196 A Superior Green Dye. 



measuring the compression of water. After having depriv- 

 ed the water of atmospheric air by ebullition, he fills a 

 glass cylinder with it, whose upper part is mounted with a 

 brass cover, hermetically sealed, and which is traversed by 

 a screw, with a small brass piston at its lower extremity 

 which presses upon the fluid. In the cylinder is placed a 

 ball with a small thermometer tube, both filled with the wa- 

 ter of the cylinder, except that in the upper part of the 

 tube, which remains open, there is a small column of mer- 

 cury, which on account of the extreme fineness of the tube, 

 keeps its place without falling into the ball. 



Now suppose that the water is compressed by turning the 

 screw of the piston ; this pressure being equal both within and 

 without the ball and its tube, they will undergo neither expan- 

 sion nor contraction, and consequently the position of the 

 mercury above the water in the thermometric tube will im- 

 mediately indicate the compression. Mr. Oersted previously 

 determined the capacity of the tube and that of the ball, by 

 taking the weight of the volume of mercury which fills 

 them. The pressure exerted upon the water by the scre\!r 

 is measured by a tube filled with air and likewise enclosed 

 in the cylinder. He has ascertained by this instrument 

 that the compressibility of water diminishes very rapidly 

 as the pressure increases, and that the mean compressibili- 

 ty under a pressure of three or four atmospheres is to oVo-o o 

 for each atmosphere, a result which very nearly accords 

 with the experiments of Canton. 



28. A superior Green Dye. — M.Noel who has a fine manu- 

 factory of paper hangings at Nancy., put into the hands of 

 H. Braconnot a specimen of a superb green colour, which 

 within a few years has acquired much reputation in com- 

 merce, and the secret of which is known only to a colour 

 manufacturer at Schweinfurt. Braconnot easily discover- 

 ed, by analysis that it consisted of arsenical acid, deutox- 

 ide of hydrated copper, and acetic acid ; thus approaching 

 in its composition to Scheele's green. But this skilful 

 chemist found it not so easy to recdmbine the materials so 

 as to equal the Schweinfurt colour. After persevering ef- 

 forts, he at length succeeded. The process which he 

 adopted, is, 1st. to dissolve in a small quantity of warm 

 water, six parts of sulphate of copper; 2nd to boil eight 

 parts of oxide of arsenic with eight parts of potash of com- 



