96 Mr Forbes's Physical Notices of the Bay of Naples. 



the grotto was cut out of the same material. However diffi- 

 cult the mineralogical characters may be to separate, the real 

 difference exists. The common tufas of the Bay of Naples 

 are consistent, homogeneous, capable of being cut and chisel- 

 led, and, though porous, appear to have no particular action 

 upon water capable of making them applicable as cements. 

 The Pozzuolana, again, is extremely friable, gritty and harsh 

 to the feel to the last degree, quarried rather like sand than 

 stone, and exercising a very remarkable power of adhesion, 

 when applied to the purposes of a cement. The limits to which 

 I must confine myself oblige me to do little more than touch 

 upon the remarkable properties of Pozzuolana, which seems to 

 deserve more attention from practical chemists than it has yet 

 received. 



The ancients were well acquainted with its value, and it is 

 the " Arena Fossicia" of Vitruvius, but also called by him 

 " pulvis Puteolana *." When it could be procured, as in the 

 neighbourhood of Rome and Naples, no less than three parts 

 of it were used to one of lime. Its intrinsic value has been 

 duly appreciated in various parts of Europe ; and in the great 

 undertaking of building theEddystone Lighthouse, Mr Smea- 

 ton employed no less than an equal quantity of Pozzuolana 

 mixed with lime, for the mortar to be used under water. 



It is remarkable that different rocks of volcanic origin have 

 been used for the same purposes. A kind of vesicular basalt 

 is quarried in great quantities at Andernach on the Rhine, 

 and transported to Holland for building under water : A si- 

 milar kind of rock, which is called trass or tarrass, (and is 

 nearly assimilated to the pumiceous conglomerates of Hun- 

 gary, ■(-) is found at Coblentz, \ and might probably be ap- 

 propriated to the same purposes. It seems to have the most 

 important characters of the grey Pozzuolana of the Bay of 

 Naples. § 



• Vitruv. Lib. ii. 4, and ii. 6. 



+ Daubeny on Volcanos, p. 171. 



X Annates des Mines, xxv. 366. 



§ It seems that some of the extinct volcanic matter in the south of 

 France is, or might be, applied to similar purposes. See Soulavie's edi- 

 tion of Hamilton's Works, p. 476. 



