IOWA ACADEMY OP SCIENCE 
167 
The third class of nuclei mentioned by Baru's are the colloidal type. Stated 
in his words, “they are a structural part of the body of gas and are reproduced 
as soon as removed. They require the highest degree of supersaturation and 
are without electrification.” To produce condensation on the colloidal nuclei, 
Barus carefully filtered the air thru a plug of cotton-wool before admitting it 
into the fog chamber so as to have it entirely free from any dust nuclei. No 
energizing source such as X-rays and radium was brought near the fog cham- 
ber. 
With exceedingly rapid exhaustions at a dp. of 26 cm. clouds were produced 
consisting of exceedingly small droplets, much smaller than those induced by 
even the fieeting nuclei. Owing to the smallness of the droplets Barus seems 
to infer that the nuclei are also smaller than those of any other kind. Barus 
speaks of the colloidal nuclei as if they were perhaps the molecules or the 
normal atoms of either the gas or vapor in the fog chamber. 
It is not the purpose of this article to go extensively into the vast amount of 
work performed by Barus and his assistants, but it might be well to mention a 
few of the facts determined. 
Comparing X-rays and radium as sources of nucleation Barus found that the 
X-rays are very variable, while the radium is decidedly constant. This fact is 
also abundantly verified in the experimental work of radio-activity. The num- 
ber of efficient nuclei induced by X-rays varies rapidly with the strength of the 
rays and also with the 'suddenness and ease of exhaustion. When the exhaust 
tube was enlarged to one and one-half inches in diameter, as many as 400,000 
efficient nuclei per cubic centimeter were obtained. 
The term “efficient nuclei” is applied only to those that induce the condensa- 
tion of vapor to form the droplets of the cloud. There are no doubt many others 
besides those that get the moisture. In fact, it was proven that when a mixture 
of nuclei of different electrical magnitudes were pre'sent in the fog chamber, the 
larger ones on the first exhaustion received all the moisture. A second exhaus- 
tion following rapidly would bring down a greater number of nuclei than the 
first one. The maximum number of nuclei induced by radium was about 60,000 
per cubic centimeter, increasing some on the higher differences of pressure and 
the enlargement of the exhaust tube. The maximum number of colloidal nuclei 
varied from 80,000 to 100,000 per cubic centimeter. 
Barus carried on a series of experiments covering a period of two years on 
the nucleations of the ordinary atmosphere. Two stations were e'stablisht, 
one at Providence, Rhode Island, and the other on Block Island, off the coast. 
Observations were taken at regular intervals each day. It was shown that the 
nuclei, dustlike and ionized in the atmosphere is far greater in the winter time 
than in the summer. The maximum is reacht in December at the winter 
solstice and the minimum in July at the summer solstice. It was also found 
that the nucleation is usually greater early in the morning, gradually diminish- 
ing during the day until the middle of the afternoon, and increasing again to- 
wards evening. Rains were always followed by a markt decrease in the num- 
ber of nuclei. It was also inferred that light pressure decreast the nucleation, 
causing in a measure the diurnal variations. There was a markt difference in 
the atmospheric nucleations of Providence and Block Island. The latter place 
lies well out at sea, where the air is not affected by local contaminations. The 
number of nuclei at Providence ran as high as 60,000 to 80,000 per cubic centi- 
