AND PROPERTiEg OF GLASS CONTAINERS 27 the simplest glass in water. All that can be measured and compared, is a rate of attack, and all the measurements that have been made represent not solubilities but rates of reaction. Rates of reaction, however. differ from equilibrium values in being highly susceptible to slight differences in experi- mental conditions, and unfortunately many of the data in the literature have been obtained under poorly defined experimental conditions. Com- parison of the results of one worker with those of another is rarely possible. EXPERIMENTAL METHODS Since it is not possible to measure definite solubilities of glasses, the suitability of a glass for a particular purpose is best determined by actual service, or by exposing it to conditions simulating as nearly as possible those that will be met. Thus, for optical glasses, actual use in the finished instru- ment is the ultimate criterion and exposure to the elements under service conditions probably the next best. Such tests require a long time, and in addition they lack one prime requisite of a test of practical value in the development of resistant glasses. This requisite is that the test shall give such quantitative information that the glass maker can have a categorical answer to the questions whether or not a given change in composition or treatment is benficial, and which of several possible changes is the one which will give the most useful glass. It is in the attempt to provide such a method that much of the information that we have on the chemical proper- ties of glass, has been obtained. Most of the methods that have been used for testing the weathering qualities of glasses are variants of a few main ones. All have been developed from the same basic thesis, namely, that the verdict of actual service may be anticipated by accelerating the corrosive action by the use of higher tempera• tures, and by using sensitive methods to detect and measure the resulting action. Several of these methods have been combined for each has its faults as well as its virtues, and no method can be safely chosen as the best to be applied universally. The choice of method is largely determined by the use to which the glass is to be put. Methods suitable for differentiating between glasses to be used in optical instruments may be far too mild to discriminate adequately between types of chemical glassware. Thus, exposure to moist air would have little effect on Pyrex, etc., but it affords a suitable method for studying optical glass. One method which has been applied extensively to optical glass is the iodeosine test of Mylius, which forms the basis of the so-called hydrolyric classification. A surface of known area is immersed in a solution of iodeosine in ether for one minute, the excess iodeosine is rinsed off with ether, the alkali salt of the dye is dissolved in water, and the amount of iodeosine combined with the alkalis is estimated colorimetrically, by comparison with

28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS a standard solution. This method is said to be one of the best for testing the chemical sensitivity of glasses and gives reliable indications with the common types, in which lack of stability is associated with an excess of alkali. With other types, however, it may be misleading. Some of the earlier methods adopted for the testing of bottles for medi- cines and drugs made use of the sensitivity of alkaloids (or other materials frequently encountered in medicines and drugs) to the influence of traces of alkali extracted from the glass, with the subsequent precipitation of the alkaloid itself. The most sensitive of these tests was Kroeber's narcotine hydrochloride test, which was adopted by the German Pharmocopoeia for the testing of medicine bottles. After washing, the bottles are filled with 0.1% aqueous narcotine hydrochloride, bottles of 100 cc capacity to the neck, larger bottles about half full. After standing for one hour, there must not be more than a barely perceptible crystalline separation in the bottle, and no cloudy precipitate or flocculent free narcotine base. This test was critically examined by Turner et al who concluded it to be a quick and satisfactory method which could be carried out in a factory without using expensive apparatus, or a high degree of scientific skill. It was not, how- ever, recommended for inclusion in the British Pharmocopoeia in 1931 on the grounds that the precipitation of an alkaloid salt only measured a change of reaction as did an indicator, so that it was simpler (and presumably of wider application) to use an indicator. The evaporation method is especially applicable for testing chemical glassware, and consists of heating with water or aqueous solution at atmos- pheric pressure. One version has been adopted by the Society of Glass Technology in their "Standard 5-Hour Boiling Test for Medicine Bottles". Six well-washed bottles, after having been warmed with hot distilled water are filled, to the bottoms of the necks, with boiling carbon dioxide-free distilled water, and closed lightly with copper capsules. The filled bottles are immediately placed in a suitable steam bath at atmospheric pressure, suspended by their necks, and heated for five hours, after which the contents of each bottle are transferred to a tested conical flask and boiled for 15 min. with a known volume of standard sulphuric acid solution. The mixture is then titrated, and the sulphuric acid which has been neutralised by the alkali dissolved from the bottle is calculated to mg Na20. 4 oz medicals are not satisfactory if the Na•O dissolved exceeds 5 mõ. The test was critically discussed by Cousen, who found the reproducibility to be about 0.2 mg Na•O/sq. dcm. and that the chief cause of variation was changes in the rate of boiling of the water. To allow for variability in the area of bottles of the same volume, (4 oz), he suggested that results be expressed as mg Na•O/sq. dcm. of surface, on which basis the above specification becomes 3.3 mg/sq. dcm. It must be noted that the total alkalinity, though reported