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Transcript
NOTES – CHAPTER 4
PYSICAL PROPERTIES – GLASS AND SOIL
HONORS FORENSIC SCIENCE
I.
Introduction
a. Our understanding of the nature of properties can be made easier by classifying
them into two categories
i. Physical properties = describe a substance without a reference to any other
substance
1. Ex. Weight color, boiling point, and melting point
ii. Chemical property = describes the behavior of a substance when it reacts
or combines with another substance
1. Ex. When wood burns, it chemically combines with oxygen in the
air to form new substances
2. Ex. Marquis reagent turns purple in the presence in heroin
II.
Metric System
a. Has basic units of measurement for length (meter) , mass (gram) and volume
(liter)
b. In the metric system, volume can be defined in terms of length
c. At times it may be necessary to convert units from the metric system into the
English System or vice versa
d. Some of the more useful equivalents:
i. 1 inch = 2.54 cm
ii. 1 meter = 39.37 inches
iii. 1 pound = 453.6 grams
iv. 1 liter = 1.06 quarts
v. I kilograms = 2.2 pounds
III.
Physical Properties
a. Temperature
i. The determination of the physical properties of any material will often
require the measurement of temperature
ii. Temperature is the measure of heat intensity, or the hotness or coldness of
a substance
iii. The two most common temperature scales used are the Fahrenheit and
Celsius Scales
1. In Fahrenheit, water boils at 212 degrees F and freezes at 32
degrees F
2. In Celsius, water boils at 100 degrees C and freezes at 0 degrees C
b.
Weight and Mass
i. The force with which gravity attracts a body is called weight
ii. Mass differs from weight because it refers to the amount of matter an
object contains and is independent to its location on earth or any other
place in the universe
iii. In the metric system, the basic unit of mass is the gram
iv. The mass of an object is determined by comparing it against the know
mass of standard objects
v. The comparison is performedon a balance
c. Density
i. Is defined as mass per unit volume
ii. Is an intensive property of matter – is the same regardless of the size of a
substance and is a characteristic property of a substance and can be used
as an aid in identification
iii. Solids are more dense than liquids, and liquids more dense than gases
iv. The volumes of gases and liquids vary considerably with temperature so
when determining density, it is important to control and record the
temperature at which the measurements are made
d. Refractive Index
i. Light waves travel in air at a constant velocity of nearly 300 million
meters per second until they penetrate another medium
ii. When light waves hit another medium, they slow, causing the rays to bend
iii. The bending of a light wave because of a change in velocity is call
refraction
iv. The ratio of the velocity of light in a vacuum to that in any medium
determines the refractive index
v.
Like density, the refractive index is an intensive physical property of
matter and can serve to characterize a substance
vi. Tests used to determine a substance’s refractive index must be performed
under carefully controlled temperature and lighting conditions
vii. Normally a solid or a liquid would be expected to exhibit only one
refractive index value however, many solids that are crystalline in nature
will have two refractive indices whose values in part depend on the
direction in which the light enters the crystal with respect to the crystal
axis.
IV.
V.
1. Crystalline solids – have definite geometric forms because of the
orderly arrangement of the fundamental particle of a solid, the
atom
viii. Not all solids are crystalline in nature, some are amorphous
1. Amorphous solids – a solid win which the constituent atoms or
molecules are arranged in random or disordered patterns
2. Glass is an amorphous solid
ix. Other characteristics of glass
1. Birefringence – a difference in the two indices of refraction
exhibited by most crystalline materials
2. Dispersion – the separation of light into its component
wavelengths
Comparing Glass Fragments
a. Glass that is broken and shattered into fragments and minute particles during the
commission of a crime can be used to place a suspect at the crime scene
b. Glass is a hard, brittle, amorphous substance that is composed of silicon oxides
mixed with various metal oxides.
c. Tempered glass = glass to which strength is added by introducing stress through
the rapid heating and cooling of the glass surface
d. Laminated glass = two sheets of ordinary glass bonded together with a plastic film
i. Most car windshields
e. Glass will possess its greatest evidential value when it can be individualized to
one source. This can only be accomplished when pieces can be assembled and
physically fitted together
f. At this time, the physical properties of density and refractive index are used most
successfully for characterizing glass particles
g. Glass can be distinguished by a density determination and refractive indices.
However, even glass fragments removed from a single sheet of glass may not
have a uniform refractive index value
h. A significant difference in either density or refractive index proves that the
glasses examined do not have a common origin
i. FBI has a database of density and refractive indices of different glasses collected
Glass Fractures
a. Glass bends in response to any force that is exerted on any one of its surfaces;
when the limit of its elasticity is reached, the glass fractures
b. Fractured glass may reveal information regarding force and direction of an impact
c. Types of fractures
i. Radial – a crack in a glass that extends outward like the spoke of a wheel
from the point at which the glass was struck
VI.
ii. Concentric – a crack in a glass that forms a rough circle around the point
of impact
d. Stress marks – are shaped like arches that are perpendicular to one glass surface
and curved nearly parallel to the opposite surface
e. Radial cracks form a right angle on the reverse side of the force
f. When there have been successive penetrations of glass, it is frequently possible to
determine the sequence of impact by observing the existing fracture lines and
their points of termination. A fracture always terminates at an existing line of
fracture.
Forensic Characteristics of Soil
a. Soil = includes any disintegrated surface material, both natural and articial, that
lies on or near the earth’s surface
i. Encompasses analysis of naturally occurring material such as rocks,
minerals, vegetation and animal matter as well as glass, plaint chips,
asphalt, brick fragments, cinders etc.
b. Value of soil as evidence rests with its prevalence at crime scenes and its
transferability between the scene and the criminal
c. Soil evidence is comparative in nature
d. Most soils can be differentiated and distinguished by their gross appearance.
Samples must be dried before comparison.
e. Low power microscopic examination of soil will reveal the presence of plant and
animal materials as well as of artificial debris
f. Minerals = a naturally occurring crystal and like any other crystal, its physical
properties are most useful for identification
g. Some forensic laboratories currently rely on the density-gradient tube technique to
compare soil specimens
i. Density- gradient tube = a glass tube filled from bottom to top with liquids
of successively lighter densities, used to determine the density
distribution of soil
h. The ultimate forensic value of soil evidence depends on its variation at the crime
scene
i. Up to this time, there have been no statistically valid forensic studies on the
variability of soil evidence