CARBONIC ACID GAS, CARBON DIOXIDE.--Now of only scientific interest, it may be noted that some years ago great claims were made for the use of carbon dioxide gas as a coagulant. In actual practice we were unable to effect coagulation by passing the dry gas into latex. It was suggested that the original investigators were misled by failure to secure a dry and clean gas. It would appear that probably the gas was prepared by the action of hydrochloric acid upon marble or limestone. Unless intervening "washers" and "driers" were used, the liberated gas, when passed into latex, would carry with it traces of hydrochloric acid, which would effect coagulation. ALCOHOL.--In the cheap form of methylated spirit, alcohol has been employed by us as a speedy coagulant for many years. Latex run slowly into alcohol coagulates instantaneously. The method has been in common laboratory use. The employment of alcohol has also been made the part-subject of a patent process of coagulation, to which reference will be made in the succeeding chapter. VEGETABLE EXTRACTS.--At various times experimental work has been directed towards the use of liquids of purely vegetable origin, such as the juices of tropical fruits, and of a waste product of tropical industry--the so-called "milk" (or water) of ripe coconuts. In the former class there is usually a natural acidity, but in coconut water the acidity is chiefly the result of fermentation of the carbohydrate (sugar) constituents. These substances were all found to effect a more or less satisfactory coagulation, but it is unlikely that they would be suitable for practical application on a large scale. As being more directly related to the subject of coagulation in general than to coagulants in particular, a discussion of several special processes will be relegated to the ensuing chapter. CHAPTER XX _SPECIAL METHODS OF PREPARATION_ Every year appears to bring forth some new ideas in the mode of rubber preparation. Some of them are based in principle upon the oldest known method--_i.e._, the native Brazilian process of making "Hard Para." Others strike a new note, and in a few cases the claims put forward are substantially confirmed by results. In other instances the claims are too pretentious, and discredit may be brought upon schemes which, although lacking in comparative success, are yet commendable for the ingenuity manifested. To the present not one of these new methods has been able to compete to any marked degree in general practice with the established methods of ordinary preparation. A few continue to find local application, but most have either been abandoned or are gradually falling into desuetude. We do not propose to discuss in fine detail all the various claims made on behalf of these special processes, or to enter into controversies. The aim is to present to the reader an outline embodying the main principles and advantages claimed.