Page:Popular Science Monthly Volume 17.djvu/588

572 is in an excitable or irritable state from ill health or any other cause, and is enough to explain all the phenomena under consideration. The visions most often correspond to our previous experiences, and therefore represent objects we know. Sometimes, however, the images are unfamiliar, and they are then referred to objects that we have seen, but have ceased to remember in our natural condition. The apparitions are thus explained as the creatures of our imagination, which, through some brain-disturbance, is enabled to project its visions forward on the seats of sense, just as the ringing in the ears, with which we are all familiar, is produced by some irritation of the hearing center of the brain.

Soils as Filters.—Dr. Victor C. Vaughan, of the University of Michigan, has described in "The Sanitarian" some experiments which he has made to determine the power of soils to prevent the filtration of organic matter in solution. They had reference to the questions, To what extent are organic substances removed from solution by filtration through the soil; and do different soils differ in their capability of thus removing organic matter? Urea was selected as the substance with which the experiments should be tried, and urine as the fluid with which filtration should be performed. The ordinary gravel soil of Michigan was found to produce but little effect in detaining the urea, while it soon became saturated; and the conclusion was drawn that the secretions from a family of six persons each day would be sufficient, when properly dissolved, to saturate more than seven cubic feet of this soil, and that only a few weeks or months would suffice, with a proper amount of rainfall, to saturate every cubic foot of soil to the depth of five or ten feet in a small yard. Gravel, however, is the poorest of filters, for the spaces between the particles allow the liquid to run through freely at certain points. Sand and loam exhibited a more satisfactory action, the loam more so than the sand, both these substances receiving a perceptibly larger quantity of urea before they were saturated. This is probably owing partly to their greater uniformity of constitution, in consequence of which water can not run as fast through them as through gravel, and partly to their greater porosity, by means of which matter passing through them is more closely exposed to the action of oxygen, the most efficient agent for the destruction of organic impurities.

Freezing of a Lake by Radiation.—A remarkable instance of the freezing of water in consequence of the radiation of heat was remarked in the Lake of Morat, Switzerland, after the cold weather of March last. The lake, of which three fifths of the surface had been covered with ice, was clear on the 8th of March, and the weather had become warm. During the night of the 10th of March, the thermometer did not descend to the freezing-point; yet on the morning of the 11th the lake was covered over with a thin sheet of ice. The Lakes of Neufchâtel and Constance were similarly covered. The freezing is accounted for by supposing it to have been occasioned by the rapid and great radiation of heat which took place on a perfectly clear night. An intense degree of cold had been necessary to cause the lakes to freeze during the cloudy weather of the previous cold spell, and the freezing was then very irregular and unequal.

Effects of Diseased Meat.—Mr. Julius Hardwicke, F. R. C. S., an English local medical officer of health, recently read a paper at a sanitary meeting on "Meat as Food for Man," in which he considered the effects of diseased meat on the human system. The evidence on this subject is of the most conflicting character. According to Dr. Letheby, enormous numbers of animals that died of rinderpest in 1863, and more recently of pleuro-pneumonia, have been sent to the London market and eaten without having produced any tangible effects; the Scotch eat "braxy" mutton with impunity, and, some say, even prefer it to sound mutton; and the people of Paris must have eaten much diseased meat during the siege, though we have no account of their having suffered from the effects of it. The symptoms or complaints of those supposed to be suffering from having eaten diseased meat are very similar to those occasioned by the use of putrid meat.