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THE POPULAR SCIENCE MONTHLY. 



water, but gradual, through every degree of 

 viscosity; and the laws aud properties of 

 viscous liquids are different from those of 

 water. Bodies will float on them which un- 

 der like differences in gravity would sink in 

 water. Now, "when the lighter surface 

 material of the earth had cooled sufficient- 

 ly, a crust would be formed, which, owing 

 either to its lighter state in the hot condi- 

 tion, or to its scoriaceous character and the 

 viscidity of the material beneath, would not 

 sink." The viscidity of the material im- 

 mediately 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 be- 

 came no heavier than the liquid, and would 

 soon reach a point when the liquid, being 

 of different composition, had a higher spe- 

 cific gravity than the crust, and no further 

 sinking could take place. We should thus 

 expect to have formed on the earth's sur- 

 face a crust which would never sink, or, if 

 it sunk, only for a comparatively shoi't dis- 

 tance, there to give rise to a solid crust float- 

 ing on a denser heterogeneous liquid. Con- 

 tractions and upheavals of this crust, not 

 unlike in tlieir effects what we sometimes 

 see take place in ice, would explain all the 

 volcanic aud earthquake phenomena that 

 need to be accounted for; while the as- 

 sumption of a liquid interior accords bet- 

 ter with the facts of petrography than any 

 other that has been made. This assump- 

 tion, according to Dr. Wadsworth's con- 

 ception, is of "a heterogeneous, viscid, 

 elastic, liquid interior, irregularly inter- 

 locked with and gradually passing into a 

 lighter heterogeneous crust." 



Valne of School Recesses. — Hard as it 

 is to believe, the idea of dispensing with 

 recesses in school has gained so much cur- 

 rency among American teachers, that occa- 

 sion has been found for bringing in a com- 

 mittee 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 ex- 

 posure ; 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 teach- 

 ers. 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 opportu- 

 nity to relieve physical wants, and affords 

 a means of ventilating the school-room with- 

 out 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 in- 

 tercourse; 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. — Phys. 

 icists 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 condensa- 

 tion 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 sup- 

 pose chat the spherules are hollow and 

 contain air, like soap-bubbles, and desig- 

 nate 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 vapor- 

 ization of water, these films of liquid can 

 separate from the surface and at once en- 

 velop small volumes of air, so as to form 

 the profusion of microscopic bubbles of 

 which the fog is composed. M. Felix Pla- 

 teau 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 



