149 
sulphur, are reddened by acids, and rendered green 
by alkalis *. 
405. M. Geoffrey, in his experiments on the oil 
of thyme, mixed with it different portions of acid ami 
alkali, and obtained various tints of yellow, red, pur- 
ple, violet, blue, green, and black. When the oil 
was dissolved in a considerable quantity of alcohol, 
it afforded a greyish colour, which became blue, by 
the addition of oil of tartar, and was rendered red by 
distilled vinegar, but recovered its blue tint, by a far- 
ther addition of alkali. In other instances, by the 
use of alkali, the colour passed to green.; so that the 
oil of tartar, says M. Geoffroy, acts differently on the 
essential oil of thyme ; for according as it is more or 
less concentrated, it renders it blue or green. As 
different colours are thus formed by the simple mix- 
ture of oils and salts, M. Geoffroy was led to suppose, 
that similar colours might be formed in plants by the 
same combinations. The principal colours, says he, 
which we observe in plants and in flowers, are green, 
lemon-yellow, orange-yellow, red, purple, violet, 
blue, black, and white ; and of these colours, differ r 
ently combined, all the others are composed. The 
blue, purple, violet, and green, he attributed to the 
operation of alkali ; and the red and yellow, to the 
predominance of acid ; the black he considered to 
be formed by the action of acid on a purple ; and the 
white to be produced by a very copious reflection of 
light, from transparent and colourless particles ! 
* Mem. Phvs. Chim. torn. iii. p. 140. 
t Mem. de 1'Acad. Rov. an. 1707. 
