f ~ 
Notices respecting New Books. 457 
The idea of Chevreul, that the colouring matter of 'turnsole is 
the resuit of a colouring principle being united to an acid, is 
experimentally disproved; but the opinion most remarkable for 
seif-contradiction and more than usual absurdity, is that quoted 
from the French translation of Dr. T. Thomson’s Chemistry. 
In it the author confounds tincture of turfsole and syrup of 
violets, and says that the acids change vegetable blue colours into 
red; but that, if these colours have been rendered green by the 
alkalies, the acids make them re-appear and restore them. Turn- 
sole is not changed into green by alkali, and even the restoring 
of the eolour to syrup of violets must depend on a very exact sa- 
turation. The tincture of turnsole, it appears, spontaneously 
changes its colour from blue to yellow, and then blue again, whe- 
ther exposed to or excluded from the air, and at the same time 
some sulphuretted hydrogen gas is evolved. These spontaneous 
changes of colour take place in the course of a few days; sub-: 
carbonate of potash or alcohol, added to the tincture, will pre-. 
vent it from changing its colonr for twe years. The change into 
yellow is attributed to the sulphuretted hydrogen, which is derived 
from the decomposition either of the vegetable or animal mat- 
ter, urine being used for the preparation of turnsole. The final 
result is, that the tincture of turnsole is subject to change its 
colour and become yellowish in more or less time; that it does 
not always experience this alteration more rapidly in conse- 
quence of being prepared with hot water; that it loses its colour 
oftener when entirely excluded from the air than when partially 
exposed; that an alkaline solution of carbonate of potash ina 
sufficient dose prevents it from losing its colour, and that alcohol 
has the same effect ; that being reddened by acid and kept ina 
¢lose vessel, it suffers no further change ; that it is discoloured 
with a little acid and takes the colour of red wine, which finally 
becomes blue on exposure to the air or to ebullition ; that by this 
means it is more capable of indicating the existence of an acid in 
a small quantity; that the red vinous colour is owing to car- 
bonic acid ; that by means of phosphorus it becomes red on ex- 
posure to the atmosphere; that when exposed to the solar rays 
it undergoes much greater changes in open than in close vessels ; 
that in repeated changes of colour it precipitates some flakes of 
‘insoluble matter; that when its colouring matter is almost en- 
tirely decomposed in a dose vessel, it has then experienced the 
greatest number of discolorations ; that.on becoming yellow in 
a vessel containing atmospheric air, it abandons part of its carbon, 
which with the oxygen of the atmosphere and caloric forms car- 
bonic acid gas; that the alkalized or acidulated tinctures do not 
sensibly alter the air with which they are in contact; that the 
discoloured tincture has sometimes the smell of sulphuretted hy- 
drogen gas, which is. manifested by paper moistened with a solu- 
tion 
