332 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



OTHER USES FOR ION EXCHANGE 



The application of ion exchange in strengthening glass is only one 

 of many examples of ionic and molecular diffusion and exchange, un- 

 familiar to most of us, that play significant parts in the manufacture 

 and uses of glass. 



The modern glass manufacturer immerses carbon electrodes into 

 his continuous tank of molten glass and helps to heat it more uni- 

 formly by using the glass melt as a resistance ; and he heats and seals 

 glass parts by taking advantage of the ionic conductivity of glass at 

 elevated temperatures. 



The familiar red-stained chemical glassware and the yellow fog 

 lenses of auto headlights are colored by reactions in which copper or 

 silver ions replace an equal number of sodium ions in the glass sur- 

 face at high temperature and are then reduced to colloidal metal by 

 hydrogen or other reducing gases. 



Even at room temperature, the performance of glass electrode pH 

 meters depends on ion exchange between hydrogen ion in the solu- 

 tion being tested and cations in the glass electrode immersed in it. 

 New glass electrodes have recently appeared on the market which also 

 measure sodium and potassium ion concentrations, and even calcium 

 concentration determinations have been reported. One of the signifi- 

 cant papers of the Glass Congress is being presented by George Eisen- 

 man, a medical doctor who is learning to interpret the structure of 

 glass from glass electrode potentials in solutions of various cations. 

 He hopes that glass electrodes can be used to understand the way in 

 which cations such as sodium and potassium move through living 

 tissues, since such movement is crucial to many metabolic processes. 



NEW INSIGHT INTO NUCLEATION 



The discoveries in the photochemistry and crystallization of glass 

 described earlier have aroused the interest of a number of scientists 

 researching the fundamental mechanisms involved in the earliest 

 stages of a photochemical reaction or of crystallization. 



An example is the finding by R. D, Maurer of Corning that the 

 smallest stable gold particle capable of growing into a gold crystal 

 consists of 3 or 4 atoms ; and that the smallest silver particle capable 

 of nucleating the growth of a lithium silicate crystal is 80 angstroms 

 in size. Such information can be valuable in testing theories as diverse 

 as the theory of the photographic latent image, or the quantitative 

 theory of nucleus formation in phase changes such as condensation of 

 vapor or crystallization. Perhaps it is coincidence, perhaps not, that 

 other investigators had previously found the "critical nucleus size" of 

 supercooled water vapor — the smallest-size stable droplet capable of 

 growing — to be also about 80 angstroms. 



