“Yes,” I replied; “but the information that it yields is excessively vague.”
“That is true,” he agreed, taking up the coat and envelope to carry them to the laboratory, “and yet, you know, Jervis, as I have often pointed out, the evidential value of dust is apt to be under-estimated. The naked-eye appearances—which are the normal appearances—are misleading. Gather the dust, say, from a table-top, and what have you? A fine powder of a characterless grey, just like any other dust from any other table-top. But, under the microscope, this grey powder is resolved into recognizable fragments of definite substances, which fragments may often be traced with certainty to the masses from which they have been detached. But you know all this as well as I do.”
“I quite appreciate the value of dust as evidence in certain circumstances,” I replied, “but surely the information that could be gathered from dust on the coat of an unknown man must be too general to be of any use in tracing the owner.”
“I am afraid you are right,” said Thorndyke, laying the coat on the laboratory bench; “but we shall soon see, if Polton will let us have his patent dust-extractor.”
The little apparatus to which my colleague referred was the invention of our ingenious laboratory assistant, and resembled in principle the “vacuum cleaners” used for restoring carpets. It had, however, one special feature: the receiver was made to admit a microscope-slide, and on this the dust laden air was delivered from a jet.
The “extractor” having been clamped to the bench by its proud inventor, and a wetted slide introduced into the receiver, Thorndyke applied the nozzle of the instrument to the collar of the coat while Polton worked the pump. The slide was then removed and, another having been substituted, the nozzle was applied to the right sleeve near the shoulder, and the exhauster again worked by Polton. By repeating this process, half-a-dozen slides were obtained charged with dust from different parts of the garment, and then, setting up our respective microscopes, we proceeded to examine the samples.
A very brief inspection showed me that this dust contained matter not usually met with—at any rate, in appreciable quantities. There were, of course, the usual fragments of wool, cotton and other fibres derived from clothing and furniture, particles of straw, husk, hair, various mineral particles and, in fact, the ordinary constituents of dust from clothing. But, in addition to these, and in much greater quantity, were a number of other bodies, mostly of vegetable origin and presenting well-defined characters in considerable variety, and especially abundant were various starch granules.
I glanced at Thorndyke and observed he was already busy with a pencil and a slip of paper, apparently making a list of the objects visible in the field of the microscope. I hastened to follow his example, and for a time we worked on in silence. At length my colleague leaned back in his chair and read over his list.
“This is a highly interesting collection, Jervis,” he remarked. “What do you find on your slides out of the ordinary?”
“I have quite a little museum here,” I replied, referring to my list. “There is, of course, chalk from the road at Raynesford. In addition to this I find various starches, principally wheat and rice, especially rice, fragments of the cortices of several seeds, several different stone-cells, some yellow masses that look like turmeric, black pepper resin-cells, one ‘port wine’ pimento cell, and one or two particles of graphite.”