Although much has been said and written on this subject, it is still a question of much interest. It is taken up by the London Engineer of the 12th of February, in which opinions long since put forth by us are enforced. It says :—" We believe that nothing short of an entire revision of the prevalent practice of feeding boilers can be satisfactory. The proper way is to purify the water before it is put in the boiler, not after. That is a full, sufficient and comprehensive solution of theprob-lem of incrustation and its prevention." These are sensible words. In our opinion, there is not a railroad, and but very few factories in our country, but could find, at no very great expense, means to supply their engine boilers with soft water. The incrustations in steam boilers are deposits from hard water used in feeding, which leave a coating of stone—sulphate and carbonate of lime —on the inside of the boiler. As this stone coating is a non-conductor of heat, of course, it causes an immense loss, by the excess of fuel required to generate the steam, while at the same time it renders the metal liable to be burned, by keeping the water from direct contact with the iron. Soft water employed for feed entirely prevents the formation of such incrustations, and if possible, no other kind should be used. There are many situations, however, such as in cities and villages, where soft feed water cannot be obtained to supply steam boilers—situations where hard water is the only supply. A simple, cheap, and efficient means of preventing incrustations in such places is a desideratum. In former volumes of the SCIENTIFIC AMERICAN, more varied and useful information regarding such means or agencies can be found, we are confident, than in any other periodical cr work whatever ; but we have been informed that the patent of Robert McCafferty, of Lanas- ter City, Pa., obtained April 14, 1857, em-braces perhaps, the most efficient method of doing this yet discovered. If so—and from its nature, we think it must be good—it should be generally introduced. From his specification we will therefore give a condensed description of the invention :— The substance employed is black gum catechu, (which is well known to dyers and tanners). Half a pound of this catechu is put into a steam boiler of 100-horse power, in which it is dissolved by the water, to which it imparts a color resembling pale brandy, or a light reddish-brown shade. This is to commence operations. While the boiler is in use during the week, the water is kept as near to this color as possible, by adding small pieces of the catechu. This is the sum of the invention. Mr. McCafferty discovered that when the water was maintained at the shade described, no incrustations were formed in the steam boiler which had been previously subject to them. And not only did the catechu prevent scale forming in the boiler, when harfl water was used, but it removed thick incrustations which had been formed, and converted them into a kind of soft slush, which was easily forced out by the blow-off cock. Catechu dissolves easily in water, and is one of the most powerful astringent gums known. It is a simple and more convenient agent for combining with the lime in the water, and preventing it forming a coat of stone on the boiler, than sawdust, blocks of oak, sal ammoniac, and many other things that have been employed ; but whether it is superior to molasses, oil, and tar, which have also been employed, we are not prepared to say. We hope it is, because we welcome every new and useful improvement. Catechu has been used before for the same purpose, but not by itself, as Mr. McCafferty has employed it. M. Delfour, of Paris, tried it some years since, in combination with various salts, as described on page 40, Vol. VI, SCIENTIFIC AMERICAN, and obtained good results therefrom. In Neioton's London Journal of last month, (February), there is a description of a composition for removing and preventing incrustations in boilers, for which a patent has been obtained by Henry Hobbs and Edward Easton, of London. It consists of arsenious acid, (white arsenic of commerce), and an alkali— the carbonate of soda (sal soda) being preferred. These are mixed together in equal quantities (by weight) in a vessel, with a small quantity of water—about a gallon to five pounds—and are kept boiling until the arsenic is dissolved ; the mixture is then cooled down slowly, and is ready for use. About one gallon per week is sufficient for a 40-horse power boiler ; and all that is required, it is stated, to keep the boiler perfectly clean when using it, is to blow-off regularly at the lower cock. This may be a very good composition for the purpose, but it is certainly a dangerous one to use ; and great caution and care are necessary, to guard against evil results. We have received a very original and interesting communication on this subject from Henry Fisher, M. D., of this city, in which he describes experiments made by him with the metal antimony to prevent incrustations in steam boilers. For good reasons we here mention the substance he has successfully experimented with, as we have not been able to find room for his article in this number