The pH responses of electrodes prepared from glasses of widely different composition have been compared with the h ygroscopicity of the glasses. The rcsulting data indicate that glasses of low hygroscopicity, such as the chemical glasswares a nd common types of optical glasses, give pH responses that fall appreciably below the theoretical predi cted from the Nerll~t e qu ation and are incapable of producing satisfactory electrodes. F ur ther, electrodes blown from glasses of intermediate

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... opicity, s uch as the common bottle and sheet glasses, give pH responses more nearly approximating the theoretical, whereas t he Corning 01 5 glaRs, whose superior pH response places it in a class hy itself, has a very high hygroscopicity. These results strongly su pport the belief that. adequate hygroscopici ty is one of the primary factors in determining the sui tahility of a glass for pH measurements. The data also suggest that the pH response might well be used as a rapid t est for the serviceability of optical glasses, i. e., t he ability to maintain a clear polished surface upon exposure to the atmosphere. For t his purpose it is necessary to determine the pH response over a range in which the "chemical durability" of the glass remains constant in order to avoid the voltage anomalies that accompany durability changes.