The paper for the black prints by the hot bath process is washed with a mixture of potassic platinous chloride and ferric oxalate, the proportion being about sixty grains of the platinum salt to one ounce of the iron solution.  It will not keep good longer than twenty minutes or so, and must be applied to the paper directly after mixing.  The ferric oxalate in the paper is reduced by the action of light to ferrous oxalate, which forms the faint visible image; this, when the paper is floated on the oxalate of potash bath, is capable of reducing the platinum salt in contact with it into metallic platinum; but the ferric salt, which remains unaltered, has no action on the platinum salt, leaving these parts, which represent the high lights of the print, untouched.  The ferric oxalate is removed by the acid baths which follow the development.  A good temperature for development is 150 deg.  Fahr., and when using this so much detail should not be apparent as when printing for the cold bath process, in which all the detail desired should be very faintly visible.  There are, however, many methods of exposing the paper and developing it, and no fixed rule can be made, but the development must in every case be suited to the exposure or the result will be a failure.  For instance, the paper may be printed until all detail is visible, but a very much cooler development must be used, say 80 deg. or 90 deg.; on the other hand, a slightly short exposure may be given, and a temperature of 180 deg. to 200 deg. used. 150 deg. should be taken as the normal temperature, and kept to until some experience has been gained, as employing all temperatures will lead to confusion, and nothing will be learned.  Some negatives require a special treatment, and both printing and development must be altered, while for a very dense negative the paper may be left out in a dampish room for some time.  It will then print with less contrast and more half tone.  A thin negative is better printed by the cold bath process, but negatives should be good and brilliant for platinotype printing.  Any one taking up platinotype and getting only weak prints would do well to look to his negatives instead of blaming the paper, as the high lights should be fairly dense, and the deep shadows nearly clear glass.

Time for complete development should always be allowed; with a hot bath fifteen seconds will be sufficient, but if a cooler development is used, or the prints are solarized in the shadows, more time should be allowed.  When the deep shadows are solarized, or appear lighter than surrounding parts, a hot and prolonged development is required to obtain sufficient blackness, as they have a tendency to look like brown paper.  I have found breathing on solarized shadows useful, as in the presence of slight moisture they begin to print out and become dark before development, getting black almost directly the print is floated on the oxalate.  Three or four acid baths of about ten minutes each are used, and the prints are washed as before.  The process throughout takes much less time than silver printing, and can be kept on all the winter, when it is nearly impossible to print in silver.  Prints can be developed in weak daylight or gaslight, and prolonged washing is dispensed with.—­N.P.  Fox, reported in Br.  Jour. of Photo.