140 



UNDULATORY FORCES. LIGHT PHOTOGRAPHY. [TUB COLLODION PROCESS. 



of household ufte ; but for the purposes of the photo- 

 grapher, a pure and strong kind is required. It La 

 generally sold by the photographic chemists, under the 

 name of glacial acetic acid, which is its strongest state, 

 and which will assume the solid form when at a tempe- 

 rature near the freezing-point of water. 



Gallic acid is produced by the decomposition of gall- 

 nuts, which are freely exposed for a few weeks to the 

 action of the air and water. Pyrogallic acid is obtained 

 by the action of heat on gallic acid ; and Is largely used 

 iii the collodion process ; whilst gallic acid is equally so 

 for the calotrpe. 



Nitric acid is employed for various purposes as a pho- 

 tographic agent. Its usual source is nitre, whence it is 

 obtained by distilling that salt with sulphuric acid and 

 water. 



Protosulphate of iron is well known under the name 

 of green copperas, although not a panicle of copper 

 is into its composition. It is composed of iron and 

 sulphuric acid, and may be obtained by adding these 

 together. In a pure state it is sold in the form of rich 

 green crystals, and is much employed for develoj Mix- 

 solution. Protonitrate of iron, which is also similarly 

 used, is composed of iron and nitric acid, and may be 

 made by mixing together a solution of sulphate of iron, 

 and one of nitrate of baryta. For making small quanti- 

 ties of this salt in solution, 100 grains of each salt should 

 be dissolved in water in separate vessels. On the two 

 solutions being mixed, a white powder, the sulphate of 

 barytes, will fall down ; and being filtered from this, the 

 solution will contain the protonitrate of iron. 



Having thus disposed of those acids and salts mostly 

 employed in the collodion process, we conclude by re- 

 ferring to two substances used for the purpose of 

 fixing the picture, or, in other words, to remove the 

 unchanged salts of silver, and so to prevent any further 

 chemical change being produced on its subsequent ex- 



Cure to light. Hyposulphite of soda is largely prepared 

 photographic "fixing," and is accordingly a cheap and 

 easily procured salt. The strength of solution required 

 will be mentioned when we further refer to its use. It 

 is obtained by transmitting sulphurous acid through a 

 solution of carbonate of soda. Sulphurous acid is a 

 product of the combustion of sulphur, and may be 

 abundantly obtained by heating sulphuric acid with the 

 metal mercury in a glass retort. 



Cyanide of Potassium. This salt has lately come into 

 great use as a fixing agent, and is also largely employed 

 in electro-plating and gilding. It is generally procured 

 by heating together, in fine powder, one part of dry 

 carbonate of potass with two parts of ferrocyanide of 

 potassium (yellow prussiate of potass) : a white deliques- 

 cent powder is the result. We may here warn the 

 student, that this salt is a most deadly poison, owing to 

 the presence in it of cyanogen (the base of prussic acid) ; 

 and he will act wisely in avoiding the smell which arises 

 from its solution: he should also be cautious in immersing 

 the hands, especially if they are at all wounded, or the 

 skin abraded. This salt should be kept in close-stoppered 

 bottles, as it readily absorbs moisture from the air. 



We have thus endeavoured to give a general idea of 

 the nature and uses of those substances employed as 

 rli'-mical agents ; and as the collodion process is one 

 which is easily managed by a little care and patience, we 

 shall commence our directions in practical photography 

 therewith, reserving any further remarks, both on appa- 

 ratus, chemicals, and other matters, to such opportu- 

 nities as may more pertinently be presented as we pro- 

 ceed through the different branches of our subject. 



THE COLLODION PROCESS. 



IN many existing treatises on this and other branches of 

 the art, it has been the custom to introduce a vast 

 variety of processes, which have been from time to time 

 altered, improved, or modified by their authors. In 

 our humble opinion, this course, whilst extremely 

 valuable to those who have somewhat advanced, is of a 

 most perplexing character to the student who may be 



i- his attempts. We shall, therefore, employ 



thu same method as we have adopted in all our 

 previous pages, endeavouring to place before our readers 

 plain and successive directions, that they may, in 

 obedience thereto, gain a satisfactory conclusion. To 

 avoid distraction of our arrangement, we shall omit MH n- 

 tioniug the names of those individuals to whom photo- 

 graphy is indebted for its advance ; earnestly Loping, that 

 if such peruse our pages, they will, with us, share the 

 desire in which we write them that of spreading, by all 

 possible means, the elements and principles of know- 

 ledge to as great an extent as possible, in a disinterested 

 and truly philosophic spirit. We shall give all the 

 hints and practical details which our own experiments 

 will allow ; and only hope that our readers may reap any 

 or every advantage from Uie facts which we have 

 gathered from numerous sources. 



Assuming that the student has provided himself with 

 all the apparatus, chemicals, <tc., which we have already 

 mentioned, we shall suppose that, under our direction, 

 he is about to take the portrait of a friend, or a positive 

 picture of any stationary object. We commence with 

 positive pictures, because, to the beginner, they afford 

 immediate fruit of his early endeavours. 



TO TAKE A POSITIVE PICTURE. 



OUR process naturally divides itself into the following 

 operations ; namely 



1. Cleaning the glass plate. 



2. Coating it with collodion. 



3. Rendering it sensitive. 



4. Exposing it in the camera. 



5. Developing the latent picture. 



6. Fixing the picture. 



The above have been arranged in the order in which 

 they succeed each other; and neglect in carrying out 

 each process individually, will, without the shadow of a 

 doubt, bring inevitable failure as its result : whilst, if 

 conducted in a quiet, persevering, and cleanly manner, 

 success may most justly be expected to reward the 

 operator's attempts. The glass plates are sold, of various 

 sizes, by the instrument-makers ; and the material should 

 have a high polish be perfectly free from specks or 

 blemishes of any kind. For this reason common window- 

 glass is rarely good enough ; and a better kind, manu- 

 factured expressly for the purpose, must therefore be 

 purchased. 



1. To Clean the Glass Plate. The methods proposed 

 for this purpose are endless in number. We, however, 

 find the following to succeed as well as any we have 

 tried. Place the plate on a soft linen towel, and alter- 

 nately rub each side till all free dust and dirt are re- 

 moved. Then, by means of a little soft cotton wool, 

 wash the two sides with a mixture of spirits of wine and 

 liquid ammonia, in equal quantities ; this will remove all 

 extraneous solid matter : afterwards wash with distilled 

 water. The plate being dried, must then be held by 

 means of a finger and thumb placed at opposite corners, 

 over a spirit-lamp, until the surface of the glass presents 

 a brilliant polished appearance, and the whole becomes 

 very hot: this removes a film of air which rests on 

 every polished surface. We have never seen this plan 

 recommended by any photographer ; but have no doubt 

 that in many cases it will prove a remedy for a common 

 evil that of the non-adhesion of the collodion film. 

 We were first led to try it from long practice with 

 Grove's voltaic batteries. The platina in such ar- 

 rangements is always coated with a film of air ; and 

 unless this is removed, the nitric acid never comes in 

 contact therewith. Having observed the effect of re- 

 moving this aeriform coat in increasing the battery 

 power, we tried it on the glass plate for photographic 

 purposes, and have found, with many whom we have 

 induced to try it, great advantage therefrom. After 

 thus heating the plate, place it on its edges, so that it 

 may rest at an angle against any support; and do not 

 attempt to use it till quite cool, but do so immediately 

 after it has regained the temperature 01 the surrounding 



