The dynamic factors in regeneration. First published in 1909 in The Biological Bulletin, Vol. 16: 265-276.

With the publication of the data here presented the series of experiments that I have carried out on Tubularia for several years may be considered as temporarily brought to a close. I take this opportunity therefore to sum up the evidence bearing on the problem of the formative factors of regeneration, as exhibited by this hydroid. In the course of my experiments tentative hypotheses have been proposed here and there that have at least served to suggest further experiments. The conflicting evidence sometimes inclined me towards one point of view, sometimes towards another; yet, all in all, the same general line of thought, if sometimes vague, can be traced through the attempts to analyze the results. It will be my endeavor here to bring more into the foreground those theoretical deductions that seem to me at present to be best in harmony with the experimental evidence...

Discovery of calcium in Mercury's atmosphere.

The composition and evolutionary history of Mercury's crust are not well determined. The planet as a whole has been predicted to have a refractory, anhydrous composition: rich in Ca, Al, Mg and Fe, but poor in Na, K, OH, and S. Its atmosphere is believed to be derived in large part from the surface materials. A combination of effects that include impact vaporization (from infalling material), volatile evaporation, photon-stimulated desorption and sputtering releases material from the surface to form the atmosphere. Sodium and potassium have already been observed in Mercury's atmosphere, with abundances that require a volatile-rich crust. The sodium probably results from photon-stimulated desorption, and has a temperature of 1,500 K (ref. 10). Here we report the discovery of calcium in the atmosphere near Mercury's poles. The column density is very low and the temperature is apparently very high (12,000 K). The localized distribution and high temperature, if confirmed, suggest that the atmospheric calcium may arise from surface sputtering by ions, which enter Mercury's auroral zone. The low abundance of atmospheric Ca may indicate that the regolith is rarefied in calcium.

Evidence for magnetospheric effects on the sodium atmosphere of mercury.

Monochromatic images of Mercury at the sodium D(2) emission line showed excess sodium emission in localized regions at high northern and southern latitudes and day-to-day global variations in the distribution of sodium emission. These phenomena support the suggestion that magnetospheric effects could be the cause. Sputtering of surface minerals could produce sodium vapor in polar regions during magnetic substorms, when magnetospheric ions directly impact the surface. Another important process may be the transport of sodium ions along magnetic field lines toward polar regions, where they impact directly on the surface of Mercury and are neutralized to regenerate neutral sodium atoms. Day-to-day variations in planetary sodium distributions could result from changing solar activity, which can change the magnetosphere in time scales of a few hours. Observations of the sodium exosphere may provide a tool for remote monitoring of the magnetosphere of Mercury.

Discovery of sodium and potassium vapor in the atmosphere of the moon.

Spectra of the region just above the bright limb of the Moon show weak emission features that are attributed to resonant scattering of sunlight from sodium and potassium vapor in the lunar atmosphere. The maximum omnidirectional emission flux above the bright limb is 3.8 +/- 0.4 kilorayleighs for sodium and 1.8 +/- 0.4 kiloray-leighs for potassium. The zenith column densities above the subsolar point are estimated to be 8 +/- 3 x 10(8) atoms cm(-2) for sodium 1.4 +/- 0.3 x 10(8) atoms cm(-2) for potassium. Corresponding surface densities are 67 +/- 12 atoms cm(-3) and 15 +/- 3 atoms cm(-3), respectively. The scale height for the sodium atmosphere is 120 +/- 42 kilometers, and for potassium 90 +/- 20 kilometers, which implies that the effective temperature of the sodium and potassium is close to the lunar surface temperature. The sodium density at the south polar region was found to be similar to that at the subsolar point, indicating wide-spread distribution of sodium vapor over the lunar surface. The ratio of the density of sodium to the density of potassium is (6 +/- 3) to 1, which is close to the sodium to potassium ratio in the lunar surface, suggesting that the atmosphere originates from the vaporization of surface minerals.