POPULAR MISCELLANY. 



281 



ences, but they are differences in some of 

 tiie results of the action going on, not in 

 causes or methods. The first of the two 

 kinds of volcanoes prepares for a new erup- 

 tion by the gradual filling up of the emp- 

 tied crater, doing this by means of one or 

 more lava-vents in the bottom, which, be- 

 sides throwing up cinders, have their little 

 outflows (as well described by Seechi 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 A^esuvius 

 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 ex- 

 ists because of the ejections by outflows 

 and upthrows of the consequent down- 

 plunges, which superficial conditions in 

 large part determine. The growing mount- 

 ain-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 obliter- 

 ated, 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 re- 

 fusions 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 upon which the crater was originally made ; 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 sur- 

 face. 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-Pipcs of Lead, Tin, and Iron. — 



On the question of the " Action of Drinking- 

 Water on Lead," Dr. Tidy, i\tr. Crookes, and 

 Dr. Odling have reported to the British As- 

 sociation 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 per- 

 •fect, 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 obstruc- 

 tion 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, durabil- 

 \ ity, and flexibility, and the ease with which 

 : they can be wrought and repaired. The au- 

 I thors recommend the systematic and con- 

 I tinuous 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 quan- 

 tity of organic matter held in solution. 



Practical Chemistry in Uonsekeepin^. — 



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 coagu- 

 lated : 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 gela- 

 tin a refined form of the same substance, 

 which is insoluble in cold water, but ab- 

 sorbs it, swelling up and becoming soft. 

 When heated with the water, gelatin dis- 

 solves, and then, when cooled again," jells " ; 

 but, if boiled too long, it loses the " jell- 



