74 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
“ Some kinds of dust (page 365) have such an affinity for water that 
they determine the condensation of vapor in unsaturated air, while other 
kinds of dust only form nuclei when the air is supersaturated; that is, 
t 
they only form free surfaces on which the vapor may condense and pre¬ 
vent supersaturation.” 
In 1888 Mr. Aitken invented and developed a method of counting the 
number of dust particles in any sample of air. These particles are in¬ 
visible to the highest powers of the microscope; they become visible when 
loaded with vapor from the supersaturated air in which they were float¬ 
ing. In order to make the number small enough to count, the sample of 
air to be tested was diluted with pure air 200 times before being drawn 
into the receiver of an airpump; a plate of silver with a ruled surface 
had been placed horizontally at a convenient distance from the top of 
the receiver; the air was cooled by exhaustion, and the density of the 
vapor kept up by evaporation from water in a dish inside; and the num¬ 
ber of drops that fell on the plate was counted. The following results 
were obtained: 
Source of air. 
Outside, raining.. 
Outside, fair. 
Room. 
Room, near ceiling 
Bunsen flame .... 
Dust particles per cubic inch. 
. 521,000 
. 119,000 
. 30,318,000 
. 88,346,000 
.489,000,000 
Subsequently the pure air at the top of Ben Nevis was found to con¬ 
tain 34,000 per cubic inch. 
Suppose, then, that we attempt to produce rain, not in a small portion 
of the atmosphere cut off from the rest by means of an airtight receiver, 
but on a large scale in the unbounded atmosphere. If the air operated 
on is at a temperature higher than its temperature of saturation, it must 
be cooled down to that temperature. Further, when the moisture con¬ 
denses it gives out latent heat, which tends to arrest the process; this 
latent heat evolved must be removed. It is not, as some rainmakers 
have imagined, “ Pull a trigger; Nature will do the rest.” The only 
trigger pulling which experiments warrant as possible consists in sup¬ 
plying the necessary fine dust for nuclei, so that condensation may take 
place without delay when the air is cooled to its temperature of satura¬ 
tion; or in supplying fine dust from such a substance as common salt, 
which has a chemical affinity for water, and may be able to accelerate 
slightly the falling of a shower. 
Suppose we take a cubic mile of the air upon which Dyrenforth oper¬ 
ated on the night of Friday, November 25, 1892. The record at the 
weather office in San Antonio at 8 p. m. gave the temperature of the air 
as 72° Fahrenheit, and the dew point as 61° Fahrenheit. To cool down 
