ELEMENTARY EXPERIMENTS.] UNDULATORY FORCES. LIGHT PHOTOGRAPH Y. 



141 



derived from the acid, the nitric, and partly from the 

 base, the potass. Hence the chemist terms it the nitrate 

 of potass ; and by this system of naming salts, he in- 

 stantly sees, by the name, what the salt is composed of. 

 Our common table-salt is called, in chemical language, 

 chloride of sodium, because it is composed of a gas called 

 chlorine, which produces the smell of bleaching powder, 

 with a metal called sodium, found in soda, itc. The 

 reader having mastered this system, will at once see its 

 great value ; and although perhaps experiencing at first 

 some difficulty in acquiring names of salts, he will 

 speedily reap the advantage of the general nomenclature. 



To save repetition of terms, we shall, in future, call a 

 solution of the salts of silver, the silver solution ; and solu- 

 tion of common salt, the talt solution ; and for our intro- 

 ductory experiments, these may be made as follows : 



To make the " Silver Solution." Dissolve fifty grains of 

 crystallised nitrate of silver in one ounce of distilled water. 



To make the "Salt Solution." Dissolve sixty grains 

 of common salt in four ounces of clean water. 



It will be observed that we have mentioned, that 

 led water must be employed to dissolve the nitrate 

 of silver. This is because river and spring water contain 

 common salt. Distilled water may be procured of the 

 chemist and druggist, or may be made by affixing a long 

 pewter tube to the spout of an ordinary tea-kettle. As 

 the steam passes from the kettle it will be condensed in 

 the tube, if the latter be kept cold by means of wet rags, 

 or by being immersed in cold water. The distilled water 

 may be received in clean glass vessels, which should be 

 kept stoppered and free from dust. 



nil still may easily be constructed by an intelligent 

 smith ; and such as is represented in Fig. 141 is a con- 

 venient form. 



Fig. 141 



A i the body of the "till, into which the water to be distilled Is Introduced. 

 Round thiii vessel the fire is placed. B is the head of the ittill, through 

 which the rapour puiws by meaiu of c, until it arrives at d. ti in the 

 worm, surrounded by cold water. In thU part the uteam U condensed, 

 and escape*, aa distilled water, at the tap j, where it may be collected 

 In bottles, which should be kept carefully corked. 



The solution of the salts of silver should always be 

 kept in a glass-stoppered bottle, and surrounded with a 

 piece of black paper, or kept in a dark place, so as to 

 prevent the action of light. 



Experiment 2. Pour some of the salt solution into a 

 wine-glass, and then add a few drops of the silver 

 solution. A heavy, curdy, white substance, or precipi 

 will fall down, which is the chloride of silver. The 

 chemical change which takes place is illustrated by the 

 subjoined diagram ; wherein the materials are named on 

 the left, ami the products on the right side. 



Materials. Products. 



(Chlorine^ 

 Sodi^ 



Common 

 Bait. 



Nitrate of Soda. 



f Nitric Acid 



Mitrate 

 raw. 



Water 



v ore o ver. 



Tin- result of the mixture is, that the chlorine and 



silver unite together to form the chloride of that metal ; 

 and the nitric acid, sodium, and water form the salt, 

 nitrate of soda, which is dissolved in the liquid. We 

 have thus an instance of chemical decomposition, and the 

 formation of new compounds. 



Experiment 3. Divide the white powder obtained in 

 the last experiment into two portions, and keep one part 

 in a dark place, and expose the other to the rays of the 

 sun. In a short time, the portion left in the light will 

 turn to a dark mulberry tint, owing to the action of the 

 chemical rays on the salt ; whilst that kept in a dark 

 place will remain entirely unchanged. 



By these experiments the student will learn the ele- 

 mentary principles of the art of photography ; and may 

 now proceed to the next step, that of preparing paper 

 wliich will be- sensitive to the action of light. 



Experiment 4. To prepare Sensitive Paper. Pour 

 the salt solution into a clean plate, and place on its 

 surface a piece of white writing-paper, leaving it there 

 for some time, so that the solution may enter the pores of 

 the paper. The salted side should then be marked with 

 a black-lead pencil, so as to distinguish it from that wliich 

 is unsalted. The paper may then be hung up to dry. 

 The next process is to render the paper sensitive to the 

 action of light ; and is effected by placing the salted side, 

 face downwards, on the silver solution contained in a 

 clean plate, and leaving it there for about ten minutes. 

 The paper is then to be dried in a place to wliich light 

 has no access. This part of the preparation of the paper 

 should be done by candlelight ; and several sheets may 

 thus be made and kept, till required, in a portfolio or book. 



Paper thus prepared will at once change its colour on 

 being exposed to diffuse daylight, or in the sun's rays ; 

 and if any opaque object be placed over it, a complete 

 copy of its form may be procured. If a penny-piece, for 

 instance, be placed on a sheet of this paper, and exposed 

 to sunlight, all the uncovered portion will be turned to 

 nearly a black colour; whilst that under the coin will 

 retain a white and unchanged appearance, because 

 there no light has reached, and therefore no chennVal 

 effect could take place. On removing the coin, the white 

 I>art will become changed, if exposed to the light ; and 

 this introduces us to another process namely, "that of 

 " fixing" the picture obtained as above. 



The fixing process is intended to dissolve away that 

 portion of the silver salt which h;is not been acted on by 

 the light; and it will thus prevent a picture, when 

 obtained, from being spoilt or lost. The solution ^ 

 for this purpose is that of a salt called hyposulphite of 

 sodium, an ounce of which may be dissolved in four 

 ounces of water. The picture is to be left on this 

 solution, face downwards, for a short time, and afterwards 

 well washed in abundance of cold water. After being 

 dried, it may be exposed, without any danger of under- 

 going further change; in fact, it becomes permanent and 

 unalterable. If, instead of a coin, a piece of lace, a fern- 

 loaf, or other such object be placed on the paper, and 

 pressed thereon by means of a piece of plate-glass, a 

 complete copy of it may be taken : a child's transparent 

 slate is a very convenient arrangement for this purpose ; 

 but the ground-glass plate must be replaced by a piece of 

 window or plate-glass, so that the light may freely i 

 through. By this simple plan a vast number of objects 

 may be copied ; the solid parts being represented by 

 white spaces, and the transparent portions by dark marks 

 on the paper. This process is so exceedingly easy, that any 

 young person may readily follow the directions we havo 

 given. Wo may here remark on the advantages which ele- 

 mentary sciei.oe. aifords in training and disciplining the 

 young mind. The ordinary course of juvenile instruction, 

 has the twofold object of teaching facts, and preparing 

 the mental powers for future and more exact duties. If, 

 however, we can find any means by which amusement, 

 interest, ami instruction can be combined, such become 

 a most valuable adjunct to an educational course. 



We have now to notice that there are two classes of 

 photographs ; namely, negative and positive. By the 

 former, the dark portions of the object arc represented 

 by light parts in the picture ; but in a positive, th^ 



