4 i 8 THE POPULAR SCIENCE MONTHLY. 



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 consid- 

 eration 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. Mean- 

 while 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 any- 

 thing, 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 ; sec- 

 ondly, because under the influence of sunlight they exhale an equiva- 

 lent 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 read- 

 ers, 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 car- 

 bonic acid than one having vegetation upon it ? 



The amount of carbonic acid in the open air has been often deter- 

 mined, 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 crite- 

 rion 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 



