<CHAP NUM="4" ID="CH.00.004">chapter 4

Physical Properties: Glass and Soil

<FM><KTSET><TTL>Key Terms</TTL>

<KT>amorphous solid</KT>

<KT>atom</KT>

<KT>Becke line</KT>

<KT>birefringence</KT>

<KT>Celsius scale</KT>

<KT>chemical property</KT>

<KT>concentric fracture</KT>

<KT>crystalline solid</KT>

<KT>density</KT>

<KT>density-gradient tube</KT>

<KT>dispersion</KT>

<KT>Fahrenheit scale</KT>

<KT>intensive property</KT>

<KT>laminated glass</KT>

<KT>mass</KT>

<KT>mineral</KT>

<KT>physical property</KT>

<KT>radial fracture</KT>

<KT>refraction</KT>

<KT>refractive index</KT>

<KT>tempered glass</KT>

<KT>weight</KT></KTSET>

<OBJSET><TTL>Learning Objectives</TTL>

<P>After studying this chapter you should be able to:

<OBJ><P><INST><              </INST>Define and distinguish the physical and chemical properties of matter</P></OBJ>

<OBJ><P><INST><              </INST>Understand how to use the basic units of the metric system</P></OBJ>

<OBJ><P><INST><              </INST>Define and understand the properties of density and refractive index</P></OBJ>

<OBJ><P><INST><              </INST>Understand and explain the dispersion of light through a prism</P></OBJ>

<OBJ><P><INST><              </INST>List and explain forensic methods for comparing glass fragments</P></OBJ>

<OBJ><P><INST><              </INST>Understand how to examine glass fractures to determine the direction of impact for a projectile</P></OBJ>

<OBJ><P><INST><              </INST>List the important forensic properties of soil</P></OBJ>

<OBJ><P><INST><              </INST>Describe the proper collection of glass and soil evidence</P></OBJ></P></OBJSET></FM>

<CASE NUM="1" TY="CS1"><TTL>Murder and the Horse Chestnut Tree</TTL>

<P>Roger Severs was the son of a wealthy English couple, Eileen and Derek Severs. The elder Serverses were reported missing in 1983. Police investigators were greeted at the Severs home by Roger, who at first explained that his parents had decided to spend some time in London. Suspicion of foul play quickly arose when investigators located traces of blood in the residence. More blood was found in Derek’s car and signs of blood spatter were on the garage door. Curiously, a number of green fibers were located throughout the house, as well as in the trunk of Derek’s car. A thorough geological examination of soil and vegetation caked onto Severs’ car wheel rims seemed to indicate that the car had been in a location at the edge of a wooded area. Closer examination of the debris also revealed the presence of horse chestnut pollen. Horse chestnut is an exceptionally rare tree in the region of the Severs residence. Using land maps, a geologist was able to locate possible areas where horse chestnut pollen might be found. In one of the locations, investigators found a shallow grave that contained the bludgeoned bodies of the elder Severses. Not surprisingly, they were wrapped in a green blanket. A jury rejected Roger’s defense of diminished capacity and found him guilty of murder.</P></CASE>

<BM><P>The forensic scientist must constantly determine the properties that impart distinguishing characteristics to matter, giving it a unique identity. The continuing search for distinctive properties ends only when the scientist has completely individualized a substance to one correct source. Properties are the identifying characteristics of substances. In this and succeeding chapters, we will examine properties that are most useful for characterizing soil, glass, and other physical evidence. However, before we begin, we can simplify our understanding of the nature of properties by classifying them into two broad categories: physical and chemical.</P>

<P><BOLD>Physical properties describe a substance without reference to any other substance.</BOLD> For example, weight, volume, color, boiling point, and melting point are typical physical properties that can be measured for a particular substance without altering the material’s composition through a chemical reaction; they are associated only with the physical existence of that substance. <BOLD>A chemical property describes the behavior of a substance when it reacts or combines with another substance.</BOLD> For example, when wood burns, it chemically combines with oxygen in the air to form new substances; this transformation describes a chemical property of wood. In the crime laboratory, a routine procedure for determining the presence of heroin in a suspect specimen is to react it with a chemical reagent known as the Marquis reagent, which turns purple in the presence of heroin. This color transformation becomes a chemical property of heroin and provides a convenient test for its identification.</P>

<P>Which physical and chemical properties the forensic scientist ultimately chooses to observe and measure depends on the type of material that is being examined. Logic requires, however, that if the property can be assigned a numerical value, it must relate to a standard system of measurement accepted throughout the scientific community.</P>

<H1>The Metric System</H1>

<P>Although scientists, including forensic scientists, throughout the world have been using the metric system of measurement for more than a century, the United States still uses the cumbersome “English system” to express length in inches, feet, or yards; weight in ounces or pounds; and volume in pints or quarts. The inherent difficulty of this system is that no simple numerical relationship exists between the various units of measurement. For example, to convert inches to feet one must know that 1 foot is equal to 12 inches; conversion of ounces to pounds requires the knowledge that 16 ounces is equivalent to 1 pound. In 1791, the French Academy of Science devised the simple system of measurement known as the metric system. This system uses a simple decimal relationship so that a unit of length, volume, or mass can be converted into a subunit by simply multiplying or dividing by a multiple of 10—for example, 10, 100, or 1,000.<SIDEIND NUM="1" ID="MN2.04.001"/><SIDEIND NUM="2" ID="MN2.04.002"/></P>

<P>Even though the United States has not yet adopted the metric system, its system of currency is decimal and, hence, is analogous to the metric system. The basic unit of currency is the dollar. A dollar is divided into 10 equal units called dimes, and each dime is further divided into 10 equal units of cents.</P>

<P>The metric system has basic units of measurement for length, mass, and volume: the meter, gram, and liter, respectively. These three basic units can be converted into subunits that are decimal multiples of the basic unit by simply attaching a prefix to the unit name. The following are common prefixes and their equivalent decimal value:

<P>Hence, 1/10 or 0.1 gram (g) is the same as a decigram (dg), 1/100 or 0.01 meter is equal to a centimeter (cm), and 1/1,000 liter is a milliliter (mL). A metric conversion is carried out simply by moving the decimal point to the right or left and inserting the proper prefix to show the direction and number of places that the decimal point has been moved. For example, if the weight of a powder is 0.0165 gram, it may be more convenient to multiply this value by 100 and express it as 1.65 centigrams or by 1,000 to show it as its equivalent value of 16.5 milligrams. Similarly, an object that weighs 264,450 grams may be expressed as 264.45 kilograms simply by dividing it by 1,000. It is important to remember that in any of these conversions, the value of the measurement has not changed; 0.0165 gram is still equivalent to 1.65 centigrams, just as one dollar is still equal to 100 cents. We have simply adjusted the position of the decimal and shown the extent of the adjustment with a prefix.</P>

<P>One interesting aspect of the metric system is that volume can be defined in terms of length. A liter by definition is the volume of a cube with sides of length 10 centimeters. One liter is therefore equivalent to a volume of 10 cm × 10 cm × 10 cm, or 1,000 cubic centimeters (cc). Thus, 1/1,000 liter or 1 milliliter (mL) is equal to 1 cubic centimeter (cc) (see <LINK LINKEND="FG.04.001">Figure <FIGIND NUM="1" ID="FG.04.001"/>4–1</LINK>). Scientists commonly use the subunits mL and cc interchangeably to express volume.</P>

<P>At times, it may be necessary to convert units from the metric system into the English system, or vice versa (see <LINK LINKEND="FG.04.002">Figure <FIGIND NUM="2" ID="FG.04.002"/>4–2</LINK>). To accomplish this, we must consult references that list English units and their metric equivalents. Some of the more useful equivalents follow:

<DM ID="DM.04.001">1 inch = 2.54 centimeters

1 meter = 39.37 inches

1 pound = 453.6 grams

1 liter = 1.06 quarts

1 kilogram = 2.2 pounds</DM></P>

<P>The general mathematical procedures for converting from one system to another can be illustrated by converting 12 inches into centimeters. To change inches into centimeters, we need to know that there are 2.54 centimeters per inch. Hence, if we multiply 12 inches by 2.54 centimeters per inch (12 in. × 2.54 cm/in.), the unit of inches will cancel out, leaving the product 30.48 cm. Similarly, applying the conversion of grams to pounds, 227 grams is equivalent to 227 g × 1 lb/453.6 g or 0.5 lb.</P>

<H1>Physical Properties</H1>

<H2>Temperature</H2>

<P>Determining the physical properties of any material often requires measuring its temperature. For instance, the temperatures at which a substance melts or boils are readily determinable characteristics that will help identify it. Temperature is a measure of heat intensity, or the amount of heat in a substance. Temperature is usually measured by causing a thermometer to come into contact with a substance. The familiar mercury-in-glass thermometer functions because mercury expands more than glass when heated and contracts more than glass when cooled. Thus, the length of the mercury column in the glass tube provides a measure of the surrounding environment’s temperature. The construction of a temperature scale requires two reference points and a choice of units. The reference points most conveniently chosen are the freezing point and boiling point of water. The two most common temperature scales used are the <KT>Fahrenheit</KT> and <KT>Celsius</KT> (formerly called <ITAL>centigrade...