More on the Nature of Matter: Thermodynamics and Entropy

We briefly mentioned entropy in the preceding section. We would like to expand on the necessity of understanding thermodynamics and its impact on the nature of physical objects.

Thermodynamics is the study of the patterns of energy change. Thermody- namics refers to energy, and “dynamics” means the patterns of change. Specifically the study of thermodynamics deals mainly with (A) energy conversion and (B) the stability of molecules and direction of change. (Thinkquest, 1996)

It is the second concept, the stability of molecules and direction of change, that interests us in the examination of physical evidence. This is addressed by the second law of thermodynamics:

A spontaneous change is accompanied by an increase in the total entropy of the system and its surroundings. Thus, the entropy of the universe always increases. (Thinkquest, 1996)

Entropy (S) is most simply a measure of disorder. While the entropy in a closed system might decrease, the entropy of the universe never decreases. For instance, the molecules in one’s body exist in great order at the expense of the ever-increasing entropy of the rest of the universe. Without specific

Figure 4.5 Sweater and fibers. When an object divides and the resulting fragment is transferred to another object, analysis of the fragment may not reveal the nature of the original item. One cannot infer the source sweater (inset) from analysis of the evidence fibers.

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input energy, things spontaneously move from order to randomness. Every moment that an object exists in the real universe, random forces from either energy or other matter impinge on it and drive it farther toward disorder. These random influences result in changes to an object that are neither predictable nor repeatable. The ever-increasing disorder at both the micro- and macrolevels results in the generation of random characteristics that will never be exactly repeated in another similar object.

This is a two-edged forensic sword. On one hand, disorder predicts the accumulation of random marks on an object (e.g., a shoe or gun barrel), unique complementary edges from a fracture, or unpredictable fingerprint ridge patterns arising from basal volar pads. On the other hand, once an item has fractured into more than one piece, each is also subject to the influences of this disorder that may result in the destruction of traits that might be useful in a forensic examination. The analyst benefits from this understand- ing both in deciding what traits might be useful for individualization and by considering the possibility of adventitious or ambiguous traits confounding the possibility of concluding a common source.

4. An Example of Divisibility

The number of “things” that can be evidence is limited to the things that exist in the physical universe! In other words, any “thing” can be evidence. To attempt an enumeration of how divisible matter might apply to every object would be ludicrous, and so we will simply provide an illustrative example and leave the reader the exercise of considering how this principle might apply to the infinite variety of matter that may be encountered.

A consideration of fibers affords an opportunity to develop some practical applications of divisible matter. Consider first a sweater made of dyed cot- ton. Cotton is a material that is easily fractured, and so little force is required to separate a fiber fragment from the parent piece. Once fractured, little opportunity exists to perform a physical match examination. First, finding the matching fiber on the parent piece would make it a “career case” (not a case that makes your career, but consumes it!). Second, the nature of cotton is such that very little input energy is required to change the fractured (complementary) ends, and soon they would not appear very complemen- tary. Instead, the analyst would more likely perform some examination of the physicochemical properties of the item. Cotton is easily discerned by a polarized light microscopic exam. After that, only the dye components would be useful for further differentiating it from any other cotton fiber. The dye itself can be examined from the perspective of divisible matter, for it once was a liquid in a large vat or lot of dye that was then used to dye many pieces of cotton. It was divided from the remaining lot when absorbed onto the fiber. Its characteristics (color, chemical constituents, chemical properties) can be examined and would be indistinguishable from any other

dye made with the same components. If either the sweater or the separated fiber had been exposed to some deteriorative process, such as bleaching, weathering, or washing, some chemical traits of the dye, such as its color, may have changed from the original values.

Divisible matter provides a basis for examining and interpreting similarities and differences between an evidence item and any putative source.

5. Impression Evidence — Does Divisible Matter Apply?

Divisible matter does not account for a large category of evidence, that of pattern transfer evidence, such as prints and impressions. Although small amounts of physical matter may be transferred, it is the pattern of transfer that concerns us, not the substance. Therefore divisible matter does not apply. The transfer of matter requires its prior division; the transfer of traits may not.

D.

Transfer Theory (Locard)

Once matter has divided, it is “available” for transfer. Two different excerpts from Locard’s writing* provide slightly different insights into his thinking on the issue of material transfer.

* Translation courtesy of Sharon Kruzik.

1. Il est impossible au malfaiteur d’agir, et sourtout d’agir avec l’intensite que suppose l’action criminelle sans laisser des traces de son passage.

(1920)

It is impossible for the criminal to act, and especially to act with the force that a crime demands, without leaving behind traces of his presence.

2. Nul ne peut agir avec l’intensite que suppose l’action sans laisser des marques multiples de son passage, tantot le malfaiteur a laissi sur les lieux des marques de son activiti, tanto par une action inverse, il a emporti sur son corps ou sur ses vetements les indices de son sejour ou de son geste. (1923)

No one can act (commit a crime) with the force (intensity) that the criminal act requires without leaving behind numerous signs (marks) of it; either the wrong-doer (felon, malefactor, offender) has left signs at the scene of the crime, or on the other hand, has taken away with him — on his person (body) or clothes — indi- cations of where he has been or what he has done.

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As much as the Locard transfer theory has been invoked, no peer-reviewed literature exists that proffers it, tests it, or refutes it. It is axiomatic in forensic science; it is accepted as true without proof.

The second of the two quotes above is a more complete explication of Locard’s sense of what happens during the commission of a crime. For example, he talks about transfer of material both from the criminal to the scene, as well as from the scene to the criminal. Significantly, this passage has been taken to mean that when two objects come in contact, material will be transferred in both directions between the two objects. That does not appear to be the plain reading of this translation. It is possible to interpret this passage to mean that different kinds of material may be transferred from the criminal to the surroundings, or from the surroundings to the criminal; carpet fibers may accumulate on the shoes of a criminal, yet there may be no trace of his on the carpet.

For a maxim that has been accepted as the lynchpin of forensic science for over 80 years, it is curious that no one has ever tested this theory by trying to falsify it. Of course, if it were completely false, then no physical evidence would exist to examine. Not wanting to tackle the disputatious “Il est impos- sible…,” we can say as a matter of common observation that at least some of the time, contact between two objects results in the transfer of material from one to the other. We leave as a challenge to the profession of forensic science to begin the process of testing this working hypothesis.

1. Physical Transfer

Locard posited that when a person was engaged in the commission of a crime, the intensity of the effort dictated that traces present on the person would be left behind on anything with which he came into contact, or that he would take away something of the crime scene on his body. One alternative to the translation of the word “intensity” is “force”; the implication is that the force required to commit a crime is such that transfer of material is inevitable.

Three objects and some energy are required for transfer.

Object 1 — The original source

Object 2 — A fragment (ultimately the evidence) divided from the source

Object 3 — A target object onto which the fragment is transferred

Energy — Some energy that facilitates movement of the fragment from the source to the target. This is typically taken to be contact between the source and the target, but it need not necessarily be so.

When the fragment is detected on the target by some responsible and author- itative person (typically but not always law enforcement), it becomes evi- dence. Largely unappreciated is the difference between the target and the 8127/frame/ch04 Page 94 Friday, July 21, 2000 11:48 AM

evidence. Later we will develop a critical distinction between individualiza- tion (how sure we are the evidence is from a particular source) and association

(the inference of contact between the source and the target). For now, it is important to recognize that there is more complexity to transfer than the deceptively simple statement derived from Locard.

a. Trace Evidence — Transfer of Very Small Physical Entities

Locard was particularly concerned with the transfer of trace materials, such as dust, dirt, and grime (1920; 1923; 1928; 1930; 1931–1940). In today’s crime laboratory, typical trace evidence includes hairs, fibers, paint, soil, and other materials that make up the world. Trace evidence is typically characterized by the need for a microscope to characterize the material adequately, even if it can be seen by the unaided eye. These are the materials that Locard felt were most important in forensic science because the criminal would not be aware that such materials would be transferred.

b. Macroscopic Evidence — Transfer of Larger Physical Entities

Because the concept of transfer arose through the study of dusts and other microscopic material, we are used to thinking about transfer only on a microscopic scale. In fact, it is inordinately useful to consider transfer on a macroscopic scale as well. Because it is impossible to draw an arbitrary line between microscopic evidence and that which is easily visible to the naked eye, we will take the liberty of creating the term “macro-transfer” to describe this situation.

Much physical match evidence falls into this category. For instance, the scrap of paper used to write the ransom note is only one half of the original intact piece, the other half of which is still in the kidnapper’s pocket. Broken glass fragments are another example of this type of evidence.

c. Factors Affecting Transfer and Detection

Several factors either promote or inhibit transfer of materials. These influence both our ability to detect crime-related materials (i.e., evidence) and our interpretation of the analytical findings. These components must be consid- ered when assessing the strength and significance of an association:

• The force for divisionand ease of divisibility (fragmentation) of a mate- rial — These influence the number and size of fragments available for transfer. For example, a cotton fabric will divide much more easily than a steel door when subjected to similar forces.

• The force for transfer during contact and the transferability of the material — These determine how likely a daughter fragment will either adhere to its parent piece or be transferred to a target.

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• The abundance of fragments transferred — The preceding factors, combined with the initial abundance, will determine how many frag- ments are transferred to a target.

• The persistence of the fragment — This refers to the ability of a frag- ment, once transferred from a source to a target, to adhere to the target.

• Secondary transfer — This refers to a fragment transferred from its source A to target B, and then transferred from target B to target C. Detecting the fragment from source A on target C leads to the infer- ence of contact between A and C, when in fact no such contact has been made.

• Unrelated transfer — This refers to the possibility of fragments trans- ferred and detected that are unrelated to the crime event.

These last two factors lead us to realize that the commission of a crime is not the only time that fragments are transferred from a source to a target. At any crime scene there is an overabundance of physical material, some related to the crime, and much that is not. Unfortunately, Locard’s formu- lation of transfer theory ignores the consideration of objects being transferred when crimes are not being committed. This is an important limitation to be incorporated into the final inference of a putative association between a source and a target.

2. Spatial Trait Transfer

a. Impression Evidence

Although we are used to thinking about transfer of physical material, many criminalistic examinations deal with traits or characteristics of one item left behind on some receptive medium, rather than any transfer of actual physical material. Dermal ridge prints, shoeprints, and tire tracks all fall into this category. This may be accomplished by impressing the traits into the medium (e.g., a toolmark), or leaving the trait as a print or track on a substrate via yet another medium (e.g., a fingerprint left in naturals oils and perspiration from the finger). One can think of these mechanisms as transfers as well; however, in these cases traits rather than matter are transferred.

b. Three- to Two- to One-Dimensional Transformations

In most types of trait transfer, some dimensional information is lost. For example, in a toolmark, the mechanism for producing a mark is usually the passing of a defect in the tool over some target material. This defect may be a gouge or a protrusion, and has three dimensions (Figure 4.6). When exam- ining the mark made on the substrate, normally only one of the dimensions 8127/frame/ch04 Page 96 Friday, July 21, 2000 11:48 AM

(width) is used for comparison. The length is lost in the process of producing the mark, and the depth is highly dependent on a number of factors, such as the relative hardness of the two media (source and target) and the angle between them. Thus, a three-dimensional defect is reduced to a one-dimen- sional mark.

In friction ridge examinations, normally only one dimension coupled with spatial information is used to compare evidence and reference prints. Some examiners will perform a more complex analysis involving the integration of pore placement and the shape of the ridges themselves. Nonetheless, the fingerprint examiner is restricted to an analysis of at most two dimensions. Shoeprints and tire prints hold the potential for including all three dimensions, but typically this examination is done using only width and length data (two dimensions). The number of dimensions available for exam- ination ultimately depends on whether the mark is an impression, where

Figure 4.6 Tool and mark. A toolmark is a three-dimensional impression. How- ever, so many variables are involved in producing the mark, that only one dimension is normally used to conclude a common source between a tool and a mark left by it.

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three-dimensional information might be retained, or a track, where some other medium (such as dust) is transferred to the target, and the dimension of depth is lost. As we have previously mentioned, some material is trans- ferred along with the print or track, but this is usually not the item of interest. Rather, the shape of this transferred material provides more information about the source than the material itself. In the case of shoe traits transferred to a floor through the medium of dust, the medium communicates nothing about the shoe itself.

The transfer of traits does not seem to fit into Locard’s writings, but we believe that this is an appropriate expansion of his philosophy. One way to think of transfer of either material or traits is at a level that unites them both, the concept that what is being transferred is information about the source of the evidence. Understanding the nature of the evidence allows the analyst to decide which information might be most useful in answering the relevant question(s). It may be that analysis of both the shoe print and the dust in which it is made will be useful in deciding where a particular shoe has trod. It may also occur that an incomplete fingerprint can be analyzed for its DNA content via the physiological fluids in which the print is made. When we think of transfer in terms of the information gleaned about the source of some evidence item, a new avenue of research is opened for forensic science that relies on information theory (Shannon, 1948). This may, for the first time, lead to some hypothesis testing of Locard’s transfer theory.

E.

Summary

We reiterate that understanding the fundamental nature of matter is the essential starting point for a forensic scientist. A good grounding in chemistry and physics provides this background knowledge. This information allows for a determination of the relevant properties of a material that might be useful in answering a legal question.

In attempting to identify those principles that are unique to forensic science, we have in this chapter articulated what we believe are the two principles involved in the origin of evidence: division of matter, and transfer of the divided fragment from the source to a target. It is certainly true that division and transfer occur whether a crime is being committed or not, or whether we are aware of it or not. When applied to forensic science, however, the division and transfer of matter becomes the division and transfer of evidence. It is arguable that the amount and types of physical evidence at a crime scene are more complex now than at the turn of the century; what cannot be argued is that the simplistic view of a criminal acquiring traces from the scene and leaving some of his own behind leaves out some inherent 8127/frame/ch04 Page 98 Friday, July 21, 2000 11:48 AM

ambiguity and complexity. Division of matter and transfer of divided mate- rial or transfer of traits provides a theoretical framework for the collection, analysis, and interpretation that occurs after a crime has been discovered.

References

Doyle, A. C., Adventures of Sherlock Holmes: The Red-headed League, The Strand

Magazine, 1891.

Kalia, R. K. et al., Role of ultrafine microstructures in dynamic fracture in nanophase silicon nitride, Phys. Rev. Lett., 78, 2144–2147, 1997.

Locard, E., L’enquete criminelle et les methodes scientifique, Ernest Flammarion, Paris, 1920.

Locard, E., Manuel de Tachnique Policiere, Payot, Paris, 1923. Locard, E., Dust and its analysis, Police J., 1, 177, 1928.

Locard, E., The analysis of dust traces, Part I–III, Am. J. Police Sci., 1, 276, 401, 496, 1930.

Locard, E., Traité de criminalistique, J. Desvigne, Lyon, 1931-1940.

Shannon, C. E., A mathematical theory of communication, Bell Syst. Tech. J., 27, 379–423, 623–656, 1948.

Thinkquest Chemystery page, Introduction to Thermodynamics, ref. Atkins, P., Gen-