OF FOREST-TREES. 21 



and will not be much concerned with the increasing heat of the season, as cHAP. l. 

 such as being crude and unferraented, are newly sown in the beginning ^^^*v^ 

 of the spring, especially in hot and loose grounds ; being already in so 

 fair a progress by this artificial preparation, and which (if the provision 

 to be made be very great) may be thus managed : Choose a fit piece of 

 ground, and with boards (if it have not that position of itself) design it 



" filled with this air, every leaf has been full of life; fresh shoots have branched out in 

 " Tarious directions, and have grown much faster than other similar plants, growing in the 

 " same exposure in common air. This observation led me to conclude, that plants, 

 " instead of affecting the air in the same manner with animal respiration, reverse the 

 " effects of breathing, and tend to keep the atmosphere sweet and wholesome, when 

 "it is become noxious, in consequence of animals, either living and breathing, or 



" dying and putrefying in it. In order to ascertain this, I took a quantity of air, 



" made thoroughly noxious, by mice breathing and dying in it, and divided it into two 

 " parts ; one of which I put into a phial immersed in water ; and to the other (which 

 " was contained in a glass jar, standing in water) I put a sprig of mint. This was 

 "about the beginning of August 1771, and after eight or nine days, I found that a mouse 

 " lived perfectly well in that part of the air, in which the sprig of mint had grown, but 

 " died the moment it was put into the other part of the same original quantity of air ; 

 " and which I had kept in the very same exposure, but without any plant growing in it. 

 " This experiment I have several times repeated ; sometimes using air in which animals 

 " had breathed and died ; sometimes using air tainted with vegetable or animal putre- 

 " faction, and generally with the same success. Once I let a mouse live and die in a 

 " quantity of air which had been noxious, but which had been restored by this process, 

 " and it lived nearly as long as I conjectured it might have done in an equal quantity of 

 "fresh air; but this is so exceedingly various, that it is not easy to form any judgment 

 " from it ; and in this case the symptom of difficult respiration seemed to begin earlier than 

 "it would have done in common air. Since the plants that I made use of manifestly 

 " grow and thrive in putrid air ; since putrid matter is well known to affovd proper 

 " nourishment for the roots of plants ; and since it is likewise certain that they receive 

 " nourishment by their leaves as well as by their roots, it seems to be exceedingly probable, 

 that the putrid effluvium is in some measure extracted from the air, by means of the 

 " leaves of plants ; and therefore that they render the remainder more fit for respiration. 

 " Towards the end of the year some experiments of this kind did not answer so well as 

 " they had done before, and I had instances of the relapsing of this restored air to its 

 "former noxious state: I therefore suspended my judgment concerning the efficacy of 

 "plants to restore this kind of noxious air, till I should have an opportunity of repeating 

 " my experiments, and giving more attention to them. Accordingly I resumed the 

 "experiments in the summer of the year 1772, when I presently had the most 

 " indisputable proof of the restoration of putrid air by vegetation ; and as the fact is of 

 " some importance, and the subsequent variation in the state of this kind of air is a Httle 

 "remarkable, I think it necessary to relate some of the facts pretty circumstantially. 



