by keeping the soil clean through good drainage, abolition of cesspools, and abundant water-supply. It would carry me too far if I were to analyze now to which of these measures the lion's part belongs; I should have to enter upon many controversies, which I have no time to fight out in this place; but this is my conviction, which I want to impress upon you, that cleanliness of the soil and diminution of organic processes in the ground of dwelling-houses are most essential.
Many have considered these processes, and their effects on the ground-air, to be a mere hypothesis. This view lies now behind us, and facts have been found proving their reality. Stimulated by the investigations of Huxley and Haeckel, further researches have followed, and shown that not only at the greatest depth of the sea, but also in every porous soil, there are everywhere those beginnings of organic life, belonging neither to the animal nor vegetable kingdom, mucous formations, which are called Moneras and Protistes. When I wrote my part of the report on the cholera in Bavaria, in 1854, I pointed out already that the air, not less than the water in the soil, ought to be drawn into the circle of experimental investigations. Neither others nor myself acted at once upon my suggestion, and it is only during the last eighteen months that I have examined the ground-air in the rubble-soil of Munich, regularly twice a week, for its varying amount of carbonic acid. The results are surprising, and for the future I shall have to trouble others and myself, not only with groundwater, but also with ground-air.
The place where the examination of the ground-air of Munich is being carried on is rubble, without any vegetation, and the carbonic acid increases with the distance from the surface. Agricultural chemistry has been aware, for a long time, that a clod of arable earth which is rich in humus is a source of carbonic acid, but no one expected that, at times, so much carbonic acid should be met with in sterile lime-rubble. A few feet under the surface there is already as much carbonic acid as in the worst ventilated human dwelling-places.
I have found that the quantity of carbonic acid is smaller at fifty-eight inches than at one hundred and fifty-six inches throughout the year, the months of June and July excepted, when an inverse proportion arises. But then there begins also, in the lower stratum, a considerable increase, so that the upper stratum soon finds itself behind again. This large quantity of carbonic acid in the ground-air of Munich has been far surpassed in Dresden. Examinations have taken place in that town under the authority of the Central Board of Public Health. Prof. Fleck's diary proves that, at least at that spot where his examinations took place, the quantity of carbonic acid was in winter already nearly twice as great as in Munich in the month of August. I might become jealous of Dresden, but we must often,
