1106 
laboratory experiments that high relative humidity of 
the air does not increase the resistivity of air-borne 
bacteria. Wells [35] and Whisler [86], on the other 
hand, maintain that ultraviolet radiations are ten to 
twenty times more germicidal in dry air than in humid 
air. These antithetical results strongly suggest that the 
differences in germicidal effects were in some way 
related to differences in the number or character of 
condensation nuclei present in the several experiments. 
In this connection it should be stressed that soil or 
sea-salt particles play a possible dual role in the main- 
tenance of viability in the organism. In addition to 
containing aggregates of microorganisms they may, 
under certain circumstances, act as condensation nuclei 
Gf hygroscopic) for water vapor in the atmosphere and 
in so doing provide favorable conditions of moisture 
for the persistence of organisms in the viable state. 
If this reasoning is valid, then the consideration of 
variations of atmospheric moisture should be important. 
Wellington [34] has reported upon a series of labora- 
tory experiments designed to determine the effects 
of substantial decreases in pressure, temperature, and 
humidity upon certain species of imsects common in 
Canada. Attempts were made to simulate those com- 
binations of pressure, temperature, and humidity that 
an organism might experience upon being lifted from 
the surface to a height of around 20 km. He concludes 
that the decrease im atmospheric pressure may be 
safely neglected as either a limiting or lethal factor 
among the elementary environmental changes experi- 
enced by insects distributed at higher levels; he does 
show, however, that most of the species tested become 
insensible at pressures below 120 mb. He found, also, 
that the flight activity of the insects varied somewhat 
with pressure, remaining constant for most insects up 
to simulated altitudes of about 7.5 km, then decreasing 
to zero at about 16 km. In the cases of Coleoptera and 
Diptera, however, there was a distinct increase in 
activity down to a pressure equivalent to 1.5 km where 
it again approached normality. This latter finding is 
significant from the standpoint of the actual vertical 
distribution of these orders. That the responses are 
barotactic and not due to distress occasioned by the 
lowered oxygen pressure was also proved by the experi- 
ment. The effects of reduced pressures on microor- 
ganisms appear not to have been similarly investigated 
but, because of their small size and simple internal 
structure, it is assumed that the effect can be neglected 
in the cases of these groups also. 
There appear to be few, if any, actual quantitative 
data concerning the fraction of organisms that are able 
to survive significant transport. Proctor [21], in his 
investigation of the number of bacteria at high levels, 
also collected and counted the number of dust particles. 
He found the ratio of microorganisms (bacteria and 
molds) to dust particles to be 1 to 108 for all levels and 
1 to 118 above 9000 ft. As the writer has previously 
pointed out [14], this would show about eight per cent 
fewer bacteria per particle for the higher levels, which 
might indicate that this proportion was killed, since the 
physical factors governing their removal from the air 
BIOLOGICAL AND CHEMICAL METEOROLOGY 
would be the same as for the dust particles. Proctor 
did not give average values for the layer below 9000 
ft; it can therefore be assumed that the proportion 
that did not survive was somewhat greater than eight 
per cent. 
METEOROLOGICAL FACTORS 
The Mechanics of Exchange of Organisms Between 
Earth and Atmosphere. It is assumed that nearly all 
surfaces, whatever their nature or composition, will 
contribute organisms to the atmosphere; but that some 
surfaces, because of greater populations of organisms, 
more extensive vegetative cover, greater mobility of 
surface materials, or more intensive atmospheric tur- 
bulence, will contribute far more than others. In the 
case of bacteria, it is a statistically remote possibility 
that any single organisms in the soil will be lifted by 
themselves into the air; m all probability they are most 
frequently associated with debris of some kind, such as 
bits of organic matter, dust motes, or, in the case of 
marine forms, with salt particles or droplets of con- 
centrated sea water. These organisms must therefore 
exist most frequently as- colonies rather than as in- 
dividuals. The laboratory method of plating and count- 
ing the bacteria and molds permits of no distinction 
between individuals and groups. Because the number of 
bacteria in a cubic centimeter of soil may be numbered 
in the millions or hundreds of millions, the potential 
supply of these organisms must be considered almost 
unlimited. 
By this reasoning, the processes governing the ex- 
change of soil bacteria between the earth and atmos- 
phere are the same as for the exchange of other terrig- 
enous materials. The quantity of material exchanged 
will be greatest in those areas where ample supplies 
of loose particles exist on the surface, surface winds are 
strong enough to stir up these materials, and steep 
lapse rates exist in the atmosphere to favor the vertical 
transport of the particles. The supply of microorgan- 
isms should be greatest where the surface is dry due to 
deficient rainfall, where there is an abundance of organic 
matter in the soil, where intensive cultivation is prac- 
ticed, and in and near wooded areas. A spring maximum 
of dust in the atmosphere is noted in the solar radiation 
measurements in the United States. This is to be ex- 
pected since the soil is driest at this season, cultivation 
is most extensive, surface winds are strongest, and the 
vertical temperature lapse rates are steepest. It can be 
assumed that the population of soil microorganisms in 
the atmosphere will be greatest during this season for 
the same reasons. 
It has previously been pointed out that Proctor 
[21], during his determinations of the number of bac- 
teria at high levels, also collected and counted the 
number of dust particles. Although he made no volu- 
metric calculations, rough computations by the writer 
indicate that he collected approximately 510 visible 
and collectable dust particles per cubic meter of air 
averaged for all levels. Above 9000 ft there were ap- 
proximately 25 per cent fewer particles than the general 
average. His ratio of 1 microorganism to 108 dust 
