NOME IV, TUFO. 423 



2,570, which is that of the black, to 2,785, rarely 

 2,8. Has an earthy smell. 



" It is not diffusible in cold water ; but in boil- 

 ing water it gradually deposites a fine earth. It 

 does not effervesce with acids. 



" Heated, it assumes a darker colour, and 

 easily melts into a black slag; or, with borax, 

 into a yellowish green glass. 



" It is magnetic before it is heated, but not 

 after. This is the most remarkable of its pro- 

 perties. 



" By Mr. Bergman's analysis, it contains from 

 55 to 6'0 per cent of silex, 19 to 20 of argil, 5 or 

 6 of lime, and from 15 to 20 of iron. 3 Bergm. 

 p. 194. 



" When mixed with a small proportion of lime 

 it quickly hardens, and this induration takes place 

 even under water. This singular property ap- 

 pears to me to proceed from the magnetic state of 

 the iron it contains ; for this iron being unoxygen- 

 ated, subtilly divided, and dispersed through the 

 whole mass, and thus offering a large surface, 

 quickly decomposes the water with which it is 

 mixed when made into mortar, and forms a hard 

 substance analogous to the specular iron ore ; as 

 it does in the iron tubes, in which water is de- 

 composed, in Mr. Lavoisier's and DP Priestley's 

 experiments. For in these the iron swells and in- 



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