Page:Popular Science Monthly Volume 43.djvu/146

13.6 there has been in connection with the Glacial period a succession of oscillations of the earth's crust nearly duplicating one another. Such oscillations seem to have occurred in various geological ages, as, for instance, during the coal period, when the successive coal beds were formed. And, indeed, much can be said in favor of the view that such an oscillation when once begun would perpetuate itself. . . . But our knowledge of these matters is too vague to reason of it with any confidence, as is that also of the other causes which have been suggested for the production of the phenomena of the period. In conclusion, it is sufficient to remark that our present state of knowledge on the subject seems so imperfect that it is not conducive to success in investigation to hold any theory as to the unity or duality of the period with great positiveness. Overconfidence on this point at the present time is likely to blind the eyes of the investigator, and to hinder progress both in the collection and in the interpretation of the multitudinous and complicated facts which everywhere invite our close attention."

Preservation of Leaves as Fossils.—In a paper on the Preservation of Plants as Fossils, Mr. Joseph F. James, of Cincinnati, names as one of the requisites to secure the preservation of any plant, that it must be in a position to be almost immediately covered by some material. A leaf or branch falling to the ground and likely to be exposed to the elements has a poor prospect of being preserved. But if it fall into the water and, sinking to the bottom of a lake or swamp or morass, be covered by mud or sand; or if it lie on the seashore and be covered by sand brought in with the tide, it may at least leave its mark. Or it may, through certain chemical properties it possesses, so act upon the stone on which it lies as to be preserved, not in actual substance, but as an intaglio. The author was impressed with the possibilities of the last process while walking along the street in the rain and looking at the fallen leaves on the pavements. He first noticed numerous irregular, discolored patches on the stone slabs. Looking more closely, he Bays: "I found that these discolorations had been caused by the leaves, which had left their impress on the stone. In many cases this impression was so distinct that there was no difficulty in recognizing the species. The leaves were those of the soft maple, one or two species of oak, tulip tree, and sycamore. There is here a possibility of the preservation of the remains of plants, or, at all events, of their impress upon stone, had it occurred under more favorable circumstances. But on a pavement, where people were passing constantly, the impressions were worn off and soon disappeared. The rain, however, did not seem to wash them away, so they were something more than mere surface markings." A similar phenomenon was observed and described in 1858 by Mr. Charles Peach in a paper on the Nature Printing of Sea-weeds, on the rocks of one of the Orkney Islands in Scotland.

Breath Figures.—Some interesting experiments are described by W. B. Croft in the production of "breath figures"—or latent impressions on contact of objects with glass and electrifying, which are made visible by breathing upon them. While there appears to be no limit to the durability of these figures if they are carefully protected, they usually become obscured by dust gathering on them after being often breathed upon. But certain changes or developments take place after the lapse of some weeks or months. In coin pictures, the object is near to the glass, but not in contact with it; for in the best specimens the rim of the coin keeps the inner part clear of the surface. Even if a coin only rest for a while on glass, an outline of the disk and sometimes faint traces of the inner detail will be produced when the spot is breathed upon. An examination paper, printed on one side, put between two plates of glass and left for ten hours, either in the dark or the daylight, will leave a perfect breath impression of the print, both on the glass that lay against the print, and on that which faced the blank side of the paper. Sometimes both impressions are white, and sometimes they are both black; or one may be part white and part black, or may even change while being examined. The impressions were very easy to produce during a sharp frost with east winds early in March, 1890. The following experiments easily succeed at any time: Stars and crosses of paper are placed for a few hours beneath a plate of