nber category 1 Chemical subcat 13 Gas

uspto classes
48 Gas: Heating and Illuminating
55 Gas Separation
95 Gas Separation: Processes
96 Gas Separation: Apparatus

Historic patent counts for subcat 13

uspto search in the 1790 to present database

ccl/48/$ OR ccl/55/$ OR ccl/95/$ OR ccl/96/$

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