Page:Popular Science Monthly Volume 26.djvu/586

570 water, but gradual, through every degree of viscosity; and the laws and properties of viscous liquids are different from those of water. Bodies will float on them which under like differences in gravity would sink in water. Now, "when the lighter surface material of the earth had cooled sufficiently, a crust would be formed, which, owing either to its lighter state in the hot condition, or to its scoriaceous character and the viscidity of the material beneath, would not sink." The viscidity of the material immediately under the crust would prevent it sinking, even if it should become heavier and break up; or, if it began to sink, it would be heated and expanded till it became no heavier than the liquid, and would soon reach a point when the liquid, being of different composition, had a higher specific gravity than the crust, and no further sinking could take place. We should thus expect to have formed on the earth's surface a crust which would never sink, or, if it sunk, only for a comparatively short distance, there to give rise to a solid crust floating on a denser heterogeneous liquid. Contractions and upheavals of this crust, not unlike in their effects what we sometimes see take place in ice, would explain all the volcanic and earthquake phenomena that need to be accounted for; while the assumption of a liquid interior accords better with the facts of petrography than any other that has been made. This assumption, according to Dr. Wadsworth's conception, is of "a heterogeneous, viscid, elastic, liquid interior, irregularly interlocked with and gradually passing into a lighter heterogeneous crust." Value of School Recesses.—Hard as it is to believe, the idea of dispensing with recesses in school has gained so much currency among American teachers, that occasion has been found for bringing in a committee report against it, to the American Council of Education. The advocates of no recess claim that the adoption of their measure will promote the conservation of the health of the pupils, by preventing exposure; that it will tend to refinement by removing the opportunities for rude and boisterous play; that it will take away the opportunity for association with the vicious and consequent corruption of morals; and that it will make things easier for the teachers. The committee find in their report that the exposure to the weather during recess will not hurt, but be beneficial. It gives a change from the close, bad air of the school-room to the free air, with opportunity to relieve physical wants, and affords a means of ventilating the school-room without chilling the scholars; that the "rude and boisterous play" of recess is only a rehearsal of what is indulged in outside of the school-room, with the advantage that the teacher is present to restrain excess, and that it gives needed exercise; that moral corruption is not generated in the open practices of recess, but in secret intercourse; and that the teacher's office is not to make things easy for himself, but by every means in his power to promote the well-being of his pupils. Experiments with Fog-Particles.—Physicists are divided respecting the mode of constitution of the minute particles of the vapor with which a portion of air becomes clouded when it is cooled and a condensation takes place. According to one view, the invisible vapor resolves itself at the moment of condensation into minute, full, liquid spherules, the aggregation of which produces rain-drops. Other persons suppose that the spherules are hollow and contain air, like soap-bubbles, and designate them as vesicles, while they point to the fact that fog-particles may be observed to rebound from water-surfaces, or from dry bodies, as soap-bubbles do; but full drops of water will do the same under some conditions. One of the strongest objections to the vesicular hypothesis is the difficulty of explaining how the vesicles are formed. It is hard to conceive that, during the vaporization of water, these films of liquid can separate from the surface and at once envelop small volumes of air, so as to form the profusion of microscopic bubbles of which the fog is composed. M. Félix Plateau succeeded, with soap and water, in dashing off thin films of liquid in such a way that a part of the film resolved itself into full drops, while another part gave rise to bubbles. MM. Georges Sire and Minary obtained, by stirring a mixture of olive-oil