ences, but they are differences in some of the results of the action going on, not in causes or methods. The first of the two kinds of volcanoes prepares for a new eruption by the gradual filling up of the emptied crater, doing this by means of one or more lava-vents in the bottom, which, besides throwing up cinders, have their little outflows (as well described by Secchi for Vesuvius), and keep at the work until the crater is filled, or nearly so; and then come the break and the greater outflows. The second kind differ only as to the cinders; and in Kilauea, as to the height of the floor before the outbreak. Both from Vesuvius and Kilauea we learn that, next to the lava-vent, the crater of a volcanic mountain is its prime or most fundamental element. It incloses the extremity of a lava-conduit of greater or less breadth that reaches down to the seat of fires; and this inclosure exists because of the ejections by outflows and upthrows of the consequent downplunges, which superficial conditions in large part determine. The growing mountain-cone can not be rid of its crater except by the gradual disappearance and healing over of the lava-vent; and, commonly, when extinction happens, the crater is still of nearly full size. If half or wholly obliterated, it may be again restored; and is likely to be, if activity is ever renewed in the region by new aggressive action below. If so renewed, it may go forward through refusions and new ingulfment. But the first step may be the opening of the old fissure upon which the crater was originally made; in this way the lava-conduit might secure for itself at once an open way to the surface. It may be that the course of the old fissure has been a chief cause in determining the form of a crater; and it may lead, in after-history, to changes in the locus of the chief vent, or an elongation of the crater in one direction rather than in another.
Water-Pipes of Lead, Tin, and Iron.—On the question of the "Action of Drinking-Water on Lead," Dr. Tidy, Mr. Crookes, and Dr. Odling have reported to the British Association that they deem it impracticable, even though it were advisable, which they doubt, to replace lead pipes by iron pipes, or even by pipes of tinned lead. In tinned-lead pipes the tinning is found not only to be detrimental to the strength of the lead, but to be likely, unless the coating is perfect, to assist the dissolution of the lead. Iron pipes, although strong, and safe so far as health is concerned, are more likely to break, more difficult to adjust and repair, very easy of oxidation, and liable to obstruction from accumulation of the oxide. Tin is also acted upon by water, though its toxic action is below that of lead. It is fairly flexible, but four times as expensive as lead, but a thinner pipe would suffice. There is reason, however, to believe that waters do not afford a protective coating to tin as they do to lead. The advantages of lead service-pipes are their cheapness, durability, and flexibility, and the ease with which they can be wrought and repaired. The authors recommend the systematic and continuous filtration of the water, with such modifications of the filter-bed as will insure its efficient silication. This will, in their belief, minimize and practically prevent the action of the water on the lead surfaces. It would, moreover, improve the brightness and color of the water, and lessen the quantity of organic matter held in solution.
Practical Chemistry in Housekeeping.—The "Popular Science News" notices some facts connected with the preparation of food that illustrate how the housekeeper is in reality a practical chemist. The object of all cooking, or application of heat to the raw material of food, is to bring about changes in the character of certain bodies of complicated organization; and this is often done without producing any difference in composition perceptible to the chemist. Such is the case when albumen is coagulated: we clear coffee with albumen or the white of an egg, through its power of inclosing particles in suspension when it becomes hard. If the coffee-maker uses fish-skin, it performs the same office by forming a kind of leather with the tannin of the coffee. Glue is a coarse, and cooking gelatin a refined form of the same substance, which is insoluble in cold water, but absorbs it, swelling up and becoming soft. When heated with the water, gelatin dissolves, and then, when cooled again,"jells"; but, if boiled too long, it loses the "jell-
