160 
IOWA ACADEMY OF SCIENCE Voi,. XXVII, 1920 
phates, as only fifteen samples produced sufficient sulphates for 
determination, and only eleven other samples showed a slight 
trace. The largest yield, from a sample collected on February 
28, was only .262 parts per million. The average for the period 
is .03 parts per million. An unduly mild winter, with the atmos- 
phere comparatively free from smoke and soot, no doubt accounts 
for this low average. 
Five samples showed a trace of phosphates, while only four 
supplied a sufficient quantity for determination. The largest 
amount, on March 4, was only .03 parts per million. The average 
is .002 parts per million. 
Table I gives the parts per million of the several nitrogen com- 
pounds and the chlorine content ; the total for each precipitation ; 
and the number of pounds supplied per acre. 
The chlorine content averages 11.12 parts per million. It varies 
from 6.10 to 25.70 parts per million. Its presence in the atmos- 
phere has been ascribed by Doctor Knight to salt particles carried 
in the air from the Atlantic ocean. No increase was experienced 
due to wind preceding or accompanying a precipitation. 
The average of total nitrogen supplied for each precipitation 
is 1.046 parts per million. The totals for the 46 precipitations 
are fairly constant. Strong winds and severe electrical discharges 
on May 6, and on June 1 and 3, did not increase the amounts 
of nitrogen, but the total amount of nitrogen supplied on those 
dates is unusually high, especially on May 6 and 7 — the highest 
of the period. 
The average parts per million for Free Ammonia is .407 ; for 
Albuminoid Ammonia, .366; for Nitrates, .255; and for Nitrites, 
.018. 
Of the total nitrogen supplied during the period, 38.85 per cent 
is in the form of Free Ammonia; 34.99 per cent. Albuminoid 
Ammonia; 24.42 per cent. Nitrates; and 1.74 per. cent, Nitrites. 
The amount of the precipitation determines largely the total 
number of pounds of nitrogen supplied per acre. But an exami- 
nation of the pounds of nitrogen supplied per acre by each of 
the 46 precipitations reveals a remarkable uniformity. When re- 
duced to pounds per acre for one inch of rain, 43 of the 46 pre- 
cipitations come within the narrow limits of .15 to .40 pounds per 
acre. The wide variance in the amounts of rain — .05 to 2.00 
inches — • indicates a marked degree of concentration in the small- 
er showers. The same standard reveals the fact that during con- 
tinued precipitations, such as occurred June 1 to June 6, the total 
