jjO. I AIR AND TUBERCULOSIS — HINSDALE 7 



small. A little study shows that the carbonic acid gas taken up by 

 a forest is a very small quantity compared with that which passes 

 the forest in the same time with the moving air. Grandeau ' esti- 

 mated the annual product of carbon by a forest of beeches, spruces, 

 or pines as about 2,700 pounds per acre. This corresponds to 9,900 

 pounds of carbonic acid gas or 69,300 cubic feet. Now, if the aver- 

 age motion of the air is five miles an hour, a low estimate, and the 

 layer of air from which the gas is taken be estimated at one hundred 

 feet thick, there would pass over an acre 550 million cubic feet in 

 one hour. This air must contain about three parts in ten thousand 

 of carbonic acid gas and the total amount of the latter per hour is 

 165,000 cubic feet. But this is two and two-thirds, or more than 

 twice as much as that taken up by the trees in the entire season, 

 so that the air could provide in thirty minutes for the wants of the 

 trees for the entire season. Prof. Harrington shows that the ratio 

 of carbonic acid used to that furnished is only one part in 8,600. 



OXYGEN IN FORESTS 



Again, the additions of oxygen to the air would form a still 

 smaller percentage of the oxygen already present, for this gas makes 

 up 20.938 per cent of the air against a thirtieth of one per cent ob- 

 tainable from this source. 



OZONE IN FORESTS 



The occurrence of ozone in the air of forests, especially coniferous 

 forests, has been credited, since its discovery by Schoenbein in 1840, 

 with affording remarkable health-giving qualities. This opinion has 

 become firmly fixed in the minds of the public and, to a large extent, 

 has been accepted by the medical profession as an evidence of high 

 oxidizing power at once corrective of decaying vegetation and exhil- 

 arating and curative to mankind. Popular behef usually has some 

 basis for its existence ; indeed, meteorologists made regular estima- 

 tions of ozone in the atmosphere by testing with sensitized papers 

 and the results were published in connection with statistics of health 

 resorts.' 



The Schonbein test is based on the power of ozone to free iodine 

 from a solution of potassium iodide in contact with starch, when a 

 violet color is developed in the sensitized paper. Unfortunately the 



' See Belgique Horticole, Vol. 35. 1885, p. 227. 



-See Transactions American Climatological Association, Vol. 5, p. 118. 



