No doubt all students of science have read more or less about Barus\u27 work on condensation phenomena as produced in a specially constructed fog chamber. As is known, his work has been done under the direction of the Carnegie Institution of Washington, founded particularly for the solution of research problems, involving much time and considerable expense. The prime purpose of his work, so far as I can interpret it, is to determine the part played by various kinds of nuclei in the atmospheric condensation of vapor. This, of course, is a very interesting problem to the student of physiography, as well as to the physicist

Begeman, L.

English

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 15 Annual Issue Article 26 1907 Nucleations According to Barus L. Begeman Copyright ©1908 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Begeman, L. (1907) "Nucleations According to Barus," Proceedings of the Iowa Academy of Science, 15(1), 165-168. Available at: https://scholarworks.uni.edu/pias/vol15/iss1/26 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. NUCLEATIONS ACCORDING TO BARUS. UY L. UEGE)IAN . No doubt all students of science have read more or l ess about Barus' work on condensation phenomena as produced i n  a specially constructed fog chamber. As is  known, his  work has been done under the direction of  the Carnegie Institution of \Vashington, founded particularly for the solution of  re­search problems, involving much time and considerable expense. The prime purpose o f  his work, so far as I can interpret it, is  to determine the part played by various kinds o f  nuclei in the atmospheric condensation o f vapor. This, of course, is a very interesting problem to the student of physiography, as well as to the physicist. It i s  very difficult to read Barus' notes and I would not commend his style to anyone .  In fact, I doubt whether anyone who has not had. some experience in the experimental production o f  such phenomena could get much out of his writings. His l iterature is apparently intended for the few who have the cour­age to attack something exceedingly dry. In my experimental work to deter­mine the charge of an electron I had occasion to refer to his work to help interpret my own phenomena. I n  the discussion of  my work I have made fre• quent reference to Barus and it i s  for thi s  reason that I present this sketch with the hope that it  will make the other paper more intelligible. Barus' fog chamber i s  sketched in a simple manner without detail in Fig. 1 .  It  consisted of a cylindrical vpssel made either of  glass or wood and sealed air tight. The lower part o f  the vessel contained a l ayer of  pure water which kept the air or  gas above in  a ·stat!' of supersaturation.  The vessel contained also a number of wet muslin partitions placed parallel to its length. The purpose o f  these was t o  prevent cross currents in the vessel when the exhaustions were made, thus keeping the clouds uniformly compact. The . tube E leads to a vacuum chamber cut off by an intervening plug valve. By the sudden opening of the plug valve an exhaustion of  the fog chamber is made, producing a conse­quent lowering of temperature which results under the proper conditions when nuclei are present i n  the formation of  a cloud. A bright beam of  l ight is ad­mitted into the fog chambers so that when a cloud is  produced a coronna, gener­ally of a green-blue-purple type, i s  p)ainly visible .  The angular diameter o f  the coronna is  measured by a p a i r  of goniometers suitably placed a n d  from thhl the number of  efficient nuclei are determined in a manner described in B arus' publication of  the Smithsonian Institute, published in 1 9 0 5 .  Barus describes threP kinds of nuclei which a t  given pressures a r e  able t o  induce t h e  condensation of  vapor in a fog chamber by exhaustion.  First, "the ordinary dust-like persistent nuclei which require the smallest degree o f  super­saturation to induce condensation. Ordinary air, particularly o f  cities, contains ( 165) 1Begeman: Nucleations According to BarusPublished by UNI ScholarWorks, 1908166 IOWA ACADE MY OF SCIENCE at all  times multitudes of minute dust pa rticles. When such air is admitted a into the fog chamber ·saturated with vapor, the dust particles on exhaustion be-come the nuclei of small droplets of  water which taken together constitute the cloud. As Barus states, such p articles induce the condensation of vapor.  These dust like nuclei are persistent. By the term "persistent" used BO much by ; Barus, is meant that they will remain suspended in the gas of the fog chamber indefinitely until an exhaustion is made. Several successive exhaustionB, how-ever, will bring them down in droplets and thus purify the air. These per-sistent nuclei are particularly efficient for nucleation at a difference of pressure of 1 6  to 18 cm. By difference of pressure is meant the difference between the barometric p res·sure in the fog chambt'r pro dured by exhaustion and that o f  the outside atmospheric pressure. The second class of  nuclei denoted by Barus are called "fleeting nuclei ." They are produced by some ionizing source, such as X-rays or radium. "They carry a charge of electrification and are called ions." These fleeting nuclei are most efficient for nucleation at a dp. of 19 cm. or  higher. When a weak radium compound i's brought near the fog chamber, th1> interior is instantly surcharged with fleeting nucleL Barus does not attempt to describe how these are produced. The experimental work of Rutherford has abundantly proven that the Alpha and Beta rays of a radio active pompound are the principal sources of its ionizing power when they are not intercepted by some intervening solid.  The Alpha rays are by far the best i onizers, but unfortunately they are easily cut off by the slightest obBtruction, such as a thin piece of 'celluloid. It  is  evident then tha t  t h E:  Alpha or  Beta rays a r e  ineffective a s  ionizers of t h e  interior gas o f  a fog chamber having such thick walls of glass or wood as that used by Barus. Barus i;ightly concluded that the interior of his fog chamber was ionized wholly by the Gamma rays and their 'Secondary effects on the walls o f  the Yessel.  The Gamma rays of radium, as we know, are like the X-rays, although decidedly more penetrating and more unifo rm in intensity. According to modern theory, the ionization of a gas by means of  X-rays or Gamma rays is due to the fnct that these rays have the power of disintegrating its atomic structure. A stable atom consists of  a multitude of negatively charged electrons moving with high velocities inside of a positively charged sphere, the whole in static equil ibrium.  The action of the Gamma rays is  to  set  free a negative electron leaving a sur ­plus po sitive charge on the atom. Such a freed electron and its corresponding positively charged atom from which it has been delivered constitute the fleeting nuclei mentioned by Barus. They are fleeting because when the ionizing source, radium or X-rays,  is removed, they at once recombine and disappear in very small intervals o f  time. Barus found also that persistent nuclei ·similar to those o f  a dust-l ike char­acter mentioned first could be induced by X-rays even in thoroughly dust frer air. These persistent nuclei varied in number with the nature of the sol id m a­terial thru which the X-rays past. They are probably disintegrated particles of  matter carrying one or several ions .  Like the dU'st nuclei,  they produce l arge droplets on exhaustion at small differences of  pressure. Those who have read the account of Wilson's  experiments to determine the charge of an electron . by means of X-rays will remember that his clouds broke up into a succession of layers.  Barus'  notion of persistent nuclei evoked by X-rays gives a simple and reaso nable explanation of these layers. ! .. 2Proceedings of the Iowa Academy of Science, Vol. 15 [1908], No. 1, Art. 26https://scholarworks.uni.edu/pias/vol15/iss1/26J IOWA ACADEMY OF SCIENCE 167 The third class of nuclei mentioned by Barus are the colloidal type.  Stated in his words, "they are a structural part of the body of gas and are reproduced aa 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 en ergizing source such as X-rays and radium was brought near the fog cham­ber. \Yith 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 fleeting nuclei .  Owing to the smallness of the droplets B arus seems to infer that the nuclei are also smaller than those of any other kind. Barus speaks of the colloidal nuclei as i f  they \Yere perhaps the molecules or  the normal atoms of either the gas or vapor in the fog chamber. It  is  not the vuri1ose of this article to go extensively into the vast amount of work performed by B arus and his ass istants, but it  might be wdl to mention a few of th e 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 num­ber of efficient nuclei induced by X-rays varies rapidly w ith the strength of  the rays and also with the ·sudclPnness and ease of exhaustion .  \Vhen the exhaust tube was enlarged to one and on e-half incoh0s in diameter, as many as 400 ,000  efficient nuclei  per cubic  centimeter were obtained.  The term "efficient nucl ei" is applied only to those that induce the condensa­tion of vapor to form the droplets of the cloud. There are no doubt many ofoers besides those that get the mo isture . I n  fact, it was proven that when a mixture of nuclei of different electrical magnitudes were pre·sent i n  the fog chamber, the larger ones on the first exhaustion received all the moisture. A secon d  exhaus­tion following rapid ly  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 station s  were establisht, one at Providence, Rhode Island, and the other on Block I sl and,  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 diminish­ing during the day until the middle of the afternoon,  and increasing again to­wards Pvening. Rains were always followed by a markt decrease in the num­ber of nuclei .  I t  was also inferred that light pressure decreast lht 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 l ies well out at sea, where the air i"s not affected by local contaminations. The number of nuclei at Providence ran as high as 60,000 to 8 0 , 000 per cuhic centi-3Begeman: Nucleations According to BarusPublished by UNI ScholarWorks, 1908168 IOWA ACADE M Y  OF SC IENCE mete r ;  at Block Island 10,000 to 15,000.  It is evident that the products of r:om· bustion of a large city furnish a vast number of nuclei and in this way supply the material necessary for the condensation of moisturP. It might be askt why is the nucleation of the atmosphf're so enormous in the winter time ? The reason, of course, is that there is V<•ry little watery vapor in the air at low ::< tempe ratures for condensation on the nuclei to bring them down. The result is that the nuclei gradually accumulate as the> weather grows colder and colder, reaching a maximum at the winter solstice. In  his later work Barus carried on a sPries of experiments to determine whether or  not the ioniz<•d nu elei of the atmasphPre at Providence were due to local causes. To do this,. the air before being admitted into the fog chamber was past thru a tubular condenser, one surface of which was charged. In  this way the charge given up by the ions in a given time for a given •quantity of air was accurately measured. D ividing this charge by 3 .4xl0-10, J .  J .  Thom­son's de.termination of the charge of an electron,  gave the number of ions per cubic centimeter. From this work Barus concluded, quoting his o wn words, "that the ionization of a giveh region is independent of artificial local contribu­tions, however abun dant thPse may be." We would infer from this that the ionization of the atmosphere results from cosmi cal rather than local conditions. 4Proceedings of the Iowa Academy of Science, Vol. 15 [1908], No. 1, Art. 26https://scholarworks.uni.edu/pias/vol15/iss1/26

Nucleations According to Barus

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Nucleations According to Barus

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