NIGHT-AIR IN CITIES.
To the Editor of the Popular Science Monthly.
YOUR April correspondent, Mr. C. W. Johnson, in his critique on my brief note which appeared in your pages of February last, either has misapprehended the issue I there made with Dr. Niemeyer's article in reference to the salubrity of night as compared with day air in cities, or he has stumbled upon the unwarrantable conclusion, solely through the bias of his own cerebration, that, because I did not mention, in the short compass of a few lines, all the forms of local urban insalubrity, therefore I do not believe some of them exist.
Having imagined that I had put the issue so plainly that no reader could foil to understand it, I am at a loss to spell it out any clearer for Mr. Johnson. Let me try it in the form of an interrogatory. Is the air in which we live, move, and breathe, more likely to be charged from local sources with pollutions that produce disease when comparatively still and calm, as during the night, during the day, when stronger currents are more apt to prevail? Dr. Niemeyer says that the night-air of cities is purer than the day-air, while I, in the language of Mr. Johnson, "assume to correct" his statement by showing by a fact or two that moving air is less apt to be intensely charged by any focus of corruption than that which is almost motionless and circumjacent to the source of contamination. The issue was made on the point as to the time most favorable for the atmosphere to acquire impurities the most largely from local sources of pollution, not as to the nature or forms these impurities may assume, or as to whether they are gaseous, granular, molecular, organic, or inorganic.
Yet, upon the putting of the issue thus plainly, Mr. Johnson represents me as believing that "the insalubrity of city air depends upon the amount of non-respirable gases (!) that may be diffused into the respirable ones"—whatever this may mean—"and is wholly independent of the condensible effluvia of the vaporous kind, or of the organic germ-dust that the heat and stir of the day may keep suspended."
In candor I must say that only those whose ideas are crude and vague upon the subject of the atmosphere as a vehicle of contagia or materies morbi would pen such a sentence. Does Mr. Johnson suppose that the contagium vivum of scarlatina, or of typhoid fever, or of small-pox, is anything like the heavy dust of the streets, or that it needs the stir of day-air to keep it suspended? Does he not know that the contagia of these diseases are so subtilely diffused and attenuated that the highest magnifying power is unable to isolate or detect them? An investigator now and then has imagined that he has discovered the spores of infection, but only one as yet has made a near approach to settled verification.
To relieve the mind of Mr. Johnson as to my benighted condition on the subject of germ-dust, I beg to refer him to the Philadelphia Medical and Surgical Reporter, January 13, 1877, in which I take the ground that the theory of a contagium vivum is the only tenable one by which to account for the genesis and spread of some infectious diseases. The unlimited self-multiplication of definite forms or special phases of force, as, e. g.. in small-pox, scarlatina, etc., is an attribute only of living matter. There is no more likelihood of the spores of scarlatina being converted into those of small-pox than there is of the germ of a dog being converted into that of a horse. All, in common, only reproduce after their kind.
Of course, the hypothetical spores of an infectious disease are not subject to the chemical law of gaseous diffusion; yet, as such contagia show in various ways a high degree of volatility, stillness of the air is obviously far more favorable to their large aggregation in any particular locality than rapid air-movement. Every intelligent physician is well aware that one of the very best methods of preventing the spread of an infection through a house is by good ventilation or running air. But all this is against Niemeyer's notion of the superior salubrity of still night-air, as compared with the rapid air-movements during the day. The day-air mobility is the analogue of house ventilation—the night-air stagnation the analogue of concentrated house impurity.
If Mr. Johnson desires to know, as it is presumable he does from his inquiry, how "noxious effluvia, if they obey the law of the gaseous diffusion of permanent (!) gases, hover over low marshes and putrefying cess-pools," he has only to study the law in any elementary treatise on chemistry. If the effluvia be sulphuretted hydrogen, it will diffuse very rapidly; if it be carbonic acid, much more slowly—specific gravity having something to do with the process. But, in either case, the hovering is greatly promoted by the stillness of the atmosphere, as dur-
