The third series of radiocarbon measurements made at the British Museum Research. Laboratory is reported in the following list. Equipment and method used are as described previously I British Museum l 1 and, as in previous lists, the error terms are not l)ased solely on counting statistics, but are widened to include contributions of +80 years for possible isotopic fractionation effects and ±100 years for the de Vries effect. Ages are calculated on a half-life of 5568 ± 30 years. \ RS oxalic acid is now used as a reference standard in place of 100-yr-old oak. The latter gave an age-corrected value almost exactly 95rc of the oxalic-acid activity and thus no corrections are required to our lrevious date lists to bring them into line with the new standard. PRETREATMENT OF BONE AND :ANTLEROne of the laboratory's long-term projects is an investigation into the reliability of bone and antler as source materials for dating. A number of measurements on such materials are reported here. In all cases, only the organic fraction of the sample was used as a source of carbon, and the procedure adopted was as follows: Sample was broken into small pieces, either by coarse grinding or pounding in a mortar, and was treated with cold dilute hydrochloric acid in order to remove carbonates and to decalcify the material. Resulting granular gel was washed thoroughly by repeated soaking with cold water and was finally (Iried, prior to combustion.ACKNOWLEDGMENT

The following list comprises measurements made to the end of September 1961, when the equipment had to be dismantled for removal into new premises at 39 Russell Square, London W.C.1. The technique used is as described previously (Barker and Mackey, 1959) and as in previous lists, the error terms are not based solely on counting statistics but are widened to include contributions of ± 80 yr for possible isotopic fractionation effects and ± 100 yr for the de Vries-effect. NBS oxalic acid is used as the reference standard and ages are calculated on a half life of 5568 yr and expressed as years b.p. (before 1950 a.d.) and also in terms of the Christian calendar.

The dates detailed below are based on measurements made from September 1962 to August 1964. Work was often seriously interrupted due to difficulties with electronic equipment and also, from the late summer of 1963, high levels of tritium in the local water supplies used in the synthesis of acetylene (from nuclear weapons tests) made it difficult to obtain accurate measurements with acetylene as a filling gas for the proportional counter. The gas preparation equipment was therefore modified for the preparation of high purity CO2, and from Sample BM-165 onwards, the proportional counter was operated with CO2 as the filling gas at a pressure of 210 cm Hg at 22°C. instead of 140 cm Hg pressure of acetylene at 22°C. as reported previously (British Museum I). Background and net modern count rates under these conditions are 3.59 c.p.m. and 8.35 c.p.m., respectively. (In practice these values are taken as the rolling mean of the past 20 weeks’ measurements and are very constant.) The calculations of age are based on the half-life of 5570 yr and error terms are widened to include contributions of ± 80 yr for possible isotopic fractionation effects and ± 100 yr for de Vries-effects. Safeguards against inaccuracies are as described previously (British Museum I).

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