390 
PHARMACEUTICAL MEETING. 
in proportion to the quantity, and the struggle between the water and the creo¬ 
sote sets in again with the characteristic figure. Here is a specimen of carbolic 
acid. The figure is very different from that of creosote, and its duration is very 
much less. And here it may be noted that some of the figures of oils, etc., are 
very durable, remaining on the surface for hours ; and even among volatile oils 
the characteristic portions of some of them remain for a long time. 
The strong adhesion between water and an oil may be shown by taking a slice 
of one of the seeds of the castor-oil plant and throwing it on the surface of clean 
water, when it will rotate feebly, after the manner of camphor on the surface 
of water, in consequence of the oil which diffuses from it over the surface in one 
direction producing rotation by reaction in another direction ; but after a few 
turns the oil becomes equally diffused all round, and the motion ceases. Castor- 
oil seeds are heavy, they partly sink in cold water, and slices sink rapidly in hot. 
The oil that escapes from the seed upon the surface of water does not form its 
cohesion figure. To get this we must allow a single drop to descend gently near 
to the surface of clean water, and the result will be a figure of extraordinary 
beauty. The drop will flatten into a disk, from which there will proceed a 
number of perfect iridescent rings, showing most of the colours in Newton’s 
seven series ; but beyond, and bounding these rainbows is a broad silvery corona 
of exquisite delicacy, which almost immediately breaks up into a charming lace¬ 
like pattern, which I have feebly attempted to represent on the diagram before 
you. These effects last some seconds, but gradually the colour disappears, the 
lace-like pattern follows, and a large colourless disk, with a well-defined edge, 
is left on the surface of the water. It undergoes some change, due to atmo¬ 
spheric exposure, but remains permanent for hours.* 
Such is the figure produced by the various specimens of castor oil that I have 
examined. They are of different growths, but are I believe all pure, and sup¬ 
posing the specific gravity of all these specimens to be about 0969 there is no 
reason why their cohesion figures should not resemble each other at about 60° F., 
which I take to be the mean temperature of an inhabited apartment both in 
winter and in summer. There are obvious reasons, with a viscid oil like castor, 
and other oils that are sensitive to cold, why this test should be applied at a 
genial temperature. Some of the animal oleines, for example, are solid at about 
40°. These may be perfectly fluid in a warm room, but if water be brought in 
from a cistern at 40°, and be poured into the test-glass, the drop, as soon as it is 
deposited, will be chilled, and even become opaque, and the experiment fail; so 
that attention to temperature is important. 
Another precaution refers to extent of surface. By increasing this we increase 
the adhesive force, and the figures may be poor and thin, and even torn up, be¬ 
fore their characters can be studied. I have found a surface of four or four and 
a half inches in diameter well adapted to this inquiry. 
Now comes the question of admixture of oils. Castor oil is so abundant aud 
so cheap that there is no great inducement, so far as I know, to mix it with an 
inferior oil, supposing one very much cheaper could be found. But let us ima¬ 
gine that lard oil, for example, were wanting employment in the market, and 
that as a secondary product it could be obtained cheaply,—could the admixture 
be detected by this method of cohesion figures ? My answer is, that in all the 
cases of admixture which I have examined, the features of the two oils are 
manifest in the cohesion figure ; and these are, in general, very distinct, if the 
standard figure be examined by the side of the suspected figure. My memory 
is sufficiently retentive of these figures to enable me to tell immediately whether 
a figure varies from my standard. Not that my standard is necessarily the cor¬ 
rect one ; because in all cases I have had to depend upon others for the integrity 
of my .specimens ; and, in like manner, the gentlemen who have supplied me, 
* An engraving, with explanatory notes, will be given in our next number. 
