“Dig some out and let’s see if it comes from someplace different from Blackwater Landing.”

Ben scraped the dirt onto another subscription card, which he held in front of Rhyme, who examined it carefully. As a forensic scientist, he knew the importance of dirt. It sticks to clothes, it leaves trails like Hansel’s and Gretel’s bread crumbs to and from a perp’s house and it links criminal and crime scene as if they were shackled together. There are approximately 1,100 different shades of soil and if a sample from a crime scene is the identical color to the dirt in the perp’s backyard the odds are good that the perp was there. Similarity in the composition of the soils can bolster the connection too. Locard, the great French criminalist, developed a forensics principle named after him, which holds that in every crime there is always some transfer between the perpetrator and the victim or the crime scene. Rhyme had found that, second to blood in the case of an invasive homicide or assault, dirt is the substance most often transferred.

However, the problem with dirt as evidence is that it’s too prevalent. In order for it to have any meaning forensically a bit of dirt whose source might be the criminal must be different from the dirt found naturally at the crime scene.

The first step in dirt analysis is to check known soil from the scene—an exemplar—against the sample the criminalist believes came from the perp.

Rhyme explained this to Ben and the big man picked up one bag of dirt, which Sachs had marked Exemplar soil—Blackwater Landing, along with the date and time of collection. There was also a notation in a hand that was not Sachs’s. Collected by Deputy J. Corn. Rhyme pictured the young deputy eagerly scurrying off to do her bidding. Ben poured some of this dirt onto a third subscription card. He set it beside the dirt he’d dug out of Garrett’s treads. “How do we compare them?” the young man asked, looking over the instruments.

“Your eyes.”

“But—”

“Just look at them. See if the color of the unknown sample is different from the color of the known.”

“How do I do that?”

Rhyme forced himself to answer calmly. “You just look at them.”

Ben stared at one pile, then the other.

Back again. Once more.

And then once again.

Come on, come on … it isn’t that tricky. Rhyme struggled to be patient. One of the hardest things in the world for him.

“What do you see?” Rhyme asked. “Is the dirt from the two scenes different?”

“Well, I can’t exactly tell, sir. I think one’s lighter.”

“’Scope them in the comparison.”

Ben mounted the samples in a comparison microscope and looked through the eyepieces. “I’m not sure. Hard to say. I guess … maybe there is some difference.”

“Let me see.”