(a) Copper and its alloys only will receive a satisfactory platinum deposit; iron, zinc, lead, or tin, coated with this metal, even after a previous coppering, give but defective results. The platinum deposits are obtained by dipping thoroughly cleansed copper articles in the following solution, kept boiling: - Distilled water, 100 parts by weight; caustic soda, 12; platinum for neutral chloride, 1. The deposit is bright, durable, and of a dark colour, resembling oxidised silver.

(6) The platinum baths for electro-deposits will succeed when platinum chloride is dissolved in a solution of a salt with alkaline, neutral, or acid reaction, but sulphites and cyanides, even those having soda for base, should be excepted. Distilled water, 100 parts by weight; soda carbonate, 40; platinum for neutral chloride, 1. Temperature of the bath, 160°-180° F. Distilled water, 100 parts by weight; soda phosphate or borate, 60; platinum for neutral chloride, 1. Distilled water, 100 parts by weight; soda pyrophosphate, or chloride or iodide, 30; platinum for dry chloride, 1. These baths only give exceedingly thin deposits; if the coating were allowed to increase most of it would be without adherence, and often in the form of scales. The deposit is black or steel grey.

(c) Fill a glass flask with 1/3 oz. finely-laminated spongy or black platinum, and a mixture of 5 1/2 oz. hydrochloric acid and 3 1/2 oz. nitric acid at 40° B. Place the flask upon a piece of sheet iron perforated in the centre, so that the bottom of the flask alone receives the heat. After an abundant production of orange-yellow fumes, the platinum will disappear and leave a red liquid, which should be heated until it becomes viscous enough to stick against the sides of the flask. This latter part of the operation may be effected in a porcelain dish, the shallow form of which aids in the evaporation of the acids in excess. After cooling, the residuum is dissolved in 17 1/2 oz. distilled water, and filtered if necessary. Dissolve 3 1/2 oz. ammonia phosphate in 17 1/2 oz. distilled water, and mix the two solutions. This produces a precipitate of ammonia and platinum phosphate in a liquid of orange colour, which should not be separated; pour into it, stirring all the while, another solution of 17 1/2 oz. soda phosphate in 1 3/4 pint distilled water. Boil the mixture, and replace the evaporated water, until no more ammonia is disengaged, which is ascertained by the smell; and until the liquor, which was previously alkaline, begins to redden blue litmus paper.

When the yellow liquor becomes colourless, it indicates the formation of a double platinum salt. The bath is then ready to deposit platinum upon articles of copper or its alloys, by the aid of heat and of an intense electric current. Copper coated with platinum resists nitric and sulphuric acids to a considerable extent. If iron, zinc, lead, or tin come in contact with the bath they will decompose it, and the metal deposited will be black. The dead lustre of platinum is pearl-grey; it is very hard, and cannot be brightened by scratch-brushes of brass, which render its surface yellow; powdered pumice or iron wires should be employed. Platinum deposits may be burnished by an energetic friction, and the lustre obtained is very durable. Platinum may be removed from copper by a very long immersion in the liquors given for ungilding, but the success is doubtful.

(d) Iron, tin, tin-plate, and brass may be easily platinised as follows: - Pour on 1 part platinum chloride 1 part water; acidulate with a little hydrochloric acid, acid add 20 parts alcohol. Evaporate to 15 parts, and add 60-90 parts ether. Moisten a rag with this solution, and rub vigorously on the clean-scoured or polished metal. Heat to 140° F., and polish by rubbing with a woollen rag.

(e) A solution of 1 part platinum chloride in 15 of alcohol and 50 of ether is rubbed over the surfaces to be platinised, the vessels being put away in a dry warm place. The surface, on being rubbed with a cotton, or, still better, a woollen rag, yields a high polish, having the appearance and lustre of steel. - (Dr. Hager.)

(/) The following method for the electro-deposition of the heavy metals, such as platinum, iridium, palladium, etc., has recently been proposed by Prof. Silvanus Thompson. The impure metal is first obtained as a chloride by the ordinary chemical processes. The excess of acid is evaporated off in a water bath, and the salt is finally redissolved in distilled water and 10-50 times its weight of a solution of sodium phosphate, either pure or mixed with borax. The solution is then raised to the boiling point, and sal ammoniac, common salt, or sodium bromide is added. The solution is then reheated, and finally neutralised with either soda carbonate or, if alkaline, with bicarbonate. In depositing the metal from a bath of the above solution, it should be heated to 140o-194° F., and the metal deposited in the ordinary way. In the case of platinum, a brilliant deposit can be obtained from a bath of the following composition: -

Parts.

Platinum chloride ..

2

Sodium borate

16

„ carbonate ..

16

Sal ammoniac

2

Water........

150

(g) The permanence and unalterabi-lity of the metal platinum - properties which make it of such inestimable value to the chemist - have likewise suggested its application as a protective covering upon the surfaces of other metals; and even in the early days of the art of electro-deposition efforts to obtain a satisfactory coating of this metal were made. The failure with which these early experiments were attended served rather to stimulate than to deter subsequent investigators, and the problem has received the attention of a number of the most noted experts in the art. The results that have been accomplished cannot be said to have been entirely satisfactory - a statement which will be fully sustained by the fact that electroplating with platinum, on the commercial scale, is practised only to a very limited extent. When the wide field of application for platinum plating is considered, the conclusion is irresistible that the processes thus far proposed for the purpose do not fully meet the requirements of practical service.