APPENDIX B. RAPID DIFFUSION OF CARBONIC ACID. In a paper published in the "Zeitschrift für Biologie," for 1873, Pettenkofer alludes to a popular idea which, in spite of theoretical and direct evidence to the contrary, still has currency; namely, that in a badly-ventilated room the worst air collects at the bottom, on account of the high specific gravity of carbonic acid. He then gives the details of some experiments which illustrate in a remarkable manner the very great rapidity with which a heavy gas diffuses into another gas with which it is in contact, even when the heavier gas is situated below the lighter. These experiments were performed at a mineral spring at Marienbad, where there is a constant evolution of carbonic acid from the surface of the basin in which the water is confined. This basin is quite large, and is used for bathing; it is covered by a light wooden building, and the evolution of carbonic acid is very considerable. It is estimated that, if no diffusion took place, at the end of an hour the building would contain over the surface of the water a layer of carbonic acid 12 feet in thickness. Pettenkofer found the gas escaping from the water contained 70 per cent. of carbonic acid; the air 3 inches above the surface contained 31 per cent. of carbonic acid; the air 10 inches above the surface contained 23 per cent. of carbonic acid; the air 40 inches above the surface contained 2 per cent. of carbonic acid; the air 55 inches above the surface contained less than per cent. of carbonic acid. As the carbonic acid is evolved continually, we see how rapidly it diffuses into the atmosphere, or how rapidly the gases of the air diffuse into the carbonic acid layer, so that, at a height of 40 inches above the surface of the water, the carbonic acid amounts to only about 2 per cent. of the volume of the air, and this, too, in a space inclosed by a structure built of boards, and not freely open to the air. VENTILATION OF RAILROAD CARS. BY THEO. W. FISHER, M. D. WITH CHEMICAL ANALYSES, Br WM. RIPLEY NICHOLS, PROFESSOR OF GENERAL CHEMISTRY IN THE MASSACHUSETTS INSTITUTE VENTILATION OF RAILROAD CARS. The object of this paper is briefly to call public attention to the insanitary condition of the passenger-cars on our railroads, and to introduce some tests and experiments on the air in smoking and other cars made by Prof. W. R. Nichols, of the Massachusetts Institute of Technology, at the request of the State Board of Health. This subject of car-ventilation may seem trivial when we consider the brief period of occupancy in individual cases, but even in this point of view, it is of some importance; and to that quite large class of persons who spend several hours daily in the cars, it is of the most vital interest. The American people are eminently a travelling public, and the aggregate of time spent in this way is worth considering. Massachusetts, according to the Railroad Commissioners' Report for 1873, has 1,735 miles of railroad, or about one mile to every 879 inhabitants, and to every four square miles of territory. The number of passengers carried annually is over forty-two millions. These facts show the importance of making all public conveyances equal, at least, in their hygienic condition, to our school-houses and other public buildings. That this is not the case, the travelling public has daily sensible demonstration. Universally defective methods of heating and ventilation of steam-cars lead to bad air, oppressed breathing, hot, heavy and aching heads, cold feet, coughs and debility, to say nothing of the discomforts of heat, dust and bad odors in summer. To these are added the deleterious effects of the concentrated fumes of tobacco, when the traveller is driven for a seat to the smoking-car. A striking illustration of how bad the condition of things may become, is found in a monograph, by the Baron N. de Derschau, a Russian engineer, upon Heating and Ventilation of Railway Cars (Paris, 1871). An experiment was made on an American car, running as third-class between St. Petersburg and Moscow, during the winter of 1866. The car was 50 feet long, and carried 80 passengers. The outside temperature at starting was 22° F. below zero, the inside 16° below, and there was no means of heating. Observations were made hourly, with the following result: The temperature rose from the accumulation of animal heat until, at the end of nine hours, it was, in the upper part of the car, 210 above zero, while on the floor it was still 6° below. The carbonic acid increased to alarming proportions; viz., from .140 per cent. at starting to .940 per cent. the last hour! The hygrometer, as well as the frost on the windows and the fog in the air, showed that the saturation point for moisture had been reached. The experimenter left the car at the ninth hour physically unable to continue his tests. This was, of course, an extreme example of what exists in a less degree in every ill-ventilated car. In cold and rainy, or in hot and dusty weather, the opening of windows is impracticable, and passengers suffer the insidious effects of bad air in avoiding the more obvious dangers from dust and draughts. The opening of doors at stations affords but little relief, since the cars are not then in motion so as to create a thorough draught. The smoking-car is a purely American institution. In England and France, smoking is forbidden in the first and second class carriages, but is connived at by the guards, on the payment of a small fee, if no one in the compartment objects. In Germany, smoking is so universal that a contrary custom prevails, and smoking is allowed everywhere, except in certain compartments marked "Für nicht Raucher" (no smoking), where tourists and ladies may avoid the fumes. of pipe and eigar, if they wish. Our custom is to collect all smokers into one car, thus concentrating the products of burning tobacco, which might otherwise be diffused through the whole train. The bad hygienic condition of these moving fumatories must be more or less familiar to all. The fact that the air is |