8

LIFE OF COLONEL MACERONL

farmer of the La Tolfa alum works, together with the2,500 acres of land. The rent he paid to the Papal govern-ment was 54,000 dollars a year. Giving some scope to hischemical and engineering propensities, my grandfathermade several important and expensive additions to theworks, amongst which was a beautiful paved road overthe whole distance (fifteen miles) from La Tolfa to Civita-vecchia , still known by the name of Via Maceroni.

The salt called alum must he familiar to most of myreaders ; I believe its principal use in the arts and manufac-tures, is in dying stuffs of wool, cotton, linen, or silk, inwhich it has the property of fixing the colours. Alum wasnot brought to Europe until the fifteenth century, from aplace called Rocca in Syria , which caused that kind to hecalled Roche Alum. It was in the beginning of the fifteenthcentury that the alum works of La Tolfa were firstestablished. The base of alum is pure clay, called argilleby the French , otherwise alumine. This earth is the basisof all basaltic lavas, of slate, and of the rocks called traps,which I look upon as being also basaltic lavas. Most of myreaders well know, that the masses composing Fingal’sCave, and all the Hebrides , the Giant’s Causeway, &c., arestreams of basaltic lava crystallized. The lavas whichflow to this day, from Vesuvius and Etna , are of the sameidentical substance, except that the latter contain a largerportion of crystallized shoerl and hornblend. The base ofall these lavas is the earth alumine, which being combinedwith the sulphuric acid, forms the salt called alum.

Many volcanic eruptions of lava are not absolutelyigneous, hut the stream flows out in the shape of hot mud,which settling and indurating, forms strata of several kindsof slate. Other eruptions of a watery kind, or while thevolcanic crater is under water, form tuffos, puzzolanas,and piperinos, as I shall afterwards indicate.