798 The Beneficial Influence of Plants. [ December, 
found that the air in the former position was appreciably dryer 
than the latter, the average complement of the dew point being 
on the whole about five degrees greater. The room adjoining 
mine, occupied by my colleague, was very kindly left for a time 
at my disposal; in it were kepta few plants in pots with a leaf 
surface of not more than twelve square feet. The dimensions of 
the rooms were similar, each being twenty feet long, eleven feet 
wide and sixteen feet high. Each had one window fronting east, 
in which the plants were kept. The average temperature and © 
dew point in both these rooms were noted simultaneously, and 
the results showed uniformly, for a period of eighteen days, that 
the complement of the dew point averaged one and a half degrees 
less in the room containing the plants. These observations were 
made during the early part of April, 1878, when very little heat 
was required, still the windows were kept closed during the day. 
Calculating from these results, the effect of twenty-four square 
feet of leaf surface on the air of a room half the size of the above 
would be to increase the humidity sufficiently to raise the dew 
point six degrees Fahrenheit higher than it would be if there 
were no plants in the room. There can be no doubt but that a 
southern exposure of the plants would make the difference even 
greater. 
As it seemed possible that the variation in the amount of 
moisture in the two rooms tested might be due to considerations 
other than the presence of plants, it was deemed necessary to 
vary the conditions and make further observations. Accordingly 
after placing some plants in the window of my own room, I too 
the average temperature and dew point, and compared them with 
those of an adjoining room containing no plants. No artificial 
heat was required during the time of these experiments. It was 
found that when the window was kept open so as to cause. 
very free ventilation, no appreciable difference in the humidity 
of the two rooms was observed; but if the windows were 
closed for a few (say three) hours, it would make a differ- 
ence of from one and a half to two degrees Fahr. in the comple- 
ment of the dew point; the room having plants showing the 
lesser complement. This difference was maintained, almost, 
when the windows were opened just enough to allow a gradual 
interchange of the contained air; but as before intimated, a draft, 
_ though it might hasten transpiration, would, by carrying off the 
