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CORROSION OF ANALYTICAL WEIGHTS. SCIENTISTS in the Metrology Division of the National Physical Laboratory were recently called on to discover the effect of chemical laboratory atmospheres on the mass and therefore the accuracy of analytical weights. They were particularly concerned with tin‐nickel plated brass weights. They used 100 g. weights and exposed them in ventilated containers suspended above head level in three typical chemical laboratories for three months. The weights least affected were made of 25/20 stainless steel and they were highly polished. Their mean change of mass in three months' exposure was only 0.12 mg. A normal production grade of the same stainless steel changed 0.20 mg. in that period. A change of 0.22 mg. was observed with rhodium‐plated brass weights. For practical purposes the new weights— tin‐nickel plated brass—were as little affected as the rhodium‐plated and 25/20 stainless steel weights. The best of three types—plated with 15µ Sn Ni on 15µ Cu—in fact gained only 0.22 mg. The other two types, 30µ Sn Ni and 15µ Sn Ni changed 0.24 and 0.28 mg. respectively.