تسجيل الدخول

تسجيل

PHYSICAL PROPERTIES

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p432-434

2026-08-18

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PHYSICAL PROPERTIES

In general, physical properties are fairly easy to measure and the tests are inexpensive and not too consumptive of material. Standard methods of analysis are available from the American Society of Testing and Materials and the US Geological Survey. The most common physical tests are color and particle size distribution. The color of soil is affected by moisture content, mineral distribution, and location. Dusty, dry soils tend to be light tan or white owing to lack of moisture. Agricultural or tropical soils tend to be dark brown owing to the high humic content. The naked eye is a very good discriminator of color when comparing soil samples and there are standard color charts to help with exact color characterization. Figure 1 shows a case involving several different soil colors. In this case, there was a theft of trees from a plant nursery. The tress was tracked to a rural location. Soil samples were taken from the earth balls at the base of the trees and compared to holes in the ground at the nursery. Although none of the soil samples could be traced to a particular tree, the comparisons showed that the soils from the trees all showed similarities to the holes in the ground at the nursery. The first and most important tool in the forensic soil scientist’s toolbox is the microscope. Soil samples should first be examined using a stereomicroscope. This will give the analyst a general idea of the organic and inorganic fractions of the soil and their relative percent. It will also provide information about the colors of the soil and the particle size distribution. The next step in the analysis is to use a polarizing microscope at 100–400× and identify the actual minerals present and their relative amounts. Particle analysis is key to understanding the nature of the samples under examination. The interested student should seek out the references and resources from the McCrone Institute on polarized light microscopy of particles and minerals. An easy method of ascertaining particle size in soil samples is by sieving. A weighed soil sample is dried and then put through a nest of sieves where each succeeding sieve has smaller holes than the one preceding it. Then each fraction is weighed and the percentage of each particle size range can be computed. Scanning electron microscopy (SEM) has become a useful tool for many crime laboratories and has been applied to the analysis of soils. Using an electron microscope, one can visualize very tiny particles that cannot be seen with a light microscope. In addition, the elemental composition of an individual particle can be determined at the same time using energy dispersive X-ray analysis.

FIGURE 1 Different-color soils. This was taken from a case where several plants were uprooted and stolen from a plant nursery. Samples of soil taken from the plants were compared with samples taken from the pots the plants were in at the nursery.