1128 
subsequently deposited. As a result of thermal convec- 
tion, this process is continuous in the newly advected 
air, and fog particles, proportional in number to the 
mass of air flowing close to the metallic surface, adhere 
to it and finally drip off into a graduated tapered vessel. 
The condensation apparatus (Fig. 2) consists of an egg- 
shaped dew-cup which is plated with Cr or Nz, is well- 
polished, and has a capacity of two liters of ice water. 
Fic. 2.—Apparatus for the analysis of traces which form 
condensation nuclei. Center, Cauer condensation dew-cup; 
at right, Assman psychrometer; below, thermohygrograph. 
During the time required for the dew deposit, 20 min 
in most cases, the mean absolute water content of the 
air must be determined in the immediate vicinity with 
the aid of an Assmann psychrometer or a thermohygro- 
graph. Depending on the number of analyses and the 
substances to be investigated, amounts of condensate 
ranging from 1 to 20 cc are collected. During ventilation 
the sphere must be shielded from direct sunlight and 
dust. Hence it cannot be used during storms; likewise, 
it cannot be used when the absolute humidity of the 
air is small. During cold weather a coating of hoarfrost 
is produced by charging the sphere with a refrigerant. 
Insofar as possible, the chemical substances are ana- 
1. The direct condensation of steam on cooled, polished, 
metal surfaces without the existence of condensation nuclei 
amounts to almost one per cent of the total possible precipita- 
tion, according to Mayer [39] (this percentage also includes the 
direct condensation on the surface of the water). This occur- 
rence is so low that the validity of the following chemical 
analysis is not affected. 
BIOLOGICAL AND CHEMICAL METEOROLOGY 
lyzed by the methods usual in water analysis [47]. For 
other substances special trace-detection methods are 
used, for example, spot analyses [15, 24]. The number 
of analyzable substances can be increased greatly with 
the help of a spectrograph. For this purpose a measured 
quantity of dew is carefully evaporated in a platinum 
dish and the residue is picked up with the heated carbon 
rods of the spectrograph. Several standard solutions of 
different concentration are treated in the same way. 
The subsequent spectrophotographs show the presence 
and order of magnitude of substances in the dew by the 
spectrographic lines and their intensity. From the meas- 
ured concentration of substances in the dew, and on the 
basis of the simultaneous measurement of the mean 
absolute humidity, the quantity of chemical substances 
capable of forming condensation nuclei is computed for 
one cubic meter of air. It was unexpectedly found that 
the highly hygroscopic compounds can be determined 
more accurately in this way than by the absorption-tube 
methods mentioned previously. When frost deposits 
are used, however, the results must be multiplied by a 
factor of three [15], a fact which was still unknown at 
the time of the first publication of the method [47], and 
for which no adequate theoretical explanation has been 
found. 
By this method the halides, sulfates, ammoniacal 
substances, soluble alkali compounds, and alkaline- 
earth compounds as well as organic materials such as 
formaldehyde can be quantitatively determined. Metals 
can be determined particularly well with the aid of the 
spectrograph. Other substances such as nitrates and 
nitrites can also be determined with the aid of the con- 
densation sphere, but only that portion of the substance 
which can act as condensation nuclei, depending on the 
relative humidity and pressure in a given case. The 
physico-chemical reasons for this phenomenon are still 
unexplained. The nonpolar gaseous components of the 
air, such as the two principal components, O2 and Ne, 
cannot be analyzed by this method. 
RESULTS FROM ABSORPTION-TUBE METHODS 
Iodine of the Air. Contrary to previous assumptions, 
the iodine in the air over central Europe [10] does not 
originate directly from the ocean but comes principally 
from the combustion of iodine-containing ocean prod- 
ucts for the extraction of iodine and for domestic heat- 
ing in coastal towns. The principal sources of the iodine 
in the air over Europe consequently are Brittany, Scot- 
land (Shetland Islands), Scandinavia (Stavanger re- 
gion), and to a minor extent Spanish Galicia (Finis- 
terre). 
The iodine is lifted off the ground by convection cells 
and is transported in the air masses as far as the Car- 
pathians; in the case of intense storms this distance is 
occasionally covered in less than twenty-four hours. 
The dilution occurring during this process is at first 
very marked, but later it proceeds at a continuously 
decreasing rate. Under normal conditions, the mean 
iodine content along the coast of Brittany is 0.4 ug 
m *; during the combustion of seaweed (laminaria and 
fucus) it runs to 6000 pg m-*. In central Europe it 
