144 



I NUULATORY FORCES. LIGHT PHOTOGRAPHY [PHOTOGRAPHIC CHEMISTRY. 



cause* them to become unsteady, and makes them utt.-i ly 

 valueless. 



Amongst the minor, but essential apparatus of the 

 photographer, are the following. 



fig. 149 represent* the dipping-bath, and frame for 

 hoMing the pUto in the silver solution contained in the 

 L.i-.li. 



Flf . H9. FU. 150 



Fig. 150 is a representation of a copying frame, by 

 means of which positive copies are obtained from nega- 

 tive pictures. It is an improvement on the simple ar- 

 rangement of the clu'ld's transparent slate, to which, 

 and its use, we have previously alluded. * 



We hare thus referred to some of the most essential 

 arrangements required by the reader when commencing 

 the study of photography. We shall allude to, and 

 illustrate, various contrivances which have been invented 

 for the purpose of facilitating several operations, as we 

 proceed. 



In our humble opinion, it is better that every student 

 cf science, whatever his means may be, should com- 

 mence with as few "assistants" as is consistent with his 

 progress. He is thus thrown on his own resources ; he 

 sees his wants ; his ingenuity is stimulated to supply 

 them ; and his progress is infinite!} more satisfactory to 

 himself, and advantageous to others in every department 

 of scientific research, than when he is helped over every 

 little obstacle that may drop across his path. Both 

 morally and mentally, "he that chiefly owes himself 

 unto himself, is the substantial man." 



Besides the different apparatus we have described, the 

 tyro will require some glass-stoppered bottles of about 

 four ounces capacity ; square dishes of glass or porcelain 

 for holding developing solutions, &c. ; two or three 

 graduated glass measures, of two ounce, one ounce, and 

 one drachm capacity ; some glass rods for stirrers ; and a 

 dozen or so of test-glasses, which, however, may be 

 replaced by ordinary uncut wine or ale-glasses. He 

 should also possess a spirit-lamp, of four or eight ounce 

 size, and a small ring-stand, for supporting glass vessels 

 in preparing solutions by heat ; some glass funnels for 

 filtering solutions, <tc. ; some filtering papers, or, what is 

 nearly as good, some stout white blotting-paper; and 

 last, and by no means the least of his requirements, is 

 that of an unlimited supply of clean, soft towels, and of 

 rain or soft river water. The quality of ordinary water 

 differs very much, according to the source whence it is 

 obtained : when hard, distilled water should always be 

 used. 



We have mentioned that distilled water is essential in 

 n. 151. many of the ope- 



rations of photo-, 

 graphy ; and we 

 refer our readers 

 to the remarks 

 made in reference 

 to this subject, t 

 When required in 

 but small quanti- 

 ties, it can be made 

 by using a pint- 

 glass retort, tmp- 

 ported on a ring- 

 stand. A spirit- 

 lamp may be employed as a source of heat, ; and the 

 stem of thr ntr.it should bo kept cool by mean* of 

 wet cloths. (See Fig. 151). 



fee ant,, p. 141. t Ant,, p. 141. 



PHOTOGRAPHIC CHEMlsiKV. 



IN the ordinary arrangement of a scientific work, it is 

 almost a question of choice as to whether photography 

 might be considered an application of chemical or of 

 optical science. As wo have adopted the latter idea, we 

 shall y have to introduce our readers to so 



much of the science of chrmistry as will cn.il.h- them to 

 understand the rationale of the processes in which they 

 must engage. This will by no means prove any hin- 

 drance; indeed, had the student to forage out, on his 

 own account, such a knowledge of chemistry as he will 

 require, wo imagine that he would thus effect a much 

 larger expenditure of time and means, and would, most 

 likely, obtain a much less satisfactory result, than if ho 

 simply studied and followed the excerpta and directions 

 of those who are more fitted by experience to point out 

 and prepare the way for him. 



With this end in view, we shall now direct briefly the 

 reader's attention to some of the leading applications of 

 chemistry, so far as they are required in the photogra- 

 phic art ; referring him, however, to the remarks which 

 we have already made on this subject in our introduc- 

 tory experiments. J 



To one or more of three substances (chlorine, iodine, 

 and bromine) the photographer is indebted for his most 

 successful results ; and the student may become ac- 

 quainted with their specific characters by trying the 

 following experiments : 



Experiment 6. Chlorine may be obtained in various 

 ways. It is a constituent of common salt, <tc. Place a 

 teaspoonful of chloride of lime (bleaching-powdur) in a 

 glass bottle or jar; pour on the powder a little hydro- 

 chloric (muriatic) acid and water ; in a little time, gas 

 will be given off, and will gradually fill the jar. It has 

 a greenish-yellow colour ; hence the name, chlorine, from 

 the Greek. Its smell is very penetrating ; and if a 

 piece of paper, coloured by litmus or any other vegetable, 

 be dipped therein, it will bo immediately bleached. 

 Hence the employment of chloride of lime in the manu- 

 facturing districts, for bleaching cotton and linen goods. 



I'.speriment 6. Iodine. This substance is obtained 

 from sea-water, and was first discovered by its action on 

 the pans in which salt was evaporated. In its ordinary 

 state, it is a dark-coloured powder. Heat a little of this 

 on a plate, and observe the colour of the vapour, which 

 will be of a rich purple ; and thence has arisen the name 

 of the substance. Combined with potassium or potass, 

 it forms the iodide of potassium, or hydriodate of potass ; 

 names which are used for the same salt. Iodide of 

 potassium is largely employed for a variety of purposes 

 in photography. 



Experiment 7. Bromine. This is always sold in a 

 liquid state ; and, like iodine, is obtained from sea- 

 water. It has a deep orange colour, and most offensive 

 smell. Expose a little of the liquid bromine to heat; 

 its characteristic vapour will be at once noticed : the 

 fumes, however, which are extremely irritating, should 

 be avoided. Bromide of potassium that is, bromine 

 and potassium united together is the combination most 

 used. l:romine and iodine are used, together with 

 collodion, in the liquid state for the collodion process; 

 whilst, as vapour, they are employed for making the 

 silver plates of the Daguerreotype process sensitive, 

 before being placed in the camera. 



These three substances, chlorine, iodine, and bromine, 

 have been termed elementary bodies, because they have 

 resisted all attempts at decomposition; that is, by the 

 exercise of all our knowledge of chemical science, wo 

 have been unable to find any other substance in them 

 different to their own nature or characters. This fact 

 leads us to remark, that the union of such elementary 

 bodies with others, forms a class called compounds. Of 

 such are common salt, composed of chlorine and sodium, 

 and the iodide and bromide of potassium, which we 

 have just referred to in our experiments. 



The most essential element of photographic art, is the 



t SM ante, p. 140, rt irq. 



