July 31, 2016 at 11:33PM

Today I Learned: 1) Did you know you have a blind spot in each eye? Did you know it isn't in the center of your eye? I didn't. Here's how you can find our own blind spot. You can do it right now. Get a piece of paper and a marker. Draw two dots, about a half inch wide each, about a hands-width apart. Hold the paper in front of you at a short arm's width, with the dots forming a horizontal line. Close one eye. Focus on the dot on the same side of your face as the closed eye (if you close your left eye, focus on the left dot, and vice versa). Now slowly move the paper toward yourself, keeping your gaze focused on the dot. At some point, the other dot will disappear from sight, then reappear. It's a pretty small spot, so you'll need to move slowly. 2) Acid rain has apparently gotten rather better since the 80s and 90s, when it was really big in the public eye. At least, in America it has. In 1992, average rain pH on the east coast of the US was between 4 and 5 -- around the acidity of apple cider vinegar. Moreover, you can tell whether a rain storm on the east coast came from the Atlantic or from inland by the pH -- Atlantic storms have a pH betwen 5 and 6, while inland acid rain is closer to 4. Since then, it looks like environmental regulations and cap-and-trade limits on sulfide and nitrate emissions have brought acid rain levels down considerably. SO2 emissions have been successfully reduced to levels mandated by late 80s legislation *ahead of schedule*. This, along with the fluorochlorocarbon ban in 1987, serves as a reminder that government intervention *can* mitigate or even reverse severe environmental damage caused by gas emissions, as long as there's the political capital to put necessary measures into place. (Now, acid rain in *China* is another matter: http://ift.tt/2aHOvvB) 3) Ever wonder why we have mines? To have a mine, you have to have a big old pocket of some valuable metal, in one spot, easily accessible (relatively speaking). Why are there deposits of metal? Why isn't all of the rock on Earth a well-mixed, homogeneous mass? Well, for the most part, it is -- lots of silica and iron and magnesium and some other metals, mixed up and spread around. Metal deposits typically happen when 1) some event brings up metals from the mantle and 2) some kind of formation exists to transport those metals to 3) a formation that can trap or accumulate those metals. Take copper ore, for example. Copper ores are usually formed from volcanic activity or some other geologic activity that brings a lot of molten magma up from the core. Often the magma ends up running through underground tunnels, where it starts to cool. The magma doesn't actually have much copper in it -- it's mostly silica and iron, like the rest of the crust. As the outside of the magma flow cools, it crystallizes into iron-silica rock crystals. Copper ions don't fit well into iron-silica rock crystals, so they're excluded from that crystallization process. As the outside of the magma tube cools and crystallizes, it concentrates copper near the center of the tube, like salt concentrating in an evaporating tidal pool. Eventually the whole thing freezes, leaving a rich vein of copper metal (or,at the very least, copper-rich rock) at the center. Bonus rock fact: One of the unusual housing hazards is radon buildup. Radon is a radioactive gas with a half-life of about four days. It's an alpha emitter, which means it spits out high-energy protons when it decays, which are very damaging to biological tissue if it gets under the skin. It also tends to leach out of bedrock, and can accumulate in houses built on top of radon-rich ground. Wait a second, if radon has a half-life of four days, how is there any left in the ground? Turns out radon is a decay product of *uranium*, which is common enough in some soils to produce measurable amounts of radium, albeit very slowly. It's not a problem normally, but if a house is built in a way that acts as a radon sink... that's a problem.