THE object of the following notes is to show the difference between fire and chemical assays carried out on a highly pyritic tin ore, and to show the impossibility of estimating mill losses by the vanning and fire method. The tin occurs in the ore under consideration as cassiterite (SnO,,), the sulphur being mostly combined with Fe, As, Sb, Cu, Pb, etc. Stannite may occur very sparingly, but, being soluble in the preliminary acid treatment, it would not play a part in either the fire or chemical assay. The preliminary treatment this ore receives is the usual crushing, and concentration with jigs, Wilfley tables, and vanners. The whole of the pyritic concentrate is calcined, and a secondary treatment of the same kind follows. The resulting concentrate for sale averages about-Sn, 71.00 %; Pb, .29 %; S, .31 %; and as, .10 %. The writer carried out .numerous experiments to determine the mill losses, both by fire and chemical assays, and a few are here quoted. The fire assay used is that of vanning after digestion, and fusion with cyanide method. The chemical assay is the Pearce-Low method, as follows:-.5 grm. finely-ground ore is digested with aqua-regia and filtered, the filter paper and contents ignited, and fused in a nickel crucible with NaOH.