disposed to give up the idea that the air in rooms can be improved by plants, because, as is well known, every green leaf absorbs carbonic acid and gives out oxygen under the influence of light. This idea may seem the more justifiable, because, although the production of carbonic acid is not perceptible in the greatest assemblages of human beings in the open air, it is always observed in confined spaces, although the actual production is but small. In the air of a closed apartment, every person and every light burning makes a perceptible difference in the increase of carbonic acid in the air. Must not, therefore, every plant in a pot, every spray, any plant with leaves, make a perceptible difference in a room? Every lover of flowers may be pardoned for wishing to see this question answered in the affirmative. Have not even medical men proposed to adorn school-rooms with plants in pots instead of ventilating them better, in order that their leaves and stems might absorb carbonic acid from the mouths of the children, and give out oxygen in its stead? But hygiene cannot agree even to this. Hygiene is a science of economics, and every such science has to ask not only what exists and whether it exists, but how much there is and whether enough. The power of twenty pots of plants would not be nearly sufficient to neutralize the carbonic acid exhaled by a single child in a given time. If children were dependent on the oxygen given off by flowers, they would soon be suffocated. It must not be forgotten what a slow process the production of matter by plants is—matter which the animal organism absorbs and again decomposes in a very short time, whereby as much oxygen is used up as has been set free in the production of it. It is for this reason that such great extents of vegetation are required for the sustenance of animals and man. The grass or hay consumed by a cow in a cow-house grows upon a space of ground on which a thousand head of cattle could stand. How slow is the process of the growth of wheat before it can be eaten as bread, which a man will eat, digest, and decompose, in twenty-four hours! The animal and human organism consumes and decomposes food as quickly as a stove burns the wood which took so many thousand times longer to grow in the forest.
It would scarcely be intelligible if I were to calculate how much carbonic acid and oxygen a rose, a geranium, or a bignonia, would absorb and give out in a room in a day, and to what extent the air might be changed by it, taking into account the inevitable change of air always going on. I will draw attention to a concrete case which every one can understand:
When the Royal Winter Garden in Munich was completed and in use, it occurred to me to make experiments on the effect of the whole garden on the air within it. There could not be a more favorable opportunity for experimenting on the air in a space full of vegetation. This green and blooming space was not exposed to the free currents