April 12, 1373.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
805 
produced in the laboratory of the chemist, and that, 
under the name of mirbane, is the artificial oil of almonds, 
which is very largely employed in the preparation of 
almond soap. 
It is now upwards of twenty years since Professor 
Hofmann and Mr. Warren De La Rue, in reporting upon 
the chemical section of the G-reat Exhibition of 1851, 
called public attention prominently to the artificial prepa¬ 
ration of fruit essences and perfumes by the skill of the 
chemist; and it is to Hofmann himself that we owe the 
discovery of one of the chief sources of the body benzol, 
which is the basis from which artificial essential oil of 
almonds is prepared. Nitrobenzol, which in the hands of 
the perfumer is re-baptized with the sweet-sounding and 
meaningless name Essence de Mirbane, was discovered by 
the chemist Misterlich in 1834, and at that time described 
by him as strongly resembling in odour the essential oil of 
bitter almonds ; but as it could then only be prepared 
from benzoic acid, there was no hope of manufacturing it at 
a cost which would enable the artificial to compete with the 
natural product in the market. In 1845, Hofmann disco¬ 
vered that benzol could be easily prepared in enormous 
quantities at a cheap rate from coal tar ; and the series of 
investigations this eminent chemist then carried out gave 
us, in the first place, the artificial oil of almonds as a com¬ 
mercial product, and later, the infinitely more valuable 
series of coal-tar colours which have now revolutionized 
the art of the dyer and the industries concerned in sup¬ 
plying the raw materials of the dye-vat. Regarding these 
artificial odours and perfumes, Dr. Lyon Playfair, in his 
lecture on the results of the Great Exhibition of 1851, 
says :— 
“ An ingenious application of the science of chemistry 
has of late years been made in the manufacture of arti¬ 
ficial essences of pears, pine-apples, and other fruits. In 
the concentrated form the smell is rather acrid, but when 
diluted, the resemblance to the fruit is recognized. The 
best imitations are the pine-apple and the Jargonelle pear; 
the green-gage, apricot, black currant, and mulberry, when 
properly mixed, are fair imitations. They are quite in¬ 
nocuous in the proportions used, namely, a drop and a half 
to the ounce; the cheap ices are flavoured with these 
essences. Their introduction originated in the discovery 
of the fact, that the peculiar flavour of “ pine-apple rum” 
was due to butyric ether, which has since been obtained 
from the fruit itself. 
“ The jury in the Great Exhibition of 1851, or rather 
two distinguished chemists of that jury, Dr. Hofmann 
and Mr. De La Rue, ascertained that some of the most deli¬ 
cate perfumes were made by chemical artifice, and not, as 
of old, by distilling them from flowers. The perfume of 
flowers often consists of oils and ethers, which the chemist 
can compound artificially in his laboratory. Commercial 
enterprise availed itself of this fact, and sent to the Ex¬ 
hibition, in the form of essences, perfumes thus prepared. 
Singularly enough, they are generally derived from sub¬ 
stances of intensely disgusting odour. A peculiar fetid 
oil, termed “ fusel oil,” is formed in making brandy and 
whisky. This fusel oil, distilled with sulphuric acid and 
acetate of potash, gives the oil of pears. The oil of apples 
is made from the same fusel oil by distillation with sul¬ 
phuric acid and bichromate of potash. The oil of pine¬ 
apples is obtained from a product of the action of putrid 
cheese on sugar, or by making a soap with butter and dis¬ 
tilling it with alcohol and sulphuric acid, and is now 
largely employed in England in the preparation of pine¬ 
apple ale. Oil of grapes and oil of cognac, used to impart 
the flavour of French cognac to British brandy, are little 
else than fusel oil. The artificial oil of bitter almonds, 
now so largely employed in perfuming soap and for 
flavouring confectionery, is prepared by the action of 
nitric acid on the fetid oil of gas tar. Many a fair fore¬ 
head is damped with eau de millefleurs without knowing 
that its essential ingredient is derived from the draining 
of cow-houses.” 
(To be continued.) 
PROFESSOR TYNDALL ON LIGHT.* 
(Continued from p. 788.) 
You have already learned that the word “light ” may 
be used in two different senses : it may mean the im¬ 
pression made upon consciousness, or it may mean the 
physical agent which makes the impression. It is with 
the agent solely that we have to occupy ourselves at 
present. That agent is the motion of a substance which 
fills all space, and surrounds the atoms and molecules of 
bodies. The phenomena to be explained suggest the 
assumption, and the next process is to ascertain whether 
the assumption is competent to explain the phenomena. 
To this interstellar and interatomic medium certain 
definite mechanical properties are ascribed, and we deal 
with it as a body possessed of these properties. In 
mechanics we have the composition and resolution of 
forces, and of motions, extending to the composition and 
resolution of vibrations. We deal with the luminiferous 
ether in accordance with the rigid laws of mechanics, 
and from the composition, resolution, and interference 
of its vibrations, deduce all the phenomena displayed by 
crystals or polarized light. 
Take, as an example, the crystal of tourmaline. Let a 
vibration cross this crystal oblique to its axis ; we have 
seen by experiment that a portion of the light will pass 
through. How much we determine in this way : Draw a 
straight line representing the amplitude of the vibration be¬ 
fore it reaches the tourmaline, and from the two ends of this 
line draw two perpendiculars to the axis of the crystal, the 
distance between the feet of these two perpendiculars will 
represent the amplitude of the transmitted vibration. 
Follow me now while I endeavour to make clear to 
you what occurs when a film of gypsum is placed between 
the Nicol prisms. But at the outset, let us establish 
still further the analogy between the action of the prisms 
and that of two plates of tourmaline. The plates are now 
crossed, and you see that, by turning the film round, I 
place it in a position where it has no power to abolish the 
darkness. Why is this ? The answer is, that in the 
gypsum there are two directions at right angles to each 
other which the waves of light are constrained to follow, 
and that now one of these directions is parallel to one of 
the axes of the tourmaline and the other parallel to the 
other axis. When this is the case the film exercises no 
sensible action upon the light. But now I turn the film 
so as to render its direction of vibration oblique to the 
axis ; then you see it has the power demonstrated in the 
last lecture of partially restoring the light. 
Let us now mount our Nicol prisms, and cross them 
as we crossed the tourmaline. Introducing our film of 
gypsum between them, you notice that in one particular 
position the film has no power whatever over the light. 
But turn the film a little way round, you notice instantly 
that the light passes. We have now to understand the 
mechanism by which this is effected. 
First, then, we have this prism which receives the light 
emergent from the electric lamp, and which is called the 
polarizer. Then we have the plate of gypsum, and then 
the prism in front, which is called the analyzer. On its 
emergence from the first prism, the light is polarized ; and, 
in this particular case, its vibrations are executed in a 
horizontal plane. The two directions of vibration of the 
gypsum are now oblique to the horizon. Draw a rectan¬ 
gular cross upon paper to represent the two directions of 
vibration within the gypsum. Draw a line to represent 
the amplitude of the vibration where it reaches the gypsum. 
Let fall from the two ends of this line two perpendiculars 
on each of the arms of the cross ; then the distance 
between the feet of these perpendiculars represents the 
amplitudes of two rectangular vibrations which are the 
exact equivalents of the first single vibration. The polar- 
* Abstract of a series of lectures delivered in the Cooper 
Institute, New York, and reported in the Nevj York 
Tribune. 
