cinal remedies plants provide us with, but the value of plants and plantations in dwellings and in the open air in conducing to health or preventing disease. We have given the subject very little consideration until quite recently, just as we have thought very little of the way in which the pleasures of the table, fine raiment, comfortable dwellings, and many other things, conduce to our well-being. Meanwhile we have been guided by our instincts, which, like Nature in general, have, on the whole, guided us rightly. Even now there is not much scientific knowledge on the subject; still there is a little, and something is gained when we begin seriously to reflect on anything, for knowledge is sure then to increase. All that man has ever aspired to and attained has always existed much earlier in idea than in reality. Ideas are never fully realized, as we all know, and it is only very gradually that they are realized at all.
It is generally asserted that vegetation purifies the air, and chiefly by three functions: firstly, because plants absorb carbonic acid; secondly, because under the influence of sunlight they exhale an equivalent in oxygen; and, lastly, because they produce ozone. These facts I need not demonstrate, as they have been placed beyond doubt by vegetable physiologists, chemists, and meteorologists. My task is to show what the direct sanitary effect of these three functions is.
I must at once state that none whatever can be proved to exist. And, as this assertion will contradict the prepossessions of many readers, I feel bound to prove my proposition.
As to carbonic acid, the first question is: What is the proper and normal proportion of this gas in the air, next how much more carbonic acid is contained in air which is notoriously bad; and, lastly, whether the air on a surface without vegetation contains essentially more carbonic acid than one having vegetation upon it?
The amount of carbonic acid in the open air has been often determined, and is confined within very narrow limits. It may be said—leaving severe storms or very thick fogs out of the question—to vary between three and four parts in each 10,000 of the volume of the air.
Experiments have also been made on the quantity of carbonic acid in apartments occupied by man, and it is generally taken as the criterion of the quality of the air, ventilation being regulated by it. In very bad air which is undoubtedly deleterious, it has been found to amount to from three to five per mille. One per mille marks the boundary-line between good and bad air in a room.
We next inquire whether the atmosphere over a vast tract of country destitute of vegetation contains more carbonic acid than one abounding in vegetation, whether in the former case the amount of carbonic acid approaches one per mille. In 1830 De Saussure began to make researches into the variations in the quantity of carbonic acid in Geneva, and they were continued about ten years later by Verver in Holland, and Boussingault in Paris; in more recent, and
