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Page 1, clause 2, replace the 3rd paragraph by: "Conversely, if the conditions of application of the layer and its density are known, the determination of the mass per unit area (surface density) can permit a calculation of the average mass and an approximate evaluation of the thickness (see clause 9, note, 3rd paragraph)." Clause 5, 1st line: replace "phosphochromic solution" by "phosporic-chromic acid". Page 2, clause 9, note: insert the following after the 2nd paragraph: "Because of uncertainty about the density value, the method only gives an approximate value of the thickness."