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Gelatine, as found in commerce, is a very variable substance, and is often impure. It is difficult to give a rule for its selection; but, speaking generally, the gelatine sold for culinary purposes answers well for carbon printing. Common glue is not suitable, and the best samples of gelatine used in cookery are unnecessarily expensive. Different gelatines vary considerably, both in the proportions required, and in the results they produce. Some samples of commercial gelatine, and some of glue, show a tendency to dissolve in cold water. These are unsuitable for the process. Impurities, such as alum or acid, are highly objectionable.