A negative may be converted into a positive by bleaching in the ordinary mercuric chloride intensifying solution, consisting of bichloride of mercury (or corrosive sublimate, a dangerous poison) 100gr., chloride of ammonium 20 gr., water 2oz., but the results are not satisfactory. An old process, known as the ababas-trine process, has also been used, but as it depends on the action of chloride of mercury it cannot be con adered successful. The formula, however, is as follows. Dissolve 40 gr. bichloride of mercury in 2oz. water, and add 20 gr. sodium chloride (common salt) and ldr. hydrochloric acid. Either of the above formulae may be used, the negative being soaked until thoroughly bleached, then well washed and, when dry, coated with any opaque black varnish. Unless the film is thoroughly freed from hypo before bleaching, the negative will be stained, or it may not bleach at all, remaining a dirty brown colour. The staining or the refusal to bleach ocurs because the chloride of silver that is formed in the film is immediately attacked by the unremoved hypo, which is very weak.