3350427 no connotation of absolute measurements but only of relative measurements. The freezing point of a pure solvent is, in effect, that temperature at which a liquid phase and a solid phase can coexist without change of mass of the individual phases. The freezing point of a solution is that temperature at which a solid, pure-solvent phase can coexist with a liquid-solution phase without change of mass of the individual phases. The difference between these two temperatures is the freezing-point depression. In other words, the temperature of a solution and of the pure solvent must be decreased below the freezing point of the pure solvent in order to establish equilibrium with respect to the solvent between the solvent in the solution and solvent in the pure solid phase. Similar remarks apply to boiling point elevation. Osmotic pressure is then simply the pressure increment that must be imposed on a solution in order that pure solvent phase and solution phase, separated by an appropriate barrier permeable only to the solvent, can coexist without change of mass of the individual phases. In other words, osmotic pressure as a pressure dif- ference is similar to the freezing-point depression and to the boiling-point elevation. This should provide an adequate nega- tive answer to the often asked question: “Does osmotic pressure exist as such in a solution?” A negative answer is also provided from the derivations we have given. It may be pointed out, in addition, that Gibbs referred to the “so-called os- motic pressure” and indicated clearly that the pressure calculated by the laws of Boyle, Charles, and Avogadro for the solute in the space occupied by the solu- tion was calculated and not experimen- tally found (/0). Pressure can be measured ina single phase, just as temperature can. But just as the determination of bardment pressure of solute molecul against an impermeable barrier w: eventually be abandoned. References and Notes 1. J. H. Hildebrand, Science 121, 116 (1955). 2. J. D. Babbitt, ibid, 122, 283 (1955). 3. We gratefully acknowledge the support pr provided by the Life Insurance Medical R search Fund and by the U.S. Atomic Ener: Commission [contract AT(30-1)-1394] for i vestigations related to this problem. 4. F. P. Chinard, G. J. Vosburgh, T. Enns Am, J. Physiol, 183, 221 (1955). 5. J. W. Gibbs, Trans. Conn. Acad. Arts Sci. 108, 343 (1878). 6. freezing point or boiling point requires the coexistence of two phases, so does the determination of osmotic pressure re- om sion and bviling-point elevation; there is One does not subtract freezing-point depression from the actual temperature of a solution to calculate an “effective 10, quire the coexistence of two phases (//). temperature.” No more should one sub- tract osmotic pressure from the actual pressureof a solutionto calculate an “effective pressure.” It is recognized, as Haldane pointed out a number of years ago (/2), that the concept of osmotic pressure as something which exists in solutions even when no external pressure is applied is firmly entrenched, It is hoped, nonetheless, that this discussion may help to clarify the meaning of osmotic pressure and that the concept of osmotic pressure as the bom- W. M. Clack, Topics in Physical Che mistr A supplementary text for students of med cine (Williams and Wilkins, Baltimore, Md ed. 2, 1952). F. P. Chinard, J. Chem. Educ, 32, 377 (1955 E. A. Guggenheim, Thermodynamics. A advanced treatise for chemists and physicis (Interscience, New York, 1949). 9. F. PL Chinard, J. Chem. Educ. 31, 66 (1954 J. W. Gibbs, previously unpublished fragmer in The Collected Works of J, Willard Gibb Vol. I. Thermodynamics. (Yale Univ. Pres New Haven, Conn., 1948), p. 419. 1. The equilibrium condition under which o motic pressure is determined is frequent] referred to as a membrane orpartial equilit rium: equilibrium obtains with respect ¢t the solvent but not necessarily with respec to the solutes. Similarly, freezing-point an boiling point equilibria are partial equilibria equilibrium is established with respect to th solvent but act necessarily with respect to th solutes. The phase boundaries in these deter minations are analogous to the semipermeabl membsanes used in determinations of osmoti pressure: it is assumed that the solutes canno enter the pure solvent phases. 12. J. S. Haldane, Gases and Liquids. A contribu tion to molecular physics. (Oliver and Boyc Edinburgh, Scotland, 1928), p. 159. simultaneously and have only the station with rain note the presence of the par ticles overhead. The gummed-filn World-Wide Travel of Atomic Debris L. Machta, R. J. List, L. F. Hubert method of collection is recognized a: being as crudeasit is simple. The nuclear explosions are treated in this article, the Mike shot on | November 1952 and the Bravo shot on | March 1954. The shots were similar in that both are described as having had energy in the megaton range, both were detonated at or near the earth’s surface on a coral island, and both had atomic clouds that penetrated into the stratosphere. To the meteorologist, the main difference of in- terest between the two events is the For centuries meteorologists have thought of exploring large-scale atmospheric circulations by means of tracers. The literature describes how man has cient radioactive debris was thrown into the atmosphere to be deposited in both hemispheres. Measurements of the de- radioactive tracers across the United and measurement techniques have been successfully tracked fluorescent particles to a distance of 100 miles (J), used States (2), and followed volcanic ash and forest fire smoke over distances of the order of 1000 miles (3). Only the dust from a major volcanic eruption, such as Krakatao, has been tracked on a truly global scale. During two of the nuclear test periods in the Pacific Proving Grounds of the U.S. Atomic Energy Commission, suffi474 season. posited radioactivity were obtained from Winds tails of the network and the sampling The winds acting on the two atomic clouds at the time of detonation are illustrated in Fig. 1. The wind structure exposed sheets of gummed film. The de- described by Eisenbud and Harley (4). It should be noted, however, that the deposition of particles on the adhesive surface depends cither on the presence of precipitation or, in dry weather, on turbulence to assist the impaction of the particles on the horizontal surface of the paper. [t is thus possible to have a cloud of radioactive particles pass two stations has been estimated, when necessary, from observations at nearby locations and times. On both days the tropopause was found at an altitude of about 55,000 feet, and it separated winds blowing from dif- ferent directions. The easterly winds The authors are on the stalf of the U.S. Weather Bureau, Washington, D.C. SCUENCE, VOL. 124