IOWA ACADEMY OF SCIENCE 
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test. In his later study he has pointed out that the extent of injury 
cannot be shown definitely by a chemical analysis, since the difference in 
sulphur trioxide content is within the limits of experimental error. He 
.states further that the injury to forest trees may extend to a distance 
from fifteen to twenty miles; also that the injury takes nlace directly 
through the leaves and that the roots are not a medium of conduction. 
Ebaugh (18) thinks that undue emphasis has been placed upon the 
sulphur dioxide given forth and that sufficient attention is not given to 
the solid emanations. Harkness & Swain (19) have pointed out that 
high stacks of large condensing flues serve the purpose of disseminating 
the sulphur dioxide over a wider area. 
Stone (20) finds that in the case of illuminating gas, that small leaks 
cause local injury to trees. Crocker & Knight (21) have put forth 
conclusive evidence to show that traces of illuminating gas and ethylene 
prevent the normal opening of the flowers of the sweet pea. Wilcox (22) 
also notes that illuminating gas has an injurious effect upon greenhouse 
plants, 
NATURE OF SMOKE. 
Smoke is not a necessary evil, and under no condition can it be con- 
sidered as a question of economy. It is tied up with the question of 
combustion. Gebhardt (25) points out that smoke is produced by the 
following methods: 4 4 (1) An insufficient amount of air for the perfect 
combustion of the volatile gases. (2) An imperfect mixture of air and 
combustible. (3) A temperature too low to permit complete oxidation of 
the volatile combustible. Breckenridge (21) cites the following: “The- 
problem of smoke prevention is the problem of perfect combustion. Im 
the complete combustion of carbon the product of combustion is C0 2 .- 
If sufficient oxygen is not provided it will happen that each carbon atom: 
will combine with one oxygen atom, thus forming carbon monoxide,. CO;. 
As a result of this incomplete combustion the heat developed is only' 
4400 British Thermal units (B. t. u.). The carbon monoxide may itself’ 
combine according to the formula C0+0=C0 2 and the heat developed 
will be the difference, 14500 — 4400^-10100 B. T. U. per pound of carbon 
in the carbon monoxide.’' This citation brings out the economic im- 
portance. He also states that in order to insure complete combustion 
in practice, an excess of air must be furnished. Furthermore, carbon 
and oxygen atoms will not unite unless a certain temperature is reached. 
Breckenridge further adds, ‘ ‘ The products of combustion, carbon dioxide, 
steam, and sulphur dioxide, are colorless gases. If nothing but these 
gases escaped, there would be no smoke problem. Visible, smoke is due 
