have to be made the operation not only becomes tedious, but the solution also ceases to clear after shaking, so that it becomes impossible to determine the finishing point. If the assay contains less than one gram of silver the first addition of the dilute salt solution of course produces no precipitate. Five milligrams of silver in solution (5 c.c.) is then added, and the assay proceeded with in the usual way; 5 milligrams of silver being deducted from the amount found. There is required for the assay a _standard solution of sodium chloride_, which is prepared by dissolving 5.4162 grams of the salt (made by neutralizing carbonate of soda with hydrochloric acid) in water and diluting to one litre. 100 c.c. of this is equivalent to 1 gram of silver. The weaker solution of salt is made by diluting 100 c.c. of the stronger one to one litre. One c.c. of this will equal 1 milligram of silver, or 0.1 c.c. of the stronger solution. A _standard solution of silver_ equivalent to the dilute salt solution is made by dissolving 1 gram of fine silver in 10 c.c. of dilute nitric acid, and diluting with water to one litre. [Illustration: FIG. 44.] The solution of salt is standardised as follows:--Weigh up 1.003 gram of fine silver and dissolve in 25 c.c. of dilute nitric acid in a bottle provided with a well-fitting flat-headed stopper. Heat on the water bath to assist solution, resting the bottle in an inclined position. When dissolved blow out the nitrous fumes with the help of a glass tube bent at right angles. Run in from a stoppered pipette (as shown in fig. 44) 100 c.c. of the standard salt solution, and shake vigorously until the solution clears. Fill an ordinary burette with the weaker standard salt solution, and run 1 c.c. into the assay bottle, letting it run down the side so that it forms a layer resting on the assay solution. If any silver remains in solution a cloudy layer will be formed at the junction where the two liquids meet. This is best observed against a black background If a cloudiness is seen, shake, to clear the liquid, and run in another c.c. of salt, and continue this until a cloudiness is no longer visible. Deduct 1.5 c.c. from the amount of the weaker sodium chloride solution run in. Divide the corrected reading by 10, and add to the 100 c.c. This will give the volume of strong salt solution equivalent to the silver taken. If the first addition of the weaker salt solution causes no cloudiness add 5 c.c. of the silver solution from an ordinary pipette, shake, and then run in the weaker salt solution, working as before. These 5 milligrams of silver added must be allowed for before calculating. As an example:--1.0100 gram of fine silver was taken for standardising a solution and 4 c.c. of the weaker salt solution were run in. Deducting 1.5 and dividing by 10 gives 0.25 c.c. to be added to the 100 c.c. 100.25 : 1.0100 :: 100 : _x_ _x_ = 1.0075 which is the standard of the salt solution.