Letters
Only in the north It is not clear in the fine article on volcanoes (“Disaster goes global,” SN: 8/30/08, p. 16) how dust from the eruption of Huaynaputina, well south of the equator, in 1600 could affect only the Northern Hemisphere. David Bronson, Biddeford Pool, Maine For one thing, there’s less real estate in the Southern Hemisphere to have been affected. Also, the apparent lack of agricultural effects probably stems from population distribution at the time this eruption popped off. Australia was inhabited only by Aborigines until 1787 or so, and Cape Town, South Africa, wasn’t settled until the mid-1600s. There’s also a dearth of written records for the hemisphere from that time. Research examining tree rings and other climate proxies at Southern Hemisphere sites would probably show effects from the eruption in that half of the world too. — Sid Perkins Not so small Contrary to the title and the first sentence of the article, hydrogen is not “the smallest atom of them all” (“Spotting the smallest atoms,” SN: 8/16/08, p. 7). Atoms with more numerous electrons display decreasing radii. As there is one proton for each electron and because the size of the proton is negligible compared with the atomic radius, the increasing number of protons in the nucleus increases the positive charge of the nucleus, drawing the electrons into tighter orbitals. Thus helium, with two protons, has the smallest atomic radius and is followed by hydrogen. Simcha Pollack, Jamaica, N.Y. The size of atoms can be measured or calculated in different ways, says Jannik Meyer of the University of Ulm in Germany. Under a transmission electron microscope, atoms “look” larger if their nuclei have more electric charge, which makes the atoms scatter the microscope’s electrons. “In our work all that matters is how much the atom scatters electrons,” Meyer says. “Hydrogen is the lightest element, and it has the smallest charge.” — Davide Castelvecchi
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