444 Gustav Rose, on Greenstone [Dec. 



end to an edge, and exactly 29J inches long, must now be 

 placed in contact with the surface of mercury in the cistern, 

 and a mark made on the tube at its upper end : A scratch 

 is sometimes hazardous ; a little paint on a camels' hair 

 pencil is safer. The lid is now to be slipped down on the box, 

 and the whole removed from the plate, on to a piece of thin 

 chamois leather ; which being brought up over the box, is to 

 be tied tight round the tube ; and it may then be set on the case, 

 the box being supported beneath, to the proper height. If 

 the barometer stood at 29 J, or being below, was brought to 

 that height by inclination, the mark is a standard : if above, 

 it must be corrected for the depression of the surface in the 

 cistern. The scale must also be corrected, for the counter- 

 elevation and depression in the cistern ; which is conveni- 

 ently ascertained by previously filling 3 inches of the tube, 

 and measuring the height it occupies in the box with the 

 tube immersed ; allowance being made for its conical form, 

 if it be such. But this may be done by different methods, 

 generally known. 



Such an instrument may be prepared by any practical 

 chemist, and may be trusted for common laboratory pur- 

 poses. For investigations of extreme delicacy, of course, 

 every possible precaution and perfection are required. 



P. 



Article V. 



On the Rocks which are distinguished hy the names of Green- 

 stone and Greenstone Porphyry, By Gustav Rose. 



( Concluded from p. 281. J 



4. Gahhro a granular mixture of labradorite and diallage. 

 The labradorite is similar to that of the hypersthene rock, 

 still it is not so completely cleavable, and has more fre- 

 quently a thick splintery fracture, in which case the trans- 

 lucency is less, and the colour greenish-white or greenish- 

 gray. 



The Diallage may be considered as an augite, which has 

 lost the faces of cleavage parallel to the front faces of a 

 rhomboidal prism of 88°. The first faces of cleavage are 

 very perfect, possess the metallic pearly lustre and streak ; 

 the edges run parallel with the second faces of cleavage ; 



