IOWA ACADEMY OF SCIENCE 
175 
such products as sulphur. The gaseous products as a result of dry 
distillation which are given oft under incomplete combustion and the 
sulphur as sulphur dioxide as a result of complete combustion furnish 
us with at least two important considerations of our problem. 
Another important feature of the smoke problem derived from incom- 
plete combustion lies in the nature of soot. According to Cohen and 
Ruston, soot consists of carbon, tar, and ash (mineral matter) together 
with small quantities of sulphur, arsenic and nitrogen compounds which 
frequently possess an acid character. This acid character is responsible 
for the corrosion features imparted to masonry structures and leaves of 
trees, shrubs and herbs. Furthermore, the corrosion assists in making 
the tarry material, which is ejected, adhere. This tar compound will 
play the same role upon the vegetation when it comes in immediate 
contact with it as the material from tarred roads. 
GENERAL OBSERVATIONS. 
Even to the casual observer, there is considerable difference in the 
vegetation as found in the heart of an industrial center and the suburbs 
or residential portions. The prevailing slowness of growth, and the 
early leaf fall of trees is noticed by almost everyone. This condition 
of affairs is well shown in traveling out from the center of Chicago to 
the neighboring suburbs. But an additional feature also comes in. Here 
and there on the outskirts of the city proper, large manufacturing con- 
cerns or mills are found. The immediate territory is ideal for a smoke 
survey. This is well illustrated in the case of the territory immediately 
surrounding the Illinois Steel and Wisconsin Steel Companies. 
The city of Des Moines, Iowa, has most of its manufacturing concerns, 
in the heart of the city itself. To the person going to the Iowa eapitol 
for the first time, the amount of smoke is of considerable annoyance. 
He probably is so concerned as to his own personal welfare that he does 
not stop to consider the trees that are conspicuously absent. This state 
of affairs is to some extent perhaps natural in that Des Moines is situ- 
ated in a valley. But even if it is the fact, yet they can be remedied 
as the further discussion will show. 
In Germany most of the observations have been made upon the con- 
ifers. On account of their deeply sunken stomata, they are much more 
susceptible than tthe majority of our deciduous forms. In many of the 
parks of the city of Chicago, such as Humbolt, Washington, the so called 
Norway-pine ( Pinus resinosa Ait.) the most common evergreen, is a small 
scrawny tree with distorted branches, which carry in' many instances 
