What the Nose Knows: The Science of Scent in Everyday Life Read Online Free PDF Page A

was “just smell it.” His own classification, proposed in 1916, had two very important selling points: it was based on empirical data, and it came with a ready-made visual representation, the “odor prism.” The image was a compelling one, orderly and neatly geometric. The six corners of the prism were each assigned a specific odor quality. Henning claimed that any odor could be located on the surface of the prism; its distance from any corner indicated the relative contribution of that odor quality.
    Unfortunately, Henning overplayed his hand. The clean geometry of the odor prism proved irresistible to the scientific psychologists in America, who tested its feasibility in laboratories at Harvard, Clark, and Vassar. Initially enthusiastic, the Americans soon found his theory to be cumbersome and too vague to yield testable predictions. In their hands, it produced inconclusive results. Henning’s initial theory was based on work with only a few experimental subjects; it now became clear that those subjects were extremely, if not suspiciously, consistent in their responses. (Wide person-to-person variability is a hallmark of odor perception; it’s unlikely that randomly selected sniffers would agree as precisely as Henning’s trio did.) In retrospect, there was always something too neat about Henning’s idealized prism: its geometric elegance is undeniably appealing, but few areas of human experience are less linear than smell.
    The dismantling of the odor prism by American psychologists ended the European tradition of armchair smell taxonomy. The search for a Universal Classification of Smell shifted entirely from philosophical reasoning to experimental research, and with it momentum crossed the Atlantic for good. Although as outmoded as the buggy whip, the odor prism persists in contemporary encyclopedias and textbooks, a testament to its iconic power.
     
    I T WAS FRUSTRATION with Henning’s prism that led the Americans Ernest Crocker and Lloyd Henderson—of the “10,000 odors” estimate—to invent a new system of smell classification. They began by selecting four “elementary odor sensations”: fragrant, acid, burnt, and caprylic. Then they assembled a set of odors to serve as reference standards, by means of which any smell could be rated on a scale of 0 to 8 for each of the elementary sensations. Rose, for example, was rated 6 on fragrant, 4 on acid, 2 on burnt, and 3 on caprylic. Those four numbers (6423) became, presto change-o, a digital identifier for that particular smell. In the same way, vinegar was 3803 and freshly roasted coffee was 7683. A numerical specification of sensory quality is not that outlandish; the Pantone color standards, for example, use numbered samples to let graphic designers and printers communicate accurately.
    The Crocker-Henderson system had wide appeal because it was based on empirical data and an open set of standards: anyone could play. Following its publication in 1927, the system was quickly commercialized; the complete set of reference odors could be ordered from Cargille Scientific, Inc., in New York City. It was soon being used by distillers, soap companies, the U.S. Army, and even the Department of Agriculture. Sensory psychologists initially gave the system positive reviews, but in 1949 researchers at Bucknell University dealt it a stunning blow. They found that untrained people couldn’t sort the thirty-two reference odors into anything resembling the four elementary sensations postulated by Crocker and Henderson. Further, people were unable to arrange the eight odors within an elementary group in order of intensity. Because the Crocker-Henderson system was premised on elementary odors and intensity-graded smells within them, the new findings effectively undermined its logic. User enthusiasm vanished and the system eventually faded away.
     
    A NOTHER BURST OF innovation in odor classification took place in the 1950s and 1960s when chemist John Amoore observed that