NATURAL PHILOSOPHY. 193 



producing a further number of impressions. This recoating operation 

 may be repeated as many times as may be required. The following is 

 the description of the whole process, which is divided into two parts, 

 consisting of a preparatory and finishing process : 



Preparatory engraving. For this operation, which is the most deli- 

 cate, it is necessary to have, 1. A saturated solution of caustic pot- 

 ash. 2. Pure nitric acid at 36 of the areometer of Beaume (spec, 

 grav. 1.33). 3. A solution of nitrate of potassa, composed of 300 

 parts of water and 5 parts of nitrate by weight. 4. A solution of 

 common salt, composed of water 100 parts, and salt 10 parts, by 

 weight. 5. A weak solution of ammoniacal chloride of silver, with an 

 excess of ammonia. The ammoniacal chloride of silver must be dilu- 

 ted with 15 or 20 parts of pure water. In the description of the pro- 

 cess this solution will be called ammoniacal chloride of silver. 6. A 

 weak solution of ammonia, containing four or five thousandths of 

 liquid ammonia. This solution will be called ammoniacal water. 7. A 

 weak solution of caustic potash, containing four or five thousandths of 

 the saturated solution, which will be called alkaline water. 8. A solu- 

 tion composed of water 4 parts, saturated solution of potash 2 parts, 

 alcohol 1 part, all in volume. This solution will be called alcoholized 

 potash. 8. Acidulated water, composed of water 100 parts, and nitric 

 acid 2 parts in volume. Besides, it is necessary to have two capsula, 

 or dishes, made of porcelain, large enough to contain the plate, and 

 covered with an air tight piece of ground plate glass, and two or three 

 more capsula which do not require to be covered ; two or three glass 

 funnels, to wash the plate ; and two or three glass holders in the shape 

 of a spoon or shovel, by which the plate is supported when put in and 

 taken out of the solution, without touching it with the fingers. 



The daguerreotype plate is submitted to the engraving process, after 

 having been washed in the hyposulphite of soda, and afterwards in 

 distilled water. 



First process for biting in or engraving the plate. The following 

 solutions must be put in the capsula in sufficient quantity, so as to en- 

 tirely cover the plate: 1. Acidulated water. 2. Alkaline water. 

 3. Alcoholized water in a covered capsula. 4. Caustic potash, in a 

 covered capsula. 5. Distilled water. 



The plate being put upon the glass holder or spoon, is plunged into 

 the acidulated water, and agitated during a few seconds, then put into 

 a glass tunnel, and washed with distilled water. It is taken again with 

 the glass spoon, and plunged in the capsula containing alcholized pot- 

 ash. This capsula is covered with a glass cover, and then heated by 

 means of a spirit lamp, to about 144 deoj. Fahrenheit. The plate must 

 remain in the capsula half an hour, during which the solution is heated 

 now and then, and agitated. During that time, the following acid so- 

 lution, which will be called normal acid, must be prepared; it is com- 

 posed as follows : Water 600 parts, nitric acid 45 parts, solution of 

 nitrate of potassa 12 parts, solution of common salt 45 parts. These 

 proportions are in volume. The normal acid must be poured in a cap- 

 sula, covered with its glass cover, and a sufficient quantity must be 

 kept in the bottle. 



