73 
order to ascertain whether any change would take place in 
its composition. Last December I examined the specimen 
and found it had been acted upon by the weather to a con- 
siderable extent, as specks of a dark blue mineral were 
distributed here and there upon its surface. This dark blue 
mineral proved to be Azurite. Messrs. Wibel and T ungel 
(Dent. Chem. Geo. Berichte IV. 138), in a short paper on the 
formation of Azurite state that it is formed from Malachite 
by the absorption of carbonic acid and elimination of water, 
as Azurite contains a much larger percentage of carbonic 
acid than Malachite, This statement still remains to be 
proved, but it is scarcely probable that the moderate heat of 
our last two summers was sufficient to cause a loss of water 
in the Malachite specimen. At some future time I shall 
endeavour to ascertain the real cause of the formation of 
Azurite, 
“ On a Direct-Vision Spectroscope of great Dispersive 
Power,” by Arthur Schuster, Ph.D. 
This instrument is made by Mr. Adam Hilger of London. 
The following are its chief advantages 
1. The compound prism has a very great dispersive 
power. The nickel line between the two sodium lines is 
easily seen in the solar spectrum. 
2. The cross wire is replaced by a very fine slit which 
can be illuminated from above to any degree of intensity. 
3. The slit is moveable by means of a very fine micro- 
meter screw; the position of the slit can be read off to 
within 0*0001 inch. 
The measurement is made by bringing the line to be 
measured against the bright slit which comes down from 
the top to the middle of the field. The position of the lines 
can be easily measured to within the fifth part of the dis- 
tance between the sodium lines. 
