of air which at once immensely rarefy all gaseous exhalations, but was kept warm under a dome of glass, through which only the light of heaven penetrated. Although not hermetically sealed, the circulation of air in such a building, compared with that in the open air, is reduced over a hundred-thousandfold.
I asked permission to make experiments for several days at various hours of the day and night, which was readily granted. Now, what was the result? The proportion of carbonic acid in the air in the winter garden was almost as high as in the open air. This greatly surprised me, but I hoped at any rate to have one of my traditional ideas confirmed: I hoped to find less carbonic acid in the day than in the night, supported by the fact that the green portions of plants under the influence of light decompose carbonic acid and develop oxygen. But even here I was disappointed. I generally found carbonic acid increasing from morning till evening, and decreasing from night till morning. As this seemed really paradoxical, I doubled my tests and care, but the result remained the same. At that time I knew nothing of the large amount of carbonic acid of the air, in the soil, the air of the ground, or I should probably have been less surprised.
One day it suddenly became clear to me why there was always more carbonic acid by day than by night. I had been thinking only of the turf, the shrubs, and trees, which consume carbonic acid and produce oxygen, and not of the men and birds in the winter garden. One day, when there were considerably more men at work there than usual, the carbonic acid rose to the highest point, and sank again to the average during the night. The production of carbonic acid by the working and breathing of human beings was so much greater than that consumed by the plants in the same time.
The oxygen in the winter garden was rather higher than in the open air; there it was about twenty-one per cent., and in the winter garden twenty-two to twenty-three per cent.
I did not make any experiments on ozone, for reasons which I will give by-and-by.
The amount of carbonic acid in the air in the winter garden cannot be reckoned as telling for or against the hygienic value of vegetation in an inclosed space. Let us inquire, then, into the value of the slight increase of oxygen.
There is a wide-spread opinion that the breathing of air rich in oxygen effects a more rapid transformation of matter, a more rapid combustion, as we say, in the body. Even great inquirers and thinkers have considered that we only eat and imbibe nourishment to satiate the oxygen streaming through us, which would otherwise consume us. We know now well enough that the quantity of oxygen which we imbibe does not depend on the quantity in the air we breathe, but far more on previous changes in and the amount of