136 



SCIENCE- GOSSIP. 



by itself is not acted on by the light, and it is only 

 in the presence of the organic matter that alteration 

 is brought about. 



The explanation of this peculiar change is as 

 follows : — Several metals, of which chromium is 

 one, form two separate and distinct oxides, the 

 higher or chromic oxide (Cr.,0 6 ), and the lower or 

 chromous oxide (Cr 2 3 ). Now the former, which 

 is practically contained in the bichromate, by the 

 action of light in the presence of the' gelatine gives 

 up a part of its oxygen to that substance, and is 

 itself changed to the lower oxide ; and the liberated 

 oxygen unites with the gelatine, which it oxidises 

 to an insoluble resinous substance —that is to say, 

 a substance only soluble in water in proportion to 

 the duration and force of the actinic rays - and it is 

 probable that the insoluble compound thus formed 

 contains chromous oxide as an essential con- 

 stituent. The length of exposure to light may be 

 diminished, by even seven-eighths, by withdrawing 

 the print from the light and leaving it in the dark, 

 as the printing action, once started, continues 

 gradually. On development after the lapse of 

 several hours the picture will be found to be fully 

 printed. It has been demonstrated that this con- 

 tinued action only takes place in a moist atmo- 

 sphere. The reduction of the higher oxide of 

 chromium is readily shown by acting on chromic 

 acid with alcohol. If a small quantity of alcohol 

 be dropped on chromic oxide, the liberation of the 

 oxygen is so violent that it ignites the alcohol. 



From the explanation of the chemical processes 

 involved, it will at once be seen that development 

 — that is, the removal of the unaltered gelatine — 

 must be performed on the back of the exposed 

 gelatine film, and on that account a temporary sup- 

 port is necessary. After development by warm 

 water the film, which with the incorporated pig- 

 ment now forms the picture, is ready to be trans- 

 ferred to its final support, these processes, of course, 

 being purely mechanical. The resulting print 

 consists only of oxidised gelatine chromous oxide, 

 and the pigment, if thoroughly washed till free 

 from soluble salts, should be quite permanent under 

 ordinary atmospheric conditions: 



2. The Ozotype Process. 



In this method of photographic printing the 

 nascent oxygen (or ozone) is transferred under the 

 influence of light from one metallic confound to 

 another, and on this account the name " ozotype " 

 is considered a fitting one for the process. 



Briefly, the details of manipulation are as 

 follows: — The gelatinised paper is sensitised with 

 a solution containing a bichromate or other similar 

 light-sensitive compound of chromium rnanganous 

 salts, together with aluminium sulphate as a 

 hardening agent, a preservative such as boric acid, 

 and a small quantity of gum or dextrine. After 

 drying, and exposing under a negative in the 

 ordinary way, a brown positive print is pro- 



duced, and after washing with cold water, to rid it 

 of unchanged salts, the brown image remains on a 

 white ground, which is developed by a somewhat 

 complicated process. Ordinary carbon tissue, or 

 tissue of a somewhat similar composition, is im- 

 mersed for about ninety seconds in a weak solu- 

 tion of acetic acid and hydroquinone, with or 

 without sulphate of iron. The washed print, after 



bein°- coated with a 2 



per 



cent, solution of 



gelatine, is also plunged into the above-men- 

 tioned acetic solution. The two are brought 

 together, removed from the solution, and squeezed 

 on a flat surface. After drying, the print and 

 its adherent tissue is ready for development. 

 This is accomplished by treatment with warm 

 water, which first removes the backing of the 

 carbon tissue, and then washes away those 

 parts not rendered insoluble by the action of the 

 manganese and chromium compounds in the print. 

 With regard to the chemistry of this method, as 

 explained when dealing with the carbon process, 

 chromium forms two very distinct oxides, and we 

 may look upon bichromate of potassium as con- 

 taining the higher Cr 2 O s . Now in the presence of 

 light and an oxidisable body, in this instance the 

 maganous salt, the chromic oxide is decomposed 

 into the lower oxide, Cr.,0 3 , and oxygen. This nascent 

 oxygen or ozone immediately attacks the man- 

 ganous salt (and for this purpose it may be looked 

 upon as containing an oxide of manganese, prob- 

 ably MnO) oxidising it to a higher oxide, MnO.,. 

 The brown positive image is practically due to 

 this oxide ; so before exposure the surface of the 

 paper contains the very soluble Cr.,0, ; and a man- 

 ganous oxide. After exposure an insoluble image 

 is produced, consisting of Cr 2 3 and Mn0 2 ; in fact, 

 a portion of the oxygen has jDassed from the 

 chromic oxide to the manganous salt, reducing 

 the former and oxidising the latter. 



Next comes the deve^ping process. The acetic 

 acid in the presence of the manganic oxide 

 (Mn0 2 ) probably converts the Cr,0 3 again into 

 Cr.,0 6 , which, being soluble, is absorbed by the 

 gelatine, and, this being acted on by the hydro- 

 quinone or other reducing agent, the gelatine is 

 again changed into Cr.,0 3 and oxygen, which, it 

 will be noticed, are the same two substances that 

 render the gelatine insoluble in the autotype 

 process. When ferrous sulphate is also used, some 

 of the liberated oxygen possibly acts on this, form- 

 ing ferric oxide, a very stable salt, which settles on 

 the image. The altered gelatine with its pigment 

 is not removed by washing, and this, together with 

 the Cr 2 0„ Mn0 2 , and oxide of iron, constitutes the 

 finished picture. 



It is stated that by acting on the finished 

 ozotype print with an acidulated solution of a 

 soluble aniline salt, pictures in various aniline 

 colouis maybe obtained; but the writer has not 

 succeeded in producing satisfactory results by 

 this method. 



