1888.] Mr A. Johnstone on Carhonic Acid Water. 437 
special attention to the action of the carbonic acid water on the 
mineral olivine, and believing that the results obtained in this case, 
along with those previously recorded, may yet throw considerable 
light on the nature and extent of the chemical alteration and 
disintegration of rocks, and in the meantime prove to possess some 
little interest for geologists generally, I venture to bring this paper 
before the Koyal Society of Edinburgh. 
Olivine, chrysolite, or peridot is in its original state a double 
silicate of magnesia and protoxide of iron, with traces of other 
bases. It usually occurs in small transparent to translucent 
rectangular prisms of the Trimetric System, embedded in basalts 
and basaltic lavas, and looks like pale olive-green glass, differing, 
however, from glass in having cleavage."^ 
Its normal hardness when fresh is close on 7. Its specific gravity 
varies from 3 '3 to 3*5. It is one of the least fusible of minerals ; 
in fact, in the ordinary blowpipe laboratory it is considered to be 
practically infusible, its fusibility, according to Yon KobelFs scale, 
being at least as high as 6. 
It is when finely powdered, decomposed and gelatinised when 
treated for some time with warm concentrated hydrochloric acid. 
Under the microscope, fresh olivine appears colourless when in 
thin sections, and its surface, especially when viewed by oblique 
illumination, is seen to be rough, like ground glass, and minutely 
pitted. It is doubly refractive, polarising in fairly strong tints, 
and numerous irregular cracks can be seen traversing it in all 
directions. 
I took a certain amount of olivine having all the characters and 
properties of the typical mineral as described above, and placed 
it in a flask containing a litre of pure distilled water saturated with 
carbonic acid gas, and allowed this liquid to act on it undisturbed 
for two months. I tested the solution with litmus paper at the 
time I put the mineral into it, and found it to be distinctly acid. 
About every two days or so after this, I again tested the water, 
and observed that it gradually became less and less acid in nature, 
until in about six weeks’ time it was quite neutral to test papers. 
There are two cleavages — the macropinacoid and hrachypinacoid ; the 
latter is the more distinct, but neither as a rule are at all strongly marked, 
at least in fresh specimens. 
