Page:Popular Science Monthly Volume 17.djvu/587

Rh capable of undergoing that fermentation. The human body may be considered as such a substance, and we may conceive of a water containing such organisms which may be as pure as can be as regards chemical analysis, and yet be, as regards the human body, as deadly as prussic acid. This is a terrible conclusion, but it is true; and, if the public are guided by percentages alone, they may often be led astray. The real value of a determination of the quantity of organic impurity in a water is that by it a shrewd notion can be obtained as to what has had access to that water. If it be proved that sewage has been mixed with it, there is a very great chance that the excreta of some diseased person may be there also. On the other hand, water may be chemically gross, and yet do harm to no one, the great danger being in the disease-germs.

Man in America.—Professor Flower, in a recent lecture on the "Anatomy of Man," before the Royal College of Surgeons, London, discussed at some-length the question of his origin on the American Continent. Till recently, opinions on the early peopling of America had been divided between the views that the inhabitants of this continent were a distinct indigenous people, and therefore not related to those of any other land; and that they were descended from an Asiatic people who, in comparatively recent times, passed into America by the way of Behring Strait, and thence spread gradually over the whole Continent. These theories have had to undergo considerable modifications in consequence of the discovery of the great antiquity of the human race in America, as well as in the Old World. The proof of this antiquity rests upon the high and independent state of civilization which had been attained by the Mexicans and Peruvians at the time of the Spanish conquest, and the evidence that that civilization had been preceded by several other stages of culture, following in succession through a great stretch of time. The antiquity of this quasi-historical period is, however, entirely thrown into the shade by the evidence now accumulating from various parts of North and South America, that man existed on the Western Continent, and under much the same conditions of life, using precisely similar weapons and tools, as in Europe during the Pleistocene or Quaternary period, and perhaps even farther back in time. Recent paleontological investigations show that an immense number of forms of terrestrial animals, that were formerly supposed to be peculiar to the Old World, are abundant in the New. Taking all circumstances into consideration, it is quite as likely that Asiatic man may have been derived from America as the reverse, or both may have had their source in a common center, in some region of the earth now covered with sea.

Illusions and Apparitions.—All illusive visions and apparitions are susceptible of a scientific explanation. They originate in some derangement of the brain and nervous system, and are for that reason most likely to occur to persons who are out of health. The apparent reality of some of these illusions is often wonderful, and might well prompt those who are not acquainted with nervous physiology, or who have not devoted careful attention to the subject, to refer them to something out of the common. Even while we are in perfect possession of our faculties, we imagine that we see objects before us as clearly as though they were actually present, or hear, with equal distinctness, sounds which have no real existence outside of ourselves. The explanation may be found in a simple study of the physiology of the nervous system, and shows that the illusions have a material basis. Our sensations are transmitted from the organ that receives them to the brain, and it is the brain, not the organ, that experiences them and is their seat. In the case of sight, it is the function of the eye to receive and adjust the rays of light coming from the object that we see, so that they shall produce an impression on the brain. The eye represents the lenses of the photographer's camera; but the brain corresponds to the sensitive plate which receives the image, and on which all subsequent alterations of the image are effected. Similar relative parts are played by the organs and the brain in the case of the other senses. Now, if a similar impression to that which is transmitted to the brain from the organ of sense is produced upon it by any other cause, the same kind of a sensation will result. This may happen when the brain