A rose is a rose and a skunk is a skunk, and the nose easily tells the difference. But it is not so easy to describe or explain this differ ence. \Ve know surprisingly little about the sense of smell, in spite of its im portant influence on our daily lives and the voluminous literature of research on the subject. Ol"le is hard put to describe an odor except by comparing it to a more familiar one. \Ve have no vard sti<:k for measuring the strength of odors, as we measure sound in decibels and light in lumens. And we have had no satisfactory general theor�; to explain how the nose and brain detect, identify and re<:ognize all odor. .\ [ore than 30 different theories have been suggested bv investigators in various disciplines, but none of them has passed the test of experiments designed to determine their validity. The sense of smell obviously is a <:hemical sense, and its sensitivity is proverbial; to a chemist the ability of the nose to sort out and characterize sub stancps is almost beyond belief. It deals with complex compounds that might take a chemist months to analyze in the laboratory; the nose identifies them in stantly, even in an amount so small (as little as a ten-millionth of a gram) that the most sensitive modern laboratorv in struments often cannot detect the sub� stance, let alone analyze and label it. Two thousand vears ago the poet Lucretius suggested a simple explana tion of the sense of smell. He speculated that the "palate" contained minute pores of various sizes and shapes. Ever�; odor ous substance, he said, gave off tinv "molecules" of a particular shape, and the odor was perceived when these molecules entered pores in the palate. Presumably the identification of ea<:h odor depended on which pores the mole� cules fitted. It now appears that Lucretius' guess