C EM

CEM

( 27 $ )

whether frefh or fait a fufficient length of time,the ufe of common mortar muft be abandoned;for lime and fand if mixed together in any pro-portions, and put while foft into water, will ina Ihort time fall to pieces.

Among the nations of antiquity the Romansappear to have been the only people who prac-tifed building in water, and efpecially in the fea,to any great extent. The bay of Baise, like ourfafhionable Watering places, was the fummerrefort of all the wealthy in Rome , who not con-tent with erecting their villas as near the Ihoreas poffible, were accuftomed to conftruct moles,and form fmall iflands in the more fhelteredparts of the bay, on which, for the fake of thegrateful coolnefs, they built their fummer houfesand pavilions. They were enable to build thusfecurely in the water by the fortunate difcoveryat the neighbouring town of Puteoli of anearthy fubftance, which, from this circumftance,was called pulvis Puteolanus (powder of Puteoli ).Puteolan powder, or as it is now denominatedPuzzolana, is a light, porous, friable mineral ofa red. colour, and is generally fuppofed to de-rive its origin from concreted volcanic afhesthrown out from Vefuvius, near to whichmountain the town of Puteoli is fituated: itfeems to confift of a ferruginous clay bakedand calcined by the force of volcanic fire, andwhen mixed with common mortar, not onlyenables it to acquire a remarkable hardnefs inthe air, but to become as firm as ftone evenunder water. The only preparation whichPuzzolana undergoes is that of pounding andfifting, by which it is reduced to a coarfepowder: in this Hate, being thoroughly beatenup with lime, either with or without fand, itforms a mafs of remarkable tenacity whichfpeedily fets under water, and becomes at leaftas ftrong as good freeftone. In the compofitionof water cements it is that the fuperior efficacyof argillo-ferruginous lime compared with thepurer kinds is moll ftrikingly manifeft, and asbuilding in water is generally very expenfiveand difficult to repair, every precaution fhouldbe taken to fecure the goodnefs of the cement.In fituations expofed to the violent ffiocks ofthe fea the difference of expence between thebell and inferior kinds of mortar is fo little,compared to the whole cod and the fatisfaclionof perfect fecurity that the cement ought to beof the very belt quality. That which was ufedby Mr. Smeaton , in the conftruction of Eddy-ftone light houfe, was compofed of equal partsby mealure of flacked Aberthaw lime and Puzzo-

* Vogel in N. Comm, Gott. iii. P-5 r -

Iana; but for works that are lefs expofed, fuchas locks and bafons for canals, &c. the propor-tions of Puzzolana may be confiderablv dimi-nifhed. A compofition of this kind which hasbeen found very effectual is 2 buffiels of flackedAberthaw lime, 1 bufhel of Puzzolana, and 3of clean fand; the whole being beaten welltogether with the proper quantity of water willyield 4.67 cubic feet of cement.

The Dutch have pradtifed building in waterto a greater extent than any other nation ofmodern Europe , and to them is due the difco-very of a cement admirably well qualified forthis purpofe, and called tarras or trafs. This isnothing more than wakke or cellular bafalt, andis procured chiefly from Bockenheim, Frankfort on the Maine, and Andernach , whence it istranfported down the Rhine in large quantitiesto Holland / It undergoes no further prepara-tion than grinding and fifting, and being thusreduced to the confiftence of coarfe fand it isbeaten up with the blue argillaceous lime fromthe banks of the Schelde, and thus compofesthe celebrated tarras mortar with which themounds and other conftru&ions for the purpofeof protecting the lowlands of Holland againft:the fea are cemented. The ftrongeft tarrasmortar is compofed of 2 meafures of flackedlime, and 1 of tarras; another kind almoft equallygood and confiderably cheaper, is made of twomeafures of flacked lime, one of tarras, and3 of coarfe clean fand ; it requires to bebeaten a longer time than the foregoing,and produces 3-j- meafures of excellent mor-tar. When the building is conftrudted o£rough irregular Hones where cavities andlarge joints are to be filled up with cement, thepebble mortar may be mod advantageoufly ap-applied: this was a favourite mode of conftruc-tion among the Romans, and has been ufed everfince their time in thofe works in which a largequantity of mortar is required. Pebble mortarwill be found of fufficient compadtnefs if com-pofed of 2 meafures of flacked argillaceous lime,half a meafure of tarras or puzzolana, 1 mea-fure of coarfe fand, 1 of fine fand, and 4 offmall pebbles fereened and waffied. Althoughthe cellular bafalt is the only kind admitted intothe preparation of Dutch tarras, yet it appearsfrom fome good experiments of Morveau, onthe fubject, that the common compadl bafalt ifprevioufly calcined will anfwer nearly the famepurpofe.& Great Britain is at a confiderableannual expence in purchafing tarras from Hol-land , it may be worth while therefore to point

* An. de Chim. xxxvii. p. 253.