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dry dip it into a solution of iodide of potassium； containing five hundred grains of that salt dissolved in a pint of water。 When perfectly saturated with this solution； it should be washed in distilled water； drained and allowed to dry。 This is the first part of the process； and the paper so prepared is called iodized paper。 It should be kept in a port…folio or drawer until required： with this care it may be preserved for any length of time without spoiling or undergoing any change。
Mr。 Cundell finds a stronger solution of nitrate of silver preferable； and employs thirty grains to the ounce of distilled water： he also adds fifty grains of common salt to the iodide of potassium； which he applies to the marked side of the paper only。 This is the first process。
Preparation of the paper for the Camera。The second process consists in applying to the above a solution which has been named by Mr。 Talbot the 〃Gallo…Nitrate of Silver；〃 it is prepared in the following manner： Dissolve one hundred grains of crystalized nitrate of silver in two ounces of distilled water； to which is added two and two…third drachms of strong acetic acid。 This solution should be kept in a bottle carefully excluded from the light。 Now； make a solution of gallic acid in cold distilled water： the quantity dissolved is very small。 When it is required to take a picture； the two liquids above described should be mixed together in equal quantities； but as it speedily undergoe decomposition； and will not keep good for many minutes； only just sufficient for the time should be prepared； and that used without delay。 It is also well not to make much of the gallic acid solution； as it will not keep for more than a few days without spoiling。 A sheet of the iodized paper should be washed over with a brush with this mixed solution； care being taken that it be applied to the marked side。 This operation must be performed by candle light。 Let the paper rest half a minute； then dip it into one of the dishes of water； passing it beneath the surface several times； it is now allowed to drain； and dried by placing its marked side upwards； on the drying apparatus。 It is better not to touch the surface with bibulous paper。 It is now highly sensitive； and ready to receive the impression。 In practice it is found better and more economical not to mix the nitrate of silver and gallic acid； but only to brush the paper with the solution of the nitrate。
Mr。 Talbot has recently proposed some modifications in his method of preparing the calotype paper。 The paper is first iodized in the usual way； it is then washed over with a saturated solution of gallic acid in distilled water and dried。 Thus prepared he calls it the io…gallic paper： it will remain good for a considerable time if kept in a press or portfolio。 When required for use； it is washed with a solution of nitrate of silver （fifty grains to the ounce of distilled water）； and it is then fit for the camera。
Exposure in the Camera。The calotype paper thus prepared possesses a very high degree of sensibility when exposed to light； and we are thus provided with a medium by which； with the aid of the photogenic camera； we may effectually copy views from nature； figures； buildings； and even take portraits from the shadows thrown on the paper by the living face。 The paper may be used somewhat damp。 The best plan for fixing it in the camera is to place it between a piece of plate glass and some other material with a flat surface； as a piece of smooth slate or an iron plate； which latter； if made warm； renders the paper more sensitive； and consequently the picture is obtained more rapidly。
Time of Exposure。With regard to the time which should be allowed for the paper to remain in the camera； no direct rules can be laid down； this will depend altogether upon the nature of the object to be copied； and the light which prevails。 All that can be said is； that the time necessary for forming a good picture varies from thirty seconds to five minutes； and it will be naturally the first object of the operator to gain by experience this important knowledge。
Bringing Out the Picture。The paper when taken from the camera； which should be done so as to exclude every ray of lightand here the dark slide of the camera plate holder becomes of great use bears no resemblance to the picture which in reality is formed。 The impression is latent and invisible； and its existence would not be suspected by any one not acquainted with the process by previous experiment。 The method of bringing out the image is very simple。 It consists in washing the paper with the gallo…nitrate of silver； prepared in the way already described； and then warming it gently； being careful at the same time not to let any portion become perfectly dry。 In a few seconds the part of the paper upon which the light has acted will begin to darken； and finally grow entirely black； while the other parts retain their original color。 Even a weak impression may be brought out by again washing the paper in the gallo…nitrate； and once more gently warming it。 When the paper is quite black； as is generally the case； it is a highly curious and beautiful phenomenon to witness the commencement of the picture； first tracing out the stronger outlines； and then gradually filling up all the numerous and complicated details。 The artist should watch the picture as it developes itself； and when in his judgment it has attained the greatest degree of strength and clearness； he shall stop further proceedings by washing it with the fixing liquid。 Here again the mixed solution need not be used； but the picture simply brushed over with the gallic acid。
The Fixing Process。In order to fix the picture thus obtained； first dip it into water； then partly dry it with bibulous paper； and wash it with a solution of bromide of potassiumcontaining one hundred grains of that salt dissolved in eight or ten ounces of distilled water。 The picture is again washed with distilled water； and then finally dried。 Instead of bromide of potassium； a solution of hyposulphite of soda； as before directed； may be used with equal advantage。
The original calotype picture； like the photographic one described in the last chapter； is negative； that is to say； it has its lights and shades reversed； giving the whole an appearance not conformable to nature。 But it is easy from this picture to obtain another which shall be conformable to nature； viz。； in which the lights shall be represented by lights； and the shades by shades。 It is only necessary to take a sheet of photographic paper （the bromide paper is the best）； and place it in contact with a calotype picture previously rendered transparent by wax or oil as before directed。 Fix it in the frame； Fig。 29； expose it in the sunshine for a short time； and an image or copy will be formed on the photogenic paper。 The calotype paper itself may be used to take the second； or positive； picture； but this Mr。 Talbot does not recommend； for although it takes a much longer time to take a copy on the photogenic paper； yet the tints of such copy are generally more harmonious and agreeable。 After a calotype picture has furnished a number of copies it sometimes grows faint； and the subsequent copies are inferior。 This may be prevented by means of a process which revives the strength of the calotype pictures。 In order to do this； it is only nesessary to wash them by candlelight with gallo…nitrate of silver； and then warm them。 This causes all the shades of the picture to darken considerably； while the white parts are unaffected。 After this the picture is of course to be fixed a second time。 It will then yield a second series of copies； and； in this way； a great number may frequently be made。
The calotype pictures when prepared as we have stated； possess a yellowish tint； which impedes the process of taking copies from them。 In order to remedy this defect； Mr。 Talbot has devised the following method。 The calotype picture is plunged into a solution consisting of hyposulphite of soda dissolved in about ten times its weight of water； and heated nearly to the boiling point。 The picture should remain in about ten minutes； it must then be removed； washed and dried。 By this process the picture is rendered more transparent； and its lights become whiter。 It is also rendered exceedingly permanent。 After this process the picture may be waxed； and thus its transparency increased。 This process is applicable to all photographic papers prepared with solutions of silver。
Having thus fully； and it is hoped clearly； considered the process； it may be necessary before dismissing the calotype from notice； to add one or two remarks from the observations and labors of some who have experimented in this art。 Dr。 Ryan in his lectures before the Royal Polytechnic Institution； has observed； that in the iodizing process the sensitiveness of the paper is materially injured by keeping it too long in the solution of iodide of potassium； owing to the newly formed iodide of silver being so exceedingly solvable in excess of iodide of potassium as in a few minutes to be completely removed。 The paper should he dipped in the solution and instantly removed。 There is another point； too； in the preparation of the iodized paper in