NATURAL PHILOSOPHY. 187 



for receiving the negative image on paper, than it is to all others 

 yet known for receiving the negative image on glass. The metallic 

 salt which combines with it has also rare merits. 



"In the first place, the manipulations are very simple and easy 

 far more so than in (he silver printing process, and thus the la!r>r 

 saved is considerable. Next, the paper, when rendered sensitive for 

 printing, or 'sensitized, 1 as the photographers say, keeps peri'eet.ly 

 for two or even three weeks, an immense boon to amateurs, who can 

 thus have their stock of printing paper ' sensitized' for them ; where- 

 as, at present, when the paper receives the sensitive preparation, it 

 has to be used almost immediately, and will not keep more than a day 

 or two. Thirdly, the color and tone obtained are very various, 

 including every shade that can be got by the ordinary silver plan ; 

 but, in addition, it has the advantage of being able to print any num- 

 ber of impressions of exactly the same color, and of doing away 

 with all such difficulties as show themselves in mealiness and irregu- 

 lar toning. The precision of result is a great point. By the silver 

 process, the results are never certain, and even when the print comes 

 out perfect from the frame, the subsequent process of washing and 

 fixing go seriously to alter it. Lastly, the permanent character of 

 the new method is very remarkable. Nobody seems to know exactly 

 why the old silver process gives way whether it be on account of the 

 albumen, or the nitrate of silver, or the hyposulphite of soda. 

 We only know, that so many of the prints prepared by the old 

 method fall away, that no reliance can be placed in those which seem 

 to stand firm." 



Since the publication of the above, the process has been patented 

 in Great Britain, and made public in all its details. These are given 

 in the specification as follows : 



To one pound of plain collodion add from 1^ to three ounces of 

 nitrate of uranium and from 20 to 60 grains of nitrate of silver. 

 The paper is prepared for printing by simply pouring the above sen- 

 sitized collodion upon its surface, and hanging the sheets to dry in 

 the dark. The printing is accomplished by exposing the paper to 

 light under the negative in the usual manner, and for about the usual 

 time required for silvered paper; print until the desired depth is 

 reached. It is not necessary, as in the ordinary process, to print 

 the positive to a greater intensity of color than the fixed picture is 

 intended to have. After printing immerse the picture in a bath of 

 acetic acid for about ten minutes, or until that portion of the salts not 

 acted upon by the light has been dissolved. The picture is now fixed 

 and finished by thorough washing or rubbing with a sponge or brush, 

 or by rinsing in pure water ; then dry. Changes in the tone of the 

 picture to suit the taste may be made before drying, by using a bath 

 of chloride of gold, or of hyposulphite of soda. 



Improvement in Photographic Printing. An improvement in pho- 

 tographic printing, brought out during the past year by Mr. Joseph 

 Swan of England, is characterized by photographers on both sides 

 of the Atlantic as something entirely in advance, as regards results of 

 any thing hitherto attained to. The following is substantially the pro- 

 cess : 



" Gelatin, in combination with a salt of chromium, becomes msolu- 



