Page:Popular Science Monthly Volume 6.djvu/782

762 with carbonic acid, and the only means of destroying that poisonous gas is found in plant-agency. Hence, if the atmosphere of a city were to be inclosed within impermeable walls, and there were no growing plants within the inclosure, the air would quickly become irrespirable. But of course the air is nowhere thus walled about, and hence the deleterious gases it contains are dissipated and carried away by the unceasing movement of the atmosphere to other regions where an abundant vegetation may deprive it of its carbonic acid. Still, there is no doubt that this purification of the air is accelerated by the presence of vegetation in the cities themselves. The writer in the Garden asserts that "Paris has now so large a number of parks, and its streets and boulevards are so profusely planted with trees, that the death-rate has been thereby reduced from one in thirty-four as it formerly was, to one in thirty-nine as it now is."

But trees are further of service in shading gutters and road-ways, thus materially retarding and preventing the action of the sun in producing noxious fermentation. Then, too, the roots of the trees take up large quantities of such matters as are washed by the rains into the interstices of the pavements. Besides these direct sanitary benefits, we must also take note of the comfort derived from the shade of the sidewalks. Last, though not least, the beauty of our cities would be greatly enhanced by the planting of trees in the streets. The author recommends the planting of the sunflower on the Harlem flats of this city. By this means the poisonous gases arising from the decaying garbage used for filling these flats would be neutralized far more effectually than by the application of either "injunctions or disinfectants."

Magnetism and the Imagination.—Dr. Volpicelli, in a communication to the French Academy of Sciences, describes certain experiments made by him to determine whether a magnet can have any influence upon persons of nervous constitution. The first person experimented on was a patient of the hospital Santo Espirito, in Rome, whom the sight of a magnet was sufficient to throw into convulsions. Volpicelli brought with him a simple piece of unmagnetized iron; this, however, produced all the effects attributed to the magnet. The second experiment was made on a person similarly affected with nervous disorder. Volpicelli placed a magnet in this person's hand, and soon the super-excitation was such that it had to be taken away. A few days later the subject of this experiment presided at a meeting of scientific men. All unknown to him, magnets had been introduced into his chair, into the drawer of his table, under his feet—in short, all around him. The meeting lasted for two hours, and, at its close, on being asked how he felt, he declared that he was perfectly well. "It appears to me," continues Dr. Volpicelli, "that these two experiments are sufficient to prove that magnetism has no effect upon the nervous system, and that the cause of the effects produced by the presence of a magnet is to be attributed only to the imagination. As I have shown, if we bring one or more powerful magnets near to a patient without his suspecting their presence, no appreciable effect is produced. For the physiologist, the most interesting circumstance connected with these experiments is the diversity of effects produced by the imagination in nervous subjects when they see a magnet, or suppose the presence of one. The diversity of these effects will, perhaps, lead to the discovery of some new truths."

A Lost Species rediscovered.—How sad the idea of the loss of a species! Suppose our robins were reduced to a single living specimen? When inevitable death should come, the going out of that one individual life would be the extinction of its race forever. There is the typical fact of the disappearance of the dodo. And at home we have the equally remarkable fact of the extinction of that noble shore-bird, the great auk. It is now fifty-five years since Major Long's expedition returned from the Rocky Mountains, bringing many unknown forms of life. Of this expedition Thomas Say was chief zoologist. Among the many new species was one especially of the Cicindela, or tiger-beetles, those beautiful insects which have always been favorites with the entomologists. Say described, and named it Cicindela limbata. At that time