1878. ] The Beneficial Influence of Plants. 799 
vapor, prevent an increase of moisture in the air of the room. On 
days when the air was laden with moisture, no difference in the 
dew point was noticed, there being at such times little or no 
exhalation of watery vapor. The observations taken at 1 o'clock 
P. M., gave the greatest variation, the morning observations usually 
the least. We do not wish to say dogmatically that there is no 
possible chance of error in these experiments, but since they 
were corroborative throughout it seems fair to conclude that they 
. are correct. Since it is allowable always to make logical deduc- 
tions from facts, we may justly conclude, from the statements made 
in the above extract concerning the rate of transpiration, coupled 
with the carefully conducted observations here detailed, that 
during the summer months when the windows are thrown widely 
open and the doors kept ajar, the influence of transpiration is 
quite inconsiderable; on the other hand, when the interchange of 
air is not too rapid a sufficient number of plants, well watered, 
have the effect (if the air be not already saturated) of increasing 
the amount of moisture to a considerable extent. 
As before intimated, it is my wish to apply the results of these 
researches particularly to the atmosphere of apartments heated 
by means of hot-air furnaces, which are known to be dryer than 
air heated by a stove or open fire-place. Not having the oppor- 
tunity myself at the Hospital of comparing the dryness of air 
thus heated with that of the outer air, my wants were made 
known to a friend residing in a house heated by a dry-air furnace. 
Through the kindness of this friend reliable observations were 
made for a period of eight days. The results showed the mean 
average complement of the dew point to be seven degrees Fahr. 
greater for the heated air than the air outside. Now, according 
to our line of reasoning, a certain number of plants would bring 
_up the humidity to that of the external air. Calculating from 
the above data, half a dozen each with a leaf surface of four 
square feet would be sufficient to produce this effect in a room 
twelve feet long and ten feet wide with a ceiling twelve feet high. 
The mean average temperature and dew point for the out-door 
observations were fifty-six degrees and forty-one degrees Fahr. 
respectively, which is a percentage still considerably below the 
healthiest standard. Some one, not a professional medical man, 
might pertinently ask: What is the effect on the systern of air 
heated by a hot-air furnace? It will be necessary to answer this _ 
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