the ancient Greeks soon became relevant. All matter is composed of small indivisible particles called "atoms", the concept opined. The concept dates back to Democritus but was resurrected in modern form by Dalton.
He observed that the rules for combining many gasses could be explained if it was assumed that certain gasses were elements and other gasses consisted of compound particles composed of a certain specific number of atomic particles of this elemental gas, a certain specific number atomic particles of that elemental gas, etc. This "atomic" idea was soon expanded to cover all elements, not just gasses. He also concluded that one of the most important properties of an atomic particle of a specific element was its weight, what we now call its "atomic weight".
This led to some extremely clever techniques being developed for determining the relative weights of various elements. An atom of Oxygen weighs almost exactly 16 times as much as an atom of Hydrogen, for instance. It was far beyond the capability of scientists of the time to determine the absolute weight of a single atom. The obvious solution was to use these ratios. The only thing necessary was to pick the base number. After several tries it was decided to arbitrarily decree that Oxygen weighed 16 and go from there.
At the time nothing was known about isotopes. An element can exist in several forms. They all have the same chemical properties but different weights. Many elements consist largely of a single isotope so, for that element, there isn't a problem. But Oxygen isn't one of them. There is lots of what we now call O-16. But there is also a goodly amount of O-18. So a mix of isotopes of Oxygen didn't work well as a standard. In 1959, too late for Asimov's book, the standard was changed so that the atomic weight of the C-12 isotope of Carbon was set to exactly 12. The "Dalton ratios" were then applied to come up with a revised atomic weight for each isotope of each element.
The first "picture" of a single atom was taken in 1955 by Mueller. That was a big deal at the time. But we can now take movies of groups of single atoms on select surfaces. We can even move individual atoms around. Something called an "atomic force microscope" can measure forces between single atoms or small collections of atoms. What we can now do in this area would look like magic to scientists in the '50s. But back to our story.
The list of elements kept growing and growing. The urge grew to put this list into some kind of order so that it would be more manageable and useful. The first version of the list just ordered them by atomic weight. But in 1862 Cannizarro arranged them into rows and columns such that elements with similar chemical properties fell into the same column. Renia did the same thing independently but the idea did not catch on until Mendeleev (and others) came up with an improved version of the same idea.
What Mendeleev in particular did was to assign more importance to preserving the regularities of his table and less to putting them in order solely by weight. He emphasized the periodicities in his "periodic table". This led him to fix various problems he saw when he strictly adhered to weight order. If the chemical properties did not align properly when certain elements were placed where their weight indicated they should go he switched things around.
In some cases the generally accepted atomic weight was wrong. In others an element weighs more than it should for complex quantum mechanical reasons. We now use "atomic number", the number of protons an element has, instead of atomic weight to organize the periodic table. Each isotope of an element has a certain number of protons and a certain number of neutrons. Each of these particles weighs approximately one atomic unit. So O-16 has 8 protons and 8 neutrons and an atomic weight of 16. O-18 has the same 8 protons but 10 neutrons for an atomic weight of approximately 18 (the discrepancy is due to quantum mechanical effects). Chemical properties are determined by the number of protons and unaffected by the number of neutrons. And none of this was known at the time (roughly 1870).
What made scientists take Mendeleev's work seriously were the holes. He left three holes in his table because he could find no element that fit. Shifting things around to close the gaps messed up the orderly progression of chemical properties. So he left those slots empty and boldly predicted that elements would eventually be found to fill each empty slot. And they were. Gallium was discovered in 1875. Scandium was discovered in 1879. And Germanium was discovered in 1886.
In 1911 Barkla discovered that each element has a unique X-ray signature. Laue found that crystals could "diffract" (bend) X-rays. They were waves and waves have a wavelength. X-ray studies of elements led to a technique for determining an element's atomic number. A gap in the sequence of atomic numbers indicated that an element was missing. A number of gaps were found this way. Some were filled quickly. Some took considerably longer.
For a long time it was assumed that the list stopped at 92 (Uranium). Since then various "artificial" (so called because they were first created using various scientific techniques and were incorrectly thought to not exist in nature) elements have been created. At the time of the book the list had been extended to 102. Since then it has been extended to 118.
Since Asimov wrote this book both theory and practice in this area has advanced by leaps and bounds. Many scientific discoveries follow from better tools. And the tools have gotten much better. The energies available to scientists are now much higher. The distances and times that can be studied have gotten spectacularly smaller. The work-horse tool of the day was the synchrotron. This was a device that applied strong magnetic fields to cause charged particles to spin in circles inside the device. We how have the LHC. It is a circular tube 27 kilometers long which uses fantastically powerful magnets. Back in the day, the largest synchrotron fit into a single room.
Computer power and speed have also increased vastly. So computations that would have taken centuries on computers of that period can now be done in minutes. And tedious processes have been automated. If you shoot a charged particle through a special tank it will leave a trail of small droplets that can be photographed. These photographs can be analyzed by having graduate students look at them and take measurements. That was then. Now solid state devices are available that can much more accurately determine the path of a charged particle and instantly analyze it. This means that billions of paths can be examined where before it was hard to examine thousands of much more low resolution pictures.
These advances and some theoretical advances have allowed us to have a much more nuanced picture of elements, chemical reactions, and the subatomic world. But I am going to defer discussion of that until I reach the appropriate sections of Asimov's book.
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