white, oil of poppy is the best.  The oil in this composition, being dissolved by the lime, wholly disappears; and, uniting with the whole of the other ingredients, forms a kind of calcareous soap.  In putting in the Spanish white, you must be careful that it is finely powdered and strewed gently over the surface of the mixture.  It then, by degrees, imbibes the liquid and sinks to the bottom.  Milk skimmed in summer is often found to be curdled; but this is of no consequence in the present preparation, as its combining with the lime soon restores it to its fluid state.  But it must on no account be sour; because, in that case, it would, by uniting with the lime, form an earthy salt, which could not resist any degree of dampness in the air.  Milk paint may likewise be used for out-door objects by adding to the ingredients before-mentioned 2 ozs. each more of oil and slaked lime, and 2 ozs. of Burgundy pitch.  The pitch should be put into the oil that is to be added to the milk and lime, and dissolved by a gentle heat.  In cold weather, the milk and lime must be warmed, to prevent the pitch from cooling too suddenly, and to enable it to unite more readily with the milk and lime.  Time only can prove how far this mode of painting is to be compared, for durability, with that in oil; for the shrinking to which coatings of paint are subject depends in great measure upon the nature and seasoning of the wood.  The milk paint used for in-door work dries in about an hour; and the oil which is employed in preparing it entirely loses its smell in the soapy state to which it is reduced by its union with the lime.  One coating will be sufficient for places that are already covered with any colour, unless the latter penetrate through it and produce spots.  One coat will likewise suffice, in general, for ceilings and stair-cases; two will be necessary for new wood.  Milk painting may be coloured, like every other in distemper, by means of the different colouring substances employed in common painting.  The quantity I have given in the receipt will be sufficient for one coat to a surface of about twenty-five square yards.

536.  Ethereal solution of gold

The following mode of effecting this solution (used chiefly for gilding steel) is recommended by Mr. H. Mill, in the “Technical Repository,” as being superior to any previously made known.  “The instructions,” he says, “given in most elementary works on chemistry for this purpose are either erroneous or not sufficiently explicit.”  The process answers equally well for either gold or platina.  Dissolve any quantity of gold or platina in nitro-muriatic acid, (aqua regia,) until no further effervescence is occasioned by the application of heat.  Evaporate the solution of gold or platina, thus formed, to dryness, in a gentle heat, (it will then be freed from all excess of acid, which is essential,) and re-dissolve the dry mass in as little water as possible:  next take an instrument which is used