Page:Popular Science Monthly Volume 12.djvu/657

Rh the ramming was blown out. The machine is described as very handy and compact, and easily used, while the cost, beyond the first outlay, is absolutely nil.

The Heliotype Process.—For some years an eminent publishing-house in Boston has been engaged in the production of "heliotype" copies of famous works of art, and the public is now more or less familiar with the products of the "heliotype" process. But what that process is we have nowhere seen explained till recently a writer in the Tribune gave a very intelligible account of its modus operandi. In what follows we propose to give in brief the main points of the author's explanation: First a photographic negative is made in the ordinary manner; but the "positive" plate is peculiar. The basis of its composition is gelatine, with a mixture of bichromate of potash and chrome alum. This mixture is dissolved in hot water, and the solution is then poured on a plate of glass or metal, and left to dry. When dry it is about as thick as ordinary parchment, and is stripped from the plate and placed in contact with the previously prepared negative; the two are then exposed to the light. The bichromate of potash makes the gelatine plate sensitive to light, and wherever the light touches it the plate becomes leathery or water-proof. The result of the exposure to light is, therefore, that a portion of the gelatine plate—the image—is water-proofed, while the remainder is absorbent of water. Now, we know what a repulsion exists between water and greasy substances of every kind—for instance, printer's ink. It follows that, if we moisten the gelatine plate, the unchanged parts will absorb the water, and, if ink is then rolled over it, it will adhere only in the altered parts. By so applying ink, the sheet of gelatine is converted into a "positive" plate from which copies can be taken on a printing-press. This plate, strange to say, is very durable, and is capable of yielding, with fair treatment, several thousand impressions. Of course, the sheet of gelatine must have a solid base given to it, to hold it firmly on the bed of the press while printing. This is accomplished by uniting it under water with a metallic plate, exhausting the air between the two surfaces, and attaching them by atmospheric pressure.

Epidemics and Ablution.—A short time ago we published some remarks of Dr. Hebra, of Vienna, depreciatory of the value of frequent bathing. A diametrically opposite opinion is held by the eminent hygienic reformer, Edwin Chadwick, C. B., who cites facts to prove that skin-cleanliness, or in other words frequent ablution of the whole person, is a powerful preservative against all infectious and contagious diseases. He asserts that in children's institutions the death-rate and cases of sickness have been reduced one-third by regular head-to-foot ablutions with tepid water. Experienced trained nurses, regularly attending scarlatina-patients, give themselves regular head-to-foot ablutions twice a day, and a change of clothes once a day. Medical men of experience, who serve amid plagues and the most terrible epidemics, do the like. Mr. Chadwick adds: "If I had again to serve as a member of a general board, and had to exercise authority in providing defenses against epidemics, I would propose regulations for the immediate and general 'tubbing' of the population, and have it seen to as sedulously as vaccination for protection against smallpox." To show the influence of skin-cleanliness on the assimilation of food, Mr. Chadwick relates the following incident: "A friend of mine," he writes, "in command of a brigade in Spain, was hemmed in, and his men were put on very short rations; and to amuse them—it being summer-time—he encouraged them to bathe daily in a river close by, and he marked, as a result he had not expected, that his men were in as good strength as the unwashed soldiers on their full rations." Similar results are observable in the inmates of well kept prisons.

New Process of Embalming.—A new and inexpensive method of embalming has been invented by Dr. Lowell, of Brooklyn. The preservative fluid he employs is a solution of zinc chloride which is injected into the body either by an artery or a vein. The apparatus required consists of a porcelain-lined vessel, which is elevated to such a height that the solution may be injected into the cadaver after the manner of a gravity-syringe. For the passage of the preservative fluid from this receptacle into the vein of the cadaver, glass and rubber