788 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[April 5, 1873. 
polarized lights. These beams, as you know, are refracted 
differently, and from this we are able to infer that under 
some circumstances the one may be totally reflected, and 
the other not. An optician, named Nicol, cut a crystal 
of Iceland spar in two in a certain direction. He polished 
the severed surfaces, and reunited them by Canada 
balsam, the surface of union being so inclined to the 
beam traversing the spar that the ordinary ray, which 
is the most highly refracted, was totally reflected by the 
balsam, while the extraordinary ray was permitted to 
pass on. The invention of the Nicol’s prism was a great 
step in practical optics, and quite recently such prisms 
have been constructed of a size which enables audiences 
like the present to witness the chromatic phenomena 
of polarized light to a degree altogether unattainable 
a short time ago. The two prisms here before you belong 
to my excellent friend Mr. Wm. Spottiswoode, and they 
were manufactured by Mr. Ladd. I have with me 
another pair of very noble prisms, still larger than these, 
manufactured for me by Mr. Browning, who has gained 
so high and well-merited a reputation in the construction 
of spectroscopes. 
These two Nicol’s prisms play the same part as the 
crystals of tourmaline. Placed with their directions of 
vibration parallel, the light passes through both. When 
these directions are crossed, the light is quenched. In¬ 
troducing a film of mica between the prisms, the light 
is in part restored. But notice, when the film of mica 
is thin, you have not only light, but coloured light. Our 
work for some time to come will be the examination of 
these colours. With this view, I will take a representa¬ 
tive crystal, one easily dealt with ; that is, the crystal 
gypsum, or selenite, which is crystallized sulphate of 
lime. Between the crossed Nicol’s I place a thick plate 
of this crystal; like the mica, it restores the light, but 
it produces no colour. With my penknife I take a thin 
splinter from this crystal and place it between the 
prisms ; its image on the screen glows with the richest 
colours. Turning the prism in front these colours gradu¬ 
ally fade, disappear, but by continuing the rotation until 
the vibrating sections of the prisms are parallel, vivid 
colours again appear, but these colours are complementary 
to the former ones. 
Some patches of the splinter appear of one colour, some 
of another; these differences being due to the different 
thicknesses of the film. If the thickness be uniform 
the colour is uniform. Here, for instance, is a stellar 
shape, every lozenge of the star being a film of gypsum 
•of uniform thickness. Each lozenge, you observe, shows 
a brilliant uniform colour. Of course it is easy, by shap¬ 
ing our films so as to represent flowers or other objects, 
to exhibit such objects in colours unattainable by art. 
Here, for example, is a specimen of heartsease, the 
colours of which you might safely defy the artist to repro¬ 
duce. By turning the front Nicol 90° round, we pass 
through a colourless phase to a series of colours comple¬ 
mentary to the former ones. Here, for example, is a 
rose on a twig ; a red flower and green leaves ; turning 
the prism 90° round, we obtain a green flower and red 
leaves. 
All these wonderful chromatic effects have definite 
mechanical causes in the motions of the ether ; the 
principle of interference, duly applied and interpreted, 
explains them all ; and if you give me your patience, 
we shall, as far as it is necessary, develop the causes of 
these effects in our next lecture. 
THE DETERMINATION OE THE MELTING AND 
SOLIDIFYING POINTS OF FATS.* 
BY F. RUEDORFF. 
On testing all the usual methods for determining the 
melting points of fats, the author obtained the most con- 
•* Pogg. Ann. cxlv., 279—290, and from the Journal of 
the Chemical Society. 
cordant results by covering a thermometer-bulb with a 
layer of fat, about 3 mm. thick, immersing it in hot water, 
and observing the temperatures at which the fat began to 
separate from the bulb and to ascend through the water. 
Although some fats, and more particularly nutmeg-butter, 
did not rise from the thermometer even at temperatures 
considerably above those at which they are perfectly 
fluid, the author prefers his method to Wimmel’s, to 
which the same cause of inaccuracy, namely, adhesion to 
the glass, attaches in a still greater degree. 
The solidifying points of some fats were determined by 
observing the temperatures at which they became solid 
whilst they were violently agitated ; but with the gly¬ 
cerides and some other fats which exhibit a rise of tem¬ 
perature during solidification, it was found best to take as 
solidifying point that temperature to which the thermo¬ 
meter rises during solidifying, as this maximum tempera¬ 
ture appeared to be more constant than the turning-point, 
which has been determined and given as the natural so¬ 
lidifying point by Wimmel. 
The following table exhibits the author’s results :—• 
Melts at 
Solidifies at 
°C. 
°c 
% 
1 
61*5 
Yellow bees-wax ... 
63-4 
62-6 
) 
62-3 
White bees-wax. 
61-8 
61-6 
, 
49-6 
49-6 
Paraffin . < 
52-5 
53 
to 54 
53 
52-9 
f 
52-7 
to 53-2 
52*7 
Spermaceti . j 
43- 5 
44- 1 
to 44-3 
43*4 
44-2 
( 
55-3 
55-2 
Stearic acid.< 
56*2 
to 56-6 
55-8 
( 
56-0 
to 54*4 
55*7 
Japan wax . 
50-4 
to 51-0 
• • • 
Cacao butter . 
33-5 
• • • 
Nutmeg butter . 
70 to 80 
• • • 
Mutton suet . 
46-5 
to 47'4 
32 to 36 
Beef suet . 
43-5 
to 45’0 
27 to 35 
The remarkable phenomenon of a rise of temperature 
during solidification was also observed in artificial mix- 
tures of fats ; for instance, of spermaceti and stearic acid, 
and of paraffin and stearic acid, and is probably due to 
the constantly varying composition of the liquid remain¬ 
ing behind during partial solidification. A change in 
composition as solidification goes on, may possibly also 
explain the great interval of temperature during which 
beef and mutton suet pass from the liquid into the solid 
state. 
DETECTION OF WATER IN ESSENTIAL OILS.* 
BY G. LEUCHS. 
All essential oils obtained by distillation with water 
contain water even when they appear quite clear. The 
author finds that when such oils are mixed with several 
times their volume of petroleum-ether (the so-called ben- 
zin), a turbidity is produced, owing to the separation of 
globules of water, the turbidity being the more marked 
the greater the quantity of water present. By this 
means he found water in the oils of lavender, cloves, 
spike, cinnamon, rosemary, sassafras, and juniper; the 
oils of lemon and bergamot likewise contained traces of 
water, as did also Portugal oil and the oil of Gaultheria 
procuvibens : on the other hand, the oils of turpentine, 
cedar, lemon, rue, and amber were found free from water. 
* J. pr. Chem. [2], vi. 159, and from the Journal of the 
Chemical Society. 
