G02 



KNO\VLEDGE 



[May 19, 1882. 



rinso with rain or distilled wator, and then wipe dry with 

 a clean linen cloth. The substiinees that may be selected 

 for study are legion ; those iigunul are very suitable for 

 a first attempt ; make solutions by putting a pinch of 

 common salt, potassium nitrate (saltpetre), o.xalic acid and 

 potassium dichromate in .separate clean test-tul)e.s, and add 

 to each a tea-spoonful of water, they will dissolve rapidly 

 to clear solutions. Take a drop out of the common salt 

 test-tube on the end of a glass rod, and place it on a clean 

 slide, spread the drop out with the rod in as thin a layer 

 as possible ; warm the slide very gently over a lamp until 

 tlie salt begins to crystallize round the edge of the drop, 

 tlieu place it under the micro.scope and watch the progress 

 of crystallisation. Little cubes of salt will be seen to 

 form, and ultimately the field will appear as shown in 

 Fig. I. 



Fig. 1.— Comn 



Fi<'. 2. — Potassinm Nitrate. 



Precisely the same experiments being made with the 

 other solutions, the shapes of the respective crystals are 

 shown in the accompanying figures. Potassium nitrate 

 difTers remarkably from the salt ; instead of the little cuVics, 

 we have the crystals arranged in long parallel feathers. 

 The oxalic acid, again, shows forms difiering from the other 

 two ; from a centre the crystals radiate out in every direc- 

 tion. Of the four specimens, however, the potassium 

 dichromate is the most beautiful ; the crystals, instead of 

 being colourless, are of a deep amber hue, while, in mode 

 of arrangement, they resemble a fern group rather than 

 mere inanimate matter. The ficrures must be looked on as 



Fig. .3.— Oxalic Aciil. Fig. 1.— Potassium Dichromate. 



giving some idea only of what is actually seen. The 

 leading outlines have been drawn, but to copy the delicate 

 tracery of the finer crystals is impossible. In the case of 

 the potassium dichromate in particular, some parts of the 

 field defy all attempts at even afibrding a conception of 

 their exquisite beauty. But were even all this possible, 

 there is yet the greater charm remaining to the actual 

 worker, and that is to see the growth proceeding. The 

 specimen btung so placed that the edge of the crystals 

 already formed is just within the field, the main lines 



shown are first rapidly filled in, and then the small, r 

 branches dart out until, the water having evaporated, the 

 whole of the salt has regained the solid state. 



PHOTOGRAPHY FOR AMATEURS, 

 By a. Brothers, F.R.A.S. 



THERE are some advantages in trimming the prints 

 before toning. One is, that the black edge would 

 take up gold wastefully, and as the paper contains silver, 

 the waste edges should not be destroyed. The saving, 

 where the quantity of paper used is large, amounts to an 

 important item in the course of a j'ear. A third advantage 

 i.s, that the prints may be mounted at once. Priuts on 

 albumenised paper have a habit of curling, and it is, there- 

 fore, less troublesome to mount them whUe damp. The 

 medium for mounting may be gelatine or starch. The 

 starch may be of about the same consistence as is used for 

 stiffening linen, and should always be used fresh. The 

 only disadvantage in the use of starch is, that if it be 

 required at any time to remove the print from the mount, 

 there is some risk of spoiling the print ; but it may be done 

 by steeping in hot water, and gradually stripping or 

 rubbing the paper or cardboard from the Ijack of the print. 

 Prints mounted with gelatine are readily detached from 

 the mount when placed in hot water. The mountant 

 should never be used if at all acid. 



When dry, the prints should be pressed with a hot flat- 

 iron, a piece of smooth paper being used between the print 

 and iron. But a beiii^. _ face will be obtained by passing 

 the prints through a suitable rolling-press. A solution of 

 indiarubber is sometimes used for mounting prints, and it 

 has the advantage of not cockling the paper, but the prints 

 are liable to peel ofT after a time. There is always a diffi- 

 culty in mounting prints on paper, or even cardboard, 

 imless very thick, on account of the cockling caused by the 

 contraction of the print in drying. If the prints are 

 larger than Sin. by 6 in., they have a much more finished 

 appearance if mounted on plate-paper with an india tint ; 

 but this cannot be done by an amateur, and the prints 

 should be sent to a professional mounter, the cost being 

 very little more than if done at home on cardboard, and 

 the superior finish is well worth the extra cost. 



Up to this point I have endeavoured to describe a 

 process by which photographs may be obtained, and if the 

 directions are carefully followed, a very little experience 

 will enable the student to become expert. The chief 

 difference between the amateur and the professional 

 photographer is that the latter has more practice, but there 

 is no reason why the work of the amateur should not equal 

 that of the profe.ssional. It may happen that our pupil 

 has no friend to whom he can apply to help him over 

 difficulties. By a recent change introduced into the 

 management of the correspondence columns of Knowledge, 

 we ofler to take the place of the friend, and we hope 

 during the present season to welcome many workers in one 

 of the most fascinating of arts. 



Before closing this part of our subject, a few general 

 remarks may be useful. 



In the early days of paper prints, the skies of landscapes 

 were almost always left white, but the effect was most 

 inartistic. It is now rare to see a landscape without ap- 

 propriate clouds, often taken at the same time as the rest 

 of the picture. This, however, is not always possible, and 

 for one reason, amongst others, that tlie sky may he quite 

 clear when our view was taken. It is quite easy to make 



