This paper describes those considerations that are of importance in undertaking large-scale oil-spreading experiments. One such experiment involving a discharge of 120 tons of Iranian Light Crude Oil in the North Atlantic is described, and the spreading pattern observed is reported in detail. The observations continued over a period of four days, after which the bulk of the oil had disappeared by natural factors, leaving only a few patches of thicker oil, largely in the form of a water-in-oil emulsion, which was also rapidly disappearing. Blokker constants have been calculated for the observed spreading of the slick, and these are reasonably constant throughout the four-day period.

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A method is described for determining silica in rocks and minerals. Most of the silica is removed by evaporation with hydrochloric acid and volatilisation with hydrofluoric acid ; the remaining traces are determined in the filtrate photometrically by a mo lybdenum-blue method. Interference by minor amounts of iron does not oc'cur a t the wavelength chosen for the photometric measurement. When present in the amounts usually encountered in silicate rocks, neither titanium nor phosphorus interferes with the determination. The photometric determination, however, cannot be made if either of these elements or iron is present in quantity. Modified procedures are described in which cations are removed from the solution before colour formation.THE classical method for determining silica in silicate rocks and minerals1 is long and tedious ; it involves two or three evaporations of the rock solution to dryness with hydrochloric acid. Silica in the filtrate from the second or third dehydration is recovered, together with iron, aluminium, titanium and other elements of this group, by precipitation with ammonia. The agreement that can be expected between duplicate results obtained by this method is about 0.1 to 0.2 per cent. of the rock. Some photometric procedures for determining silica have been applied to silicate rocks and rnineral~.2,3~4,6~~ These are based on formation of a yellow molybdosilicate complex or its subsequent reduction with a suitable reagent to molybdenum blue. As silica is the major constituent of silicate rocks, generally exceeding 50 per cent. and commonly 60 to 75 per cent., great care must be taken to avoid serious error. The precision is not high. Riley: for example, recommends determination in quadruplicate as a means of increasing the precision and the accuracy.In the method described below, most of the silica is recovered by a single evaporation with hydrochloric acid, and the small amount. remaining in the filtrate from these operations is determined photometrically. This permit.5 an accurate gravimetric determination to be * Presented at the meeting of the Society on 'Wednesday, April 6th, 1960. t

Salicylideneamino-2-thiophenol can easily be prepared by the reaction of 2-aminobenzenethiol with salicylaldehyde. This reagent reacts with silver, copper, molybdenum, tin and several other metals to give coloured complexes, but by a suitable choice of conditions it can be made selective for tin. The tin complex is readily extracted into organic solvents and can be used for the photometric determination of this metal in ores, rocks and minerals.

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