Page:Popular Science Monthly Volume 22.djvu/734

716 the fall would be at least to 50° below zero of Fahrenheit. "We see," says Professor Langley, "if these results be true, that the temperature of a planet may, and not improbably does, depend far less upon its neighborhood to or remoteness from the sun than upon the constitution of its gaseous envelope; and, indeed, it is hardly too much to say that we might approximately indicate the constitution of an atmosphere which would make Mercury a colder planet than the earth, or Neptune as warm and habitable a one." A much greater value than has hitherto been accepted appeared to be given by the observations to the solar constant, amounting to one half more than that determined by Pouillet and by Herschel near the sea-level, and even to more than the recent values assigned by M. Violle. The bolometer observations at the summit and base of Mount Whitney indicate a different distribution of solar energy at the upper station from that which prevails at the lower one. They also indicate, as the author states in a communication to the French Academy of Sciences, that only one quarter of the solar energy which vivifies the world is found in the familiar field of the visible spectrum and the ultra-violet; and that the other three quarters are found in the infra-red. Thus the action of our atmosphere, and, as is inferred from the observations, that of the solar atmosphere, is to absorb the short rays more than the long ones. The real color of the photosphere is blue; and "white light is not the 'sum of all radiations,' nor even of all visually recognizable ones, but a composition of the small groups of special rays, which, starting from this essentially blue sun, by virtue of their large coefficients, and by a kind of survival of the fittest, have struggled through the solar and terrestrial atmospheres to us, while others of short wave-length have failed on the way."

Infectious Consumption.—Dr. Alexander McAldowie has considered the much-debated question whether pulmonary consumption is an infectious disease in the light of his own infirmary and private practice. lie is of the opinion that it is infectious, although it is not so frequently communicated by infection as it would be were the lungs less well protected than they are against the access of germs. He mentions four cases where the wife, previously healthy, and with no family history of tubercular disease, became affected while attending to her phthisical husband, and two cases in which persons suffering from the pneumonic form of the disease appeared to communicate the tubercular form to healthy persons. Phthisis is not often communicated in this manner by ordinary intercourse, because the germs are sifted out in the air-passages by the vibrating action of the cilia situated there, and are removed by expectoration. The germs floating in the air are, moreover, commonly dry, and of feeble infective power. The lungs are liable to infection only when the inhaled germs escape the filtering action of the bronchi and reach the air cells, where they come in contact with a surface highly favorable for their absorption. This happens only under exceptional conditions. The parts of the alveoli most exposed to the attacks of inhaled germs are those near the entrance, at the points where the small bronchial tubes lose their cylindrical character and become covered on all sides with the cells; and pathological observation has proved that these are frequent starting-points in phthisis.

Subterraneous Effects of Atmospheric Pressure.—Hardly sufficient account has been taken of the variations in the pressure of the atmosphere as a force competent to produce important effects within the earth and on its surface. The pressure on a man's body amounts to thirty thousand pounds, and that exerted upon a table ten feet long and five feet wide is equivalent to more than one hundred thousand pounds. In both these cases the pressure varies alike on all sides, and changes are not directly felt; but the cover of an air-tight box, the pressure in the interior of which could not vary, would act very differently, and would respond to the slightest changes. The crust of the earth probably—certainly where cavities exist—is like such a cover. The consideration of this fact may help to explain the connection which many persons think they have found between earthquakes and coal-mine explosions and low stages of the barometer. A part of the weight of