160 Misccllaneovs Intelligence. 



riety may be thus described : Colour iron-grey, somewhat 

 lighter than that of native plumbago; texture scaly; structure 

 mamellated, usually in very close aggregation — some specimens 

 exhibit this structure on the large scale, but generally it re- 

 quires the lens to be seen; hardness variable, but always 

 greater than the best native plumbago — scratches gypsum, but 

 is scratched by calc spar ; lustre of the exterior surface sometimes 

 very considerable, lustre of the fracture usually but small ; the 

 powder uniformly resembles that of common plumbago, but is 

 somewhat less brilliant. The quantity of iron in it seldom ap- 

 peared to amount to 9 per cent. It is hardly fit for finer pur- 

 poses of art, but it is proposed to use it in diminishing friction, 

 in making crucibles, furnaces, &c. — Ann. Phil. v. 51. 



The artificial production of plumbago is by no means an 

 unusual event. See p. 321, Vol. IX. of this Journal. In fine, 

 iron castings where charcoal in fine powder has been used as 

 the facing, the cast may be observed every where covered with 

 a thin coat of plumbago. — Ed. 



11. Test of the Dryness of Air or Gases. — M. Serullas re- 

 commends the alloy of bismuth and potassium, obtained by 

 heating together 60 parts carbonized cream of tartar, 120 of 

 bismuth, and 1 of nitre, for two hours, as an excellent test of 

 the dryness of gas in certain circumstances. A small fragment 

 of the alloy is to be introduced into the gas over mercury, and 

 the least moisture in it will tarnish the metal immediately. 

 The alloy is so rich in potassium, that the smallest fragment, 

 when cut with scissors, scintillates. If a piece be bruised, it 

 burns, leaving a green oxide. 



12. Variation of Thermometers. — II Signor Bellani refers to 

 the following experiment as a proof of the changeableness of 

 a thermometer, with regard to the temperatures it expresses, 

 and in illustration of the cause of those changes. Take a mer- 

 curial thermometer, including a range at least from freezing to 

 boiling water, having degrees of such magnitude, that -j'g of a 

 degree may readily be perceived, and not having been exposed 

 for some months to a temperature near that of boiling water. 

 Mark exactly the point at which the mercury stands in thawing 

 ice, then plunge the bulb in boiling water, and then again 

 mark the temperature indicated in thawing ice ; it will indicate 

 above a tenth of a degree lower this time than the former. 



