=285. Specific Gravity.=—The specific gravity of an oil is readily
determined by a westphal balance (=53=), by a spindle, by a sprengel
tube, or more accurately by a pyknometer. The general principles
governing the conduct of the work have already been given (=48-59=).
The methods described for determining the density of sugar solutions
are essentially the same as those used for oils, but it is to be
remembered that oils and fats are lighter than water and the
graduation of the sinkers for the hydrostatic balance, and the
spindles for direct determination must be for such lighter liquids.
The necessity of determining the density of a fat at a temperature
above its melting point is manifest, and for this reason the use of
the pyknometer at a high temperature (40° to 100°) is to be preferred
to all the other processes, in the case of fats which are solid at
temperatures below 25°.
[Illustration: FIG. 82.—BALANCE AND WESTPHAL SINKER.]
When great delicacy of manipulation is desired, combined with
rapid work, an analytical balance and westphal sinker may be used
conjointly.[235] In this case it is well to have two or three sinkers
graduated for 20°, 25°, and 40°, respectively. Nearly all fats,
when melted and cooled to 40°, remain in a liquid state long enough
to determine their density. The sinkers are provided with delicate
thermometers, and the temperature, which at the beginning is a little
above the degree at which the sinker is graduated, is allowed to
fall to just that degree, when the equilibrium is secured in the
usual manner. The sinker is conveniently made to displace just five
grams of distilled water at the temperature of graduation, but it is
evident that a round number is not necessary, but only convenient for
calculation.
=286. Expression of Specific Gravity.=—Much confusion arises in the
study of data of densities because the temperatures at which the
determinations are made are not expressed. The absolute specific
gravity would be a comparison of the weight of the object at 4°, with
water at the same temperature. It is evident that such determinations
are not always convenient, and for this reason the determinations of
density are usually made at other temperatures.
In the case of a sinker, which at 35° displaces exactly five grams of
water, the following statements may be made: One cubic centimeter of
water at 35° weighs 0.994098 gram. The volume of a sinker displacing
five grams of water at that temperature is therefore 5.0297 cubic
centimeters. This volume of water at 4° weighs 5.0297 grams. In a given
case the sinker placed in an oil at 35° is found to displace a weight
equal to 4.5725 grams corresponding to a specific gravity of 35°/35°
= 0.9145. From the foregoing data the following tabular summary is
constructed:
Weight of 5.0287 cubic centimeters of oil at 35°, 4.5725 grams.
” ” 5.0297 ” ” ” water at 35°, 5.0000 ”
” ” 5.0297 ” ” ” ” ” 4°, 5.0297 ”
Relative weight of oil at 35°, to water at 35°, 0.9145 grams.
” ” ” ” ” 35°, ” ” ” 4°, 0.9092 ”
=287. Coefficient of Expansion of Oils.=—Oils and fats of every
kind have almost the same coefficient of expansion with increasing
temperature. The coefficient of expansion is usually calculated by the
formula
_D_₀ - _D_₀ʹ
δ = ----------------
(_tʹ_ - _t_)_D_₀
in which δ represents the coefficient of expansion, _D_₀ the density at
the lowest temperature, _D_₀ʹ the density at the highest temperature,
_t_ the lowest, and _tʹ_ the highest temperatures.
In the investigations made by Crampton it was shown that the formula
would be more accurate, written as follows:[236]
_D_₀ - _D_₀ʹ
δ = ---------------------------
(_tʹ_ - _t_) × _D_₀ + _D_₀ʹ
---------------
2
The absolute densities can be calculated from the formula Δ = δ +
_K_, in which Δ represents the coefficient of absolute expansion, δ
the apparent coefficient of expansions observed in glass vessels, and
_K_ the cubical coefficient of expansion of the glass vessel. The
mean absolute coefficient of expansion for fats and oils, for 1° as
determined by experiment, is almost exactly 0.0008, and the apparent
coefficient of expansion nearly 0.00077.[237]
=288. Standard of Comparison.=—In expressing specific gravities it is
advisable to refer them always to water at 4°. The temperature at which
the observation is made should also be given. Thus the expression of
the specific gravity of lard, determined at different temperatures, is
made as follows:
15°.5 40°
_d_ ------ = 0.91181; _d_ ----- = 0.89679;
4° 4°
100°
and _d_ ---- = 0.85997,
4°
indicating the relative weights of the sample under examination at
15°.5, 40°, and 100°, respectively, to water at 4°.
=289. Densities of Common Fats and Oils.=—It is convenient to have at
hand some of the data representing the densities of common fats and
oils, and the following numbers are from results of determinations made
in this laboratory:[238]
15°.5 40° 100°
Temperature. _d_ = -----. _d_ = ---. _d_ = ----.
4° 4° 4°
Leaf lard 0.91181 0.89679 0.85997
Lard stearin 0.90965 0.89443 0.85750
Oleostearin 0.90714 0.89223 0.85572
Crude cottonseed oil 0.92016 0.90486 0.86739
Summer ” ” 0.92055 0.90496 0.86681
Winter ” ” 0.92179 0.90612 0.86774
Refined ” ” 0.92150 0.90573 0.86714
Compound lard ” 0.91515 0.90000 0.86289
Olive oil 0.91505 0.89965 0.86168
=290. Melting Point.=—The temperature at which fats become sensibly
liquid is a physical characteristic of some importance. Unfortunately,
the line of demarcation between the solid and liquid states of this
class of bodies is not very clear. Few of them pass _per saltum_ from
one state to the other. In most cases there is a gradual transition,
which, between its initial and final points, may show a difference
of several degrees in temperature. It has been noted, further, that
fats recently melted behave differently from those which have been
solid for several hours. For this reason it is advisable, in preparing
glycerids for the determination of their melting point, to fuse them
the day before the examination is to be made. The temperature at which
a glycerid passes from a liquid to a solid state is usually higher than
that at which it resumes its solid form. If, however, the change of
temperature could be made with extreme slowness, exposing the sample
for many hours at near its critical temperature, these differences
would be much less marked.
Many methods have been devised for determining the melting point of
fats, and none has been found that is satisfactory in every respect. In
some cases the moment at which fluidity occurs is assumed to be that
one when the small sample loses its opalescence and becomes clear.
In other cases the moment of fluidity is determined by the change of
shape of the sample or by observing the common phenomena presented by
a liquid body. In still other cases, the point at which the sample
becomes fluid is determined by the automatic completion of an electric
circuit, which is indicated by the ringing of a bell. This latter
process has been found very misleading in our experience. Only a few of
the proposed methods seem to demand attention here, and some of those,
depending on the visible liquefaction of a small quantity of the fat or
based on the physical property, possessed by all liquids when removed
from external stress, of assuming a spheroidal state will be described.
Other methods which may demand attention in any particular case may be
found in the works cited.[239]
=291. Determination in a Capillary Tube.=—A capillary tube is dipped
into the melted fat and when filled one end of the tube is sealed in
the lamp and it is then put aside in a cool place for twenty-four
hours. At the end of this time the tube is tied to the bulb of a
delicate thermometer the length used or filled with fat being of the
same length as the thermometer bulb. The thermometer and attached fat
are placed in water, oil, or other transparent media, and gently warmed
until the capillary column of fat becomes transparent. At this moment
the thermometric reading is made and entered as the melting point of
the fat. In comparative determinations the same length of time should
be observed in heating, otherwise discordant results will be obtained.
As in all other methods, the resulting members are comparative and not
absolute points of fusion, and the data secured by two observers on the
same sample may not agree, if different methods of preparing the fat
and different rates of fusion have been employed.
[Illustration: FIG. 83.—MELTING POINT TUBES.]
Several modifications of the method just described are practiced, and
perhaps with advantage in some cases. In one of these a small particle
of the fat is solidified in a bulb blown on a small tube, as indicated
in Fig. 83, tube _a_. The tube, in an upright position, is heated in a
convenient bath until the particle of fat just begins to run assuming
soon the position shown in tube _b_. This temperature is determined
by a thermometer, whose bulb is kept in contact with the part of the
observation tube containing the fat particle. The rise of temperature
is continued until the fat collected at the bottom of the bulb is
entirely transparent. This is called the point of complete fusion.[240]
Pohl covers the bulb of a thermometer with a thin film of fat, and the
instrument is then fixed in a test tube, in such a way as not to touch
the bottom, and the film of fat warmed by the air-bath until it fuses
and collects in a droplet at the end of the thermometer bulb.[241]
Carr has modified this process by inserting the thermometer in a round
flask in such a way that the bulb of the thermometer is as nearly
as possible in the center. By this device the heating through the
intervening air is more regular and more readily controlled.[242]
A particle of fat placed on the surface of clean mercury will melt
when the mercury is raised to the proper temperature. Where larger
quantities of the fat are employed, a small shot or pellet of mercury
may be placed upon the surface and the whole warmed until the metal
sinks. Of the above noted methods, the analyst will find some form
of capillary tube or the use of a film of the fat on the bulb of a
thermometer the most satisfactory.[243]
Hehner and Angell have modified the sinking point method by increasing
the size of the sinker without a corresponding increase in weight.
This is accomplished by blowing a small pear-shaped float, nearly
one centimeter in diameter and about two long. The stem of the pear
is drawn out and broken off, and while the bulb is still warm, the
open end of the stem is held in mercury, and a small quantity of this
substance, sufficient in amount to cause the float to sink slowly
through a melted fat, is introduced into the bulb of the apparatus
and the stem sealed. The whole bulb should displace about one cubic
centimeter of liquid and weigh, after filling with mercury, about
three and four-tenths grams. In conducting the experiment about thirty
grams of the dry melted fat are placed in a large test tube and cooled
by immersing the tube in water at a temperature of 15°. The tube
containing the solidified fat is placed in a bath of cold water and the
sinker is placed in the center of the surface of the fat. The bath is
slowly heated until the float disappears. The temperature of the bath
is read just as the bulb part of the float disappears. The method is
recommended especially by the authors for butter fat investigations.[244]
=298. Melting Point Determined by the Spheroidal State.=—The method
described by the author, depending on the assumption of the spheroidal
state of a particle of liquid removed from all external stress, has
been found quite satisfactory in this laboratory, and has been adopted
by the official chemists.[245] In the preparation of the apparatus there
are required:
(_a_) a piece of ice floating in distilled water that has been recently
boiled, and (_b_) a mixture of alcohol and water of the same specific
gravity as the fat to be examined. This is prepared by boiling
distilled water and ninety-five per cent alcohol for a few minutes to
remove the gases which they may hold in solution. While still hot, the
water is poured into the test tube described below until it is nearly
half full. The test tube is then nearly filled with the hot alcohol,
which is carefully poured down the side of the inclined tube to avoid
too much mixing. If the alcohol is not added until the water has
cooled, the mixture will contain so many air bubbles as to be unfit for
use. These bubbles will gather on the disk of fat as the temperature
rises and finally force it to the top.
[Illustration: FIG. 84.—APPARATUS FOR THE DETERMINATION OF MELTING
POINT.]
The apparatus for determining the melting point is shown in Fig. 84,
and consists of (_a_) an accurate thermometer reading easily tenths of
a degree; (_b_) a cathetometer for reading the thermometer (but this
may be done with an eye-glass if held steadily and properly adjusted);
(_c_) a thermometer; (_d_) a tall beaker, thirty-five centimeters
high and ten in diameter; (_e_) a test tube thirty centimeters long
and three and a half in diameter; (_f_) a stand for supporting the
apparatus; (_g_) some method of stirring the water in the beaker (for
example, a blowing bulb of rubber, and a bent glass tube extending to
near the bottom of the beaker).
The disks of fat are prepared as follows: The melted and filtered fat
is allowed to fall from a dropping tube from a height of about twenty
cubic centimeters on a smooth piece of ice floating in recently boiled
distilled water. The disks thus formed are from one to one and a half
centimeters in diameter and weigh about 200 milligrams. By pressing the
ice under the water the disks are made to float on the surface, whence
they are easily removed with a steel spatula, which should be cooled in
the ice water before using. They should be prepared a day or at least a
few hours before using.
The test tube containing the alcohol and water is placed in a tall
beaker, containing water and ice, until cold. The disk of fat is then
dropped into the tube from the spatula, and at once sinks until it
reaches a part of the tube where the density of the alcohol-water is
exactly equivalent to its own. Here it remains at rest and free from
the action of any force save that inherent in its own molecules.
The delicate thermometer is placed in the test tube and lowered until
the bulb is just above the disk. In order to secure an even temperature
in all parts of the alcohol mixture in the vicinity of the disk, the
thermometer is gently moved from time to time in a circularly pendulous
manner.
The disk having been placed in position, the water in the beaker is
slowly heated, and kept constantly stirred by means of the blowing
apparatus already described.
When the temperature of the alcohol-water mixture rises to about
6° below the melting point, the disk of fat begins to shrivel, and
gradually rolls up into an irregular mass.
The thermometer is now lowered until the fat particle is even with the
center of the bulb. The bulb of the thermometer should be small, so as
to indicate only the temperature of the mixture near the fat. A gentle
rotatory movement from time to time should be given to the thermometer
bulb. The rise of temperature should be so regulated that the last
2° of increment require about ten minutes. The mass of fat gradually
approaches the form of a sphere, and when it is sensibly so the reading
of the thermometer is to be made. As soon as the temperature is taken
the test tube is removed from the bath and placed again in the cooler.
A second tube, containing alcohol and water, is at once placed in the
bath. The test tube (ice water having been used as a cooler) is of
low enough temperature to cool the bath sufficiently. After the first
determination, which should be only a trial, the temperature of the
bath should be so regulated as to reach a maximum of about 1°.5 above
the melting point of the fat under examination.
The edge of the disk should not be allowed to touch the sides of the
tube. This accident rarely happens, but in case it should take place,
and the disk adhere to the sides of the tube, a new trial should be
made.
Triplicate determinations should be made, and the second and third
results should show a near agreement.
_Example._—Melting point of sample of butter:
Degrees.
First trial 33.15
Second trial 33.05
Third trial 33.00
The fatty acids, being soluble in alcohol, cannot be treated as the
ordinary glycerids. But even those glycerids which are slightly soluble
in alcohol may be subjected to the above treatment without fear of
experiencing any grave disturbance of the fusing points.
=293. Solidifying Point.=—The temperature at which a fat shows
incipient solidification is usually lower than the point of fusion.
The same difficulties are encountered in determining the temperature
of solidification as are presented in observing the true melting
point. The passage from a transparent liquid to an opaque solid is
gradual, showing all the phases of turbidity from beginning opalescence
to complete opacity. The best the analyst can do is to determine,
as accurately as possible, the temperature at which the more solid
glycerids of the mixture begin to form definite crystals. This point is
affected to a marked degree by the element of time. A fat cooled just
below its melting point will become solid after hours, or days, whereas
it could be quickly cooled far below that temperature and still be
limpid.
The methods of observation are the same for the glycerids and fatty
acids, and the general process of determination is sufficiently set
forth in the following description of the method as used in this
laboratory.[246]
[Illustration: FIG. 85.—APPARATUS FOR DETERMINING CRYSTALLIZING POINT.]
The melted fat or fat acid is placed in a test tube contained in a
large bottle, which serves as a jacket to protect the tube from sudden
or violent changes of temperature. The efficiency of the jacket may
be increased by exhausting the air therefrom, as in the apparatus
for determining the heat of bromination, hereafter described. A very
delicate thermometer, graduated in tenths of a degree, and having a
long bulb, is employed. By means of the reading glass, the reading can
be made in twentieths of a degree. The arrangement of the apparatus
is shown in Fig. 85. The test tube is nearly filled with the melted
matter. The bottom of the jacket should be gently warmed to prevent a
too rapid congelation in the bottom of the test tube containing the
melted fat, and the tube is to be so placed as to leave an air space
between it and the bottom of the bottle. The thermometer is suspended
in such a manner as to have the bulb as nearly as possible in the
center of the melted fat. The thermometer should be protected from air
currents and should be kept perfectly still. In case the congealing
point is lower than room temperature the jacket may be immersed in a
cooling mixture, the temperature of which is only slightly below the
freezing point of the fatty mass.
When crystals of fat begin to form, the descent of the mercury in the
stem of the thermometer will become very slow and finally reach a
minimum, which should be noted. As the crystallization extends inwards
and approaches the bulb of the thermometer a point will be reached when
the mercury begins to rise. At this time the partially crystallized
mass should be vigorously stirred with the thermometer and again left
at rest in as nearly, the original position as possible. By this
operation the mercury will be made to rise and its maximum position
should be noted as the true crystallizing point of the whole mass.
In comparing different samples, it is important that the elements of
time in which the first crystallization takes place should be kept, as
nearly as possible, the same. A unit of one hour in cooling the mixture
from a temperature just above its point of fusion until the incipient
crystallization is noticed, is a convenient one for glycerids and for
fat acids.
=294. Determination of Refractive Power.=—The property of refracting
light is possessed by fats in different degrees and these differences
are of great help in analytical work. The examination may be made by
the simple refractometer of Abbe or Bertrand, or by the more elaborate
apparatus of Pulfrich.
The comparative refractive power of fats can also be observed by
means of the oleorefractometer of Amagat-Jean or the differential
refractometer of Zune.[247]
For details of the construction of these apparatus, with a description
of the optical principles on which they are based, the papers above
cited may be consulted. In this laboratory the instruments which have
been employed are three in number, _viz._, Abbe’s small refractometer,
Pulfrich’s refractometer using yellow light, and the oleorefractometer
of Amagat-Jean. A brief description of the methods of manipulating
these instruments is all that can be attempted in this manual.
=295. Refractive Index.=—Refractive index is an expression employed to
characterize the measurement of the degree of deflection caused in a
ray of light in passing from one transparent medium into another. It is
the quotient of the sine of the angle of the incident, divided by the
sine of the angle of the refracted ray.
In the case of oils which remain liquid at room temperatures, the
determinations can be made without the aid of any device to maintain
liquidity. In the case of fat which becomes solid at ordinary room
temperatures, the determination must either be made in a room
artificially warmed or the apparatus must have some device, as in
the later instruments of Abbe and Pulfrich, and in the apparatus of
Amagat-Jean, whereby the sample under examination can be maintained in
a transparent condition. In each case the accuracy of the apparatus
should be tested by pure water, the refractive index of which at 18°
is 1.333. The refractive index is either read directly on the scale
as in Abbe’s instrument, or calculated from the angles measured as in
Pulfrich’s apparatus.
[Illustration: FIG. 86.—ABBE’S REFRACTOMETER.]
=296. Abbe’s Refractometer.=—For practical use the small instrument
invented by Abbe will be found sufficient. The one which has been
in use for many years in this laboratory is shown in Fig. 86. The
illustration represents the apparatus in the position preliminary to
reading the index. In preparing the sample of oil for observation the
instrument is turned on its axis until the prisms between which the
oil is placed assume a horizontal position, as is seen in Fig. 87. The
movable prism is unfastened and laid aside, the fixed prism covered
with a rectangular shaped piece of tissue paper on which one or two
drops of the oil are placed. The movable prism is replaced in such a
manner as to secure a complete separation of the two prisms by the film
of oiled tissue paper. A little practice will enable the analyst to
secure this result.
After the paper disk holding the fat is secured by replacing the upper
prism, the apparatus is placed in its normal position and the index
moved until the light directed through the apparatus by the mirror
shows the field of vision divided into dark and light portions. The
dispersion apparatus is now turned until the rainbow colors on the part
between the dark and light fields have disappeared. Before doing this,
however, the telescope, the eyepiece of the apparatus, is so adjusted
as to bring the cross lines of the field of vision distinctly into
focus. The index of the apparatus is now moved back and forth until the
line of the two fields of vision falls exactly at the intersection of
the cross lines. The refractive index of the fat under examination is
then read directly upon the scale by means of a small magnifying glass.
To check the accuracy of the first reading, the dispersion apparatus
should be turned through an angle of 180° until the colors have again
disappeared, and, after adjustment, the scale of the instrument again
read. These two readings should nearly coincide, and their mean is the
true reading of the fat under examination.
[Illustration: FIG. 87.—CHARGING POSITION OF REFRACTOMETER.]
For butter fats the apparatus should be kept in a warm place, the
temperature of which does not fall below 30°. For reducing the results
obtained to a standard temperature, say 25°, the factor 0.000176 may be
used. As the temperature rises the refractive index falls.
_Example._—Refractive index of a butter fat determined at 32°.4 =
1.4540, reduced to 25° as follows: 32.4 -25 = 7.4; 0.000176 × 7.4 =
0.0013; then 1.4540 + 0.0013 = 1.4553.
The instrument used should be set with distilled water at 18°, the
theoretical refractive index of water at that temperature being 1.333.
In the determination above given, the refractive index of pure water
measured 1.3300; hence the above numbers should be corrected for theory
by the addition of 0.0030, making the corrected index of the butter fat
mentioned at the temperature given, 1.4583.
=297. Pulfrich’s Refractometer.=—For exact scientific measurements,
Pulfrich’s apparatus has given here entire satisfaction. In this
instrument a larger quantity of the oil is required than for the
abbe, and this quantity is held in a cylindrical glass vessel luted
to the top of the prism. The method of accomplishing this and also an
illustration of the refraction of the rays of light are shown in Fig.
88.
[Illustration: FIG. 88.—PRISM OF PULFRICH’S REFRACTOMETER.]
The angle _i_ is measured by a divided circle read with the aid of a
small telescope. The index of the prism of highly refractive glass
_N_ is known. The oil is seen at _n_. The light used is the yellow
sodium ray (_D_). From the observed angle the refractive index of n is
calculated from the formula
______________
_n_ = √_N_² - sin²_i_.
For convenience the values of _n_ for all usual values of _i_ are
computed once for all and arranged for use in tabular form. The latest
model of Pulfrich’s apparatus, arranged both for liquid and solid
bodies, and also for spectrometric observation is shown in Fig. 89.
When the sodium light is used it is placed behind the apparatus and
the light is collected and reflected on the refractive prism by the
lens _N_. Through _H_ and _G_ is secured the micrometric reading of the
angle on the scale _D_ by means of the telescopic arrangement _F E_.
For regulating the temperature of the oil and adjacent parts, a stream
of water at any desired temperature is made to circulate through _L_
and _S_ in the direction indicated by the arrows. The manner in which
this is accomplished is shown in the cross section of that part of the
apparatus as indicated in Fig. 90.
[Illustration: FIG. 89.—PULFRICH’S NEW REFRACTOMETER.]
[Illustration: FIG. 90.—HEATING APPARATUS FOR PULFRICH’S REFRACTOMETER.]
[Illustration: FIG. 91.—SPECTROMETER ATTACHMENT.]
For further details of the construction and operation of the apparatus
the original description may be consulted.[248]
In case a spectrometric observation is desired the _H_ ray, for
instance, is produced by the geissler tube _Q_, Fig. 91. The light is
concentrated and thrown upon the refractive prism by the lens _P_, the
lens _N_, Fig. 89, being removed for this purpose.
Tables, for correcting the dispersion and for calculating the indices
for each angle and fraction thereof, and for corrections peculiar to
the apparatus, accompany each instrument.
=298. Refractive Indices of some Common Oils.=—The following numbers
show the refractive indices obtained by Long for some of the more
common oils. The light used was the yellow ray of the sodium flame.[249]
Refractive Calculated
Name. Temperature. index. for 25°.
Olive oil (France) 26°.6 1.4673 1.4677
” ” (California) 25°.4 1.4677 1.4678
Cottonseed oil 24°.8 1.4722 1.4721
” ” 26°.3 1.4703 1.4709
” ” 25°.3 1.4718 1.4719
Sesamé oil 24°.8 1.4728 1.4728
” ” 26°.8 1.4710 1.4716
Castor ” 25°.4 1.4771 1.4773
Lard ” 27°.3 1.4657 1.4666
Peanut ” 25°.3 1.4696 1.4696
In case of the use of Abbe’s apparatus, in which diffused sunlight
is the source of the illumination, the numbers obtained cannot be
compared directly with those just given unless the apparatus be first
so adjusted as to read with distilled water at 18°, 1.333. In this case
the reading of the scale gives the index as determined by the yellow
ray. The numbers obtained with Abbe’s instrument for some common oils
are given below.[250]
In the determinations the instrument was set with water at 18°, reading
1.3300, and they were corrected by adding 0.0030 in order to compensate
for the error of the apparatus.
Material. Calculated for 25°. Corrected index.
Lard 1.4620 1.4650
Cotton oil 1.4674 1.4704
Olive oil stearin 1.4582 1.4610
Lard stearin 1.4594 1.4624
=299. Oleorefractometer.=—Instead of measuring the angular value of
the refractive power of an oil it may be compared with some standard
on a purely arbitrary scale. Such an apparatus is illustrated by the
oleorefractometer of Amagat-Jean, or by Zeiss’s butyrorefractometer.
In the first named instrument, Fig. 92, the oil to be examined is
compared directly with another typical oil and the shadow produced by
the difference in refraction is located on a scale read by a telescope
and graduated for two different temperatures.[251] The internal
structure of the apparatus is shown in Fig. 93.
[Illustration: FIG. 92.—OLEOREFRACTOMETER.]
[Illustration: FIG. 93.—SECTION SHOWING CONSTRUCTION OF
OLEOREFRACTOMETER.]
In the center of the apparatus a metal cylinder, _A_, is found carrying
two plate glass pieces, _C B_, so placed as to form an angle of 107°.
This cylinder is placed in a larger one, provided with two circular
glass windows. To these two openings are fixed to the right and left,
the telescopic attachments, _G_, _V, S, E_, and the apparatus _M, H,
Sʹ_, _Eʹ_, for rendering the rays of light parallel. The field of
vision is divided into two portions, light and dark, by a semicircular
stop inserted in the collimator, and contains the double scale shown
in the figure placed at _H_. The field of vision is illuminated by a
gas or oil lamp placed at a convenient distance from the collimator.
The inner metallic cylinder _A_ is surrounded with an outer one, to
which the optical parts are attached at _D Dʹ_ by means of plane glass
plates. This cylinder is in turn contained in the large water cylinder
_P P_, carrying a thermometer in the opening shown at the top on the
left. The manipulation of the apparatus is very simple. The outer
cylinder is filled with water, at a temperature below 22°, the middle
one with the typical oil furnished with the instrument, the cover of
the apparatus carrying the thermometer placed in position and the
cup-shaped funnel inserted in the cylinder _A_, which is at first also
filled with the typical oil. The whole system is next brought slowly
to the temperature of 22° by means of the lamp shown in Fig. 92. The
telescope is adjusted to bring the scale of the field of vision into
focus and the line dividing the light and shadow of the field should
fall exactly on 0°_a_. If this be not the case the 0° is adjusted by
screws provided for that purpose until it is in proper position. The
typical oil is withdrawn from _A_ by the cock _R_, the cylinder washed
with a little of the oil to be examined and then filled therewith. On
again observing the field of vision the line separating the shadow from
the light will be found moved to the right or left, if the oil have
an index different from that of the typical oil. The position of the
dividing line is read on the scale.
For fats the temperature of the apparatus is brought exactly to 45°
and the scale 0°b is used. In other respects the manipulation for
the fats is exactly that described for oils. In the use of 0°a, in
case the room be warmer than 22°, all the liquids employed should be
cooled below 22° before being placed in the apparatus. It is then only
necessary to wait until the room temperature warms the system to 22°.
In the case of fats it is advisable to heat all the liquids to about
50° and allow them to cool to 45° instead of heating them to that
temperature by means of the lamp.
One grave objection to this instrument is found in the absence of the
proper scientific spirit controlling its manufacture and sale, as
evidenced by the attempt to preserve the secret of the composition
of the typical oil and the negligence in testing the scale of the
instruments which will be pointed out further along.
According to Jean[252] the common oils, when purified, give the
following readings at 22°:
Peanut oil +3.5 to +6.5
Colza ” +17.5 ” +21.0
Cotton ” +18.0 ” +18.0
Linseed ” +47.0 ” +54.5
Lard ” +5.5 ” +5.5
Olive ” +1.5 ” 0.0
Sesamé ” +17.5 ” +19.0
Oleomargarin -15.0 ” -15.0
Butter fat -30.0 ” -30.0
Mutton oil 0.0 ” 0.0
Fish ” +38.0 ” +38.0
In this instrument, therefore, vegetable and fish oils, as a rule, show
a right hand, and animal fats a left hand deviation.
The oleorefractometer has been extensively used in this laboratory and
the data obtained thereby have been found useful. We have not found,
however, the values fixed by Jean to be constant. The numbers for lard
have varied from -3.0 to -10.0, and other fats have shown almost as
wide a variation from the values assigned by him.
Jean states that the number for lard, determined by the
oleorefractometer, is -12, and he gives a definite number for each of
the common oils and fats. On trying the pure lards of known origin in
this instrument, I have never yet found one that showed a deviation of
-12 divisions of the scale; but I have no doubt that there are many
such lards in existence. The pure normal lards derived from the fat
of a single animal would naturally show greater variations in their
chemical and physical properties, than a typical lard derived from
the mixed fats of a great many animals. In leaf lard, rendered in the
laboratory, the reading of the oleorefractometer was found to be -10°,
while with the intestinal lard it was -9°. On the other hand, a lard
rendered from the fat from the back of the animal showed a reading of
only -3°, and a typical cottonseed oil a reading of +12°. According
to the statement of Jean, a lard which gives even as low a refractive
number as -9, by his instrument, would be adjudged at least one-quarter
cottonseed oil.
After a thorough trial of the instrument of Jean, I am convinced that
it is of great diagnostic value, but if used in the arbitrary manner
indicated by the author it would lead to endless error and confusion.
In other words, this instrument is of greater value in analyses than
Abbe’s ordinary refractometer, because it gives a wider expansion in
the limits of the field of vision, and therefore can be more accurately
read, but it is far from affording a certain means of discovering
traces of adulteration with other fats.
=300. Variations in the Instruments.=—In the use of the
oleorefractometer, attention should be called to the fact that, through
some negligence in manufacture, the instruments do not give, in all
instances, the same reading with the same substance. Allen obtained the
following data with a sample of lard examined in three instruments,
_viz._, 4°.5, 6°, and 11°. Such wide differences in the scales of
the instruments cannot fail to disparage the value of comparative
determinations.
The variations in samples of known origin, when read on the same
instrument, however, will show the range of error to which the
determinations made with the oleorefractometer are subject. Pearmain
has tabulated a large number of observations of this kind, covering 240
samples of oils.[253]
Following are the data relating to the most important oils.