However, Koch never credited the Hesses for their discovery of bacteriological agar, perhaps because, at the time, he failed to recognize its importance. Even after he received the insight about agar from the Hesses, Koch stuck with gelatine for years. In 1883-84, during his first medical expedition to Egypt and India to investigate cholera, he tried and failed to grow the cholera bacterium on gelatine media in the hot climate of Cairo (despite using a half-open fridge for incubation), only succeeding in the colder winter of Calcutta.
新春里，侗寨年味浓。听侗族大歌，游鼓楼花桥，贵州黎平县肇兴侗寨迎来八方游客。
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The “Knocking on Wood” Wikipedia page mentions Roud’s conclusion, and this claim of a 19th century origin has circulated online as the final say on the matter. If you go back to the original, though, Roud’s proposal actually ends on an ambivalent note: “Before this theory [of 19th century origins] can be finally accepted, however, an examination of the history of European forms of this custom would be advisable.”