16 
Field Museum op Natural History 
for about an hour. The acid is then brought to boil- 
ing for from 15 to 20 minutes or, in some cases, 1 to 
2 hours. This latter operation must be carried on with 
considerable care, owing to the poisonous nature of 
fumes given off by the acid and the explosive action 
which occurs if water comes in contact with it. If 
one wishes to avoid boiling the acid, the desired effect 
can be obtained by allowing the stone to remain in 
warm acid for an hour, then cooling and warming it 
again for an equal length of time. Owing to the 
strong affinity which sulphuric acid has for water, the 
acid remaining in the stone will generally cause it to 
become moist after it has been removed from the acid. 
Drying the stone 1 or 2 days or immersing it for 5 or 
6 hours in cold water and then drying it at a moder- 
ate temperature will remove this tendency to "sweat." 
For coloring blue, a solution is made of 9 ounces of 
yellow prussiate of potash (potassium ferro-cyanide) 
in a quart of water. In this lukewarm solution the 
stone to be colored is left for from one to two weeks, 
according to its absorptivity. After being well washed, 
the stone is then put into a solution of iron vitriol 
(ferrous sulphate) made by adding to a quart of water 
as much of the iron vitriol as will dissolve. Left in 
this for 8 to 10 days the stone is then removed, well 
washed and dried slowly. If a satisfactory color has 
not been obtained, the stone can be returned to the 
vitriol bath until the desired shade is reached. A 
dark blue is produced by a few drops of sulphuric 
and a few of nitric acid added to the iron vitriol solu- 
tion, or red prussiate of potash (potassium ferricya- 
nide) can be used instead of the yellow prussiate of 
potash for the first solution. The blue coloring is 
much used to imitate lapis lazuli, the so-called German 
or Swiss lapis being artificially colored chalcedony or 
agate. 
tl20] 
