Page:Popular Science Monthly Volume 23.djvu/880

860 and by which their foot-prints are distinguished, are wholly absent from the impressions. The absence of any evidence that the maker of the tracks had more than two feet is also insisted upon. "The curve of the foot is so regular and so constant as to show that in every instance the hind-foot—if of a quadruped—was at all times placed exactly upon the forward foot, or that both the forward and the hind feet were of exactly the same form—conditions which, to say the least, are extremely improbable." Evidence appears to be afforded in the shape of the tracks of one or two of the series that they were made by a yielding material which, like leather softened by moisture, gave way and was bent up at the sides. Unless something of this kind is admitted, the tracks must be believed to have been made by animals of different species. Other variations in the shapes of the tracks may be more readily accounted for by supposing them to have been made by sandals of different cuts than by quadrupeds having differently shaped feet. Dr. Harkness accounts for the width of the straddle, which has been urged against the human origin of the tracks, by suggesting that it would be cue of the natural results of the exertion of walking in mud with the feet encumbered by such an unwieldy load as the enormous sandals.

Hygienic Qualities of Electric and Gas Lights.—Mr. B. H. Thwaite suggested, some years ago, that the intense heat developed in the arc electric light would produce a rearrangement in the gaseous contents of the atmosphere, by changing a mechanical combination into a chemical one, with the resultant development of deleterious nitrogen oxides. Mr. Wills, F. C. S., showed by experiment that ten to twelve grains of nitric acid were developed in an hour by the electric lamp. This rearrangement of gases is not produced in the incandescent lamps, for, besides the less development of heat, the filaments are kept in a vacuum. Hence, per se, the electric light of the incandescent type is hygienically satisfactory; but neither the incandescent nor the arc electric light assists vegetation. Besides the nitrogen oxides produced by the arc-light, says Mr. Thwaite, probably as much carbon dioxide is produced for the same illuminating power as is produced by the combustion of coal-gas. In both lights, the luminosity proceeds from the same cause—carbon heated to incandescence. The light produced by incandescent lamps is in almost perfect accord with the laws of visual or ocular hygiene, for it permits a choice of colors, but the arc-light is not so satisfactory, because* it induces fatigue by its variations and its glare. The products of the combustion of coal-gas are aqueous vapor and carbon dioxide, with sulphuric acid resulting from the oxidation of the bisulphide of carbon contained in the gas. Besides these, nitrogen is set free from its mechanical combination with oxygen, but it is practically harmless. These gases may be removed by putting over the burners pipes for conveying them to the open air; and, if this were done, as it ought always to be done, the greatest disadvantage of the system of coal-gas lighting would be removed, and adequate ventilation would at the same time be provided. With regenerative burners, the intensity of combustion could be increased to such a degree that the light would be white and neutral, permitting colors of the most delicate hues to be easily distinguished. We should then have a light not only hygienically perfect, but, to the extent that it is utilized for assisting ventilation, superior in that respect to the best electric light.

Aëration of Peaty Water.—Professor W. N. Hartley and Mr. Gerard A. Kinchan, of Dublin, have made experiments with reference to the alleged power of aëration to purify the water of rivers from peaty matter they may have in solution, from which they are led to deny the existence of such power to any measurable extent. Their first experiment was made at the Powerscourt Fall of the Dargle River, where the water descends 300 feet vertically, and mostly in the form of spray. Here, if anywhere, aëration should have been general and effective; yet analyses of specimens from above and below the fall showed no variation in the amount of carbon beyond what could be attributed to experimental error. Next, samples of the water of Carawaystick Brook were taken, from distances 1,600 feet apart,