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saturated cold solution of bichloride mercury. Before a precipitate 

 has had time to form, the solution is brushed over paper, which 

 should have a yellowish rather than a blueish cast, and dried. This 

 paper keeps well, and when used is exposed to light, until a faint 

 but perfectly visible picture is impressed. It is then brushed over 

 as rapidly as possible with a saturated solution of prussiate of 

 potash, diluted with three times its bulk of gum water, so strong as 

 just to flow freely without adhesion to the lip of the vessel. The 

 wash must be spread with one application, evenly and very quickly, 

 over every part of the paper. It is fixed by drying. Beautiful posi- 

 tive pictures are thus produced, which will bear immediate exposure 

 tolerably well, but which after a few days will bear strong sunshine 

 uninjured. If the impression be overdone, the darker shades will 

 disappear : if too little, the whole runs into blot. The exact time 

 of exposure can only be learnt by practice. 



There are several other varieties of these processes, which are 

 not sufficiently important to be included here : the formula may be 

 seen by reference to Sir John Herschel's Papers in the ' Philoso- 

 phical Transactions/ The following process communicated by him 

 to the British Association in 1843, is, however, so curious, that we 

 are induced to insert it here. If nitrate of silver, specific gravity 

 1.200, be added to fei ro-tartarie acid, specific gravity 1.023, a 

 precipitate falls, which is in a great measure re-dissolved by a 

 gentle heat, leaving a black sediment, which, being cleared by 

 subsidence, a liquid of a pale yellow colour is obtained, in which 

 a further addition of the nitrate causes no turbidness. When the 

 total quantity of the nitrated solution added, amounts to about 

 half the bulk of the ferro-tartaric acid, it is enough. 



The liquid so prepared, does not alter by keeping in the dark. 

 Spread on paper and exposed wet to sunshine (partly shaded) for 

 a few seconds, no impression seems to have been made ; but by 

 degrees, although withdrawn from the light, it developes itself 

 spontaneously, and at length becomes very intense. But if the 

 paper be thoroughly dried in the dark (in which state it is of a very 

 pale greenish yellow colour,) it possesses the singular property of 

 receiving a dormant or invisible picture; to produce which (if it be 

 for instance an engraving that is to be copied) from thirty seconds 

 to a minute's exposure in the sunshine is requisite. It should not 

 be continued too long, as not only is the ultimate effect less striking, 

 but a picture begins to be visibly produced, which darkens spon- 

 taneously after it is withdrawn. But if the exposure be discon- 



