The recent discovery by Cross, Bevan, and Green of a new process of colour-photography is not a method of photographing in natural colours, but one which gives a means of producing prints in a great variety of tints.

During a certain course of tuition, it became necessary to illustrate visibly the action of light, and for this purpose were used some of the aniline dyes which fade in the light when treated with nitrous acid. The idea then occurred to utilise this fact for photographic purposes, finally resulting in the production of prints upon silk, cotton, woollen, velveteen, and other fabrics in seven colours - red, maroon, yellow, orange, brown, purple, blue - and in the shades produced by an admixture of these colours.

The material, which is of primary importance, is primuline. This is a yellow substance discovered by Green in 1887; he obtained it by the action of sulphur upon paratoluidine - one of the coal-tar bases closely allied to aniline. At least two bases result from this reaction, and the sodium salt of the mono-sulphonic acid of the more complex of one of these has the peculiar property of dyeing unmordanted cotton a fast primrose-yellow. This product is largely employed in cotton-dyeing on account of the great range of fast shades that may be obtained with it.

Primuline, in common with all coal-tar bases, combines with nitrous acid to form a species of nitrite, which is termed a "diazo-compound." All that need be noted about this diazo-com pound is that it corresponds to the intermediate stage in the transition of ammonium nitrite to the condition of nitrogen (and water) thus: -

Photography In Aniline Colours 10084

In other words, when ammonium-nitrite is decomposed by heat, first one molecule of water is abstracted, leaving H.N: N: OH (the intermediate stage), and then another is taken away, leaving only nitrogen (the final stage). With this intermediate stage we may compare the constitution of diazo-primu-line.

The symbol X in the formula X.N: N: 0 II really stands for a very complicated group of atoms, which, however, it will serve no useful purpose to consider here.

Now, although primuline is very stable under the action of light, the diazo-compound - namely, diazo-primu-line - is very sensitive, and readily undergoes decomposition, giving off nitrogen gas - that is to say, just as ammonia nitrite, under the decomposing influence of heat, gives off nitrogen, so diazo-primuline, under the influence of light, also evolves this gas. Further, it and similar compounds obtained from other dyes of the primuline groups, have what has been called a great avidity for constructive reaction with two large groups of coal-tar compounds - the amines and the phenols. With these they chemically combine, and the products are the azo-coloaring matters.

The more important of the azocolouring matters obtained from primuline are seen in the following scheme: -

Colours respectively.

Colours respectively.

All the diazo compounds can be employed for photographic purposes, and their normal sensitiveness to light is increased by combination with the complex colloids which constitute animal or vegetable textile fabrics.

In the case of primuline, the essential conditions necessary for its photographic application are the following: -

(a) That the reactions above described take place after the application of the primuline to the surface of such material as wool, silk, etc, as a dye, without in the least affecting its union with the elements of this material.

(6) That the diazo-compound produced in combination with these materials by treating the primuline-dyed surface with nitrous acid is photo-sensitive to a high degree.

Although the chemical reactions are rather difficult to follow, the practical working of the process itself is extremely simple; it can be done by any intelligent person; no elaborate apparatus' is necessary, nor are the materials either harmful or expensive. It consists of four stages, namely: -

1. Dyeing or coating the surface upon which it is required to photograph with a particular compound.

2. Converting the applied compound into a photo-sensitive derivative.

3. Exposing the surface thus prepared to light under the usual conditions for producing the picture.

4. Converting the sensitive compound wherever it survives, through having been protected by the shadows of the object photographed, into colouring matters - shades of red, orange, brown, purple, or blue, which are stable and fixed.

The apparatus required consists of a few glass beakers or small basins, a photographic printing-frame, a sheet of glass exactly fitting this frame, and a few glass stirring-rods. Having procured these, and the necessary chemicals, which are so cheap that a square yard of material can be treated at the cost of one farthing, the remaining operations are as follows: -

The material, say a piece of silk, is dyed in a hot solution of primuline, containing 1-2 per cent. of this substance, to which some common salt has been added, causing the whole to take a primrose-yellow colour. The silk is allowed to soak in this liquid for about 1/2 minute; it is then taken out, washed in cold water, and placed for another } minute in a cold bath containing sodium nitrite (J part to 100 of water) which has been acidified with sulphuric acid. This bath changes the character of the primuline; it diazotises it, and makes it thus extremely sensitive to the action of light.

The silk is now ready to receive the picture; that is to say its surface is now ready for exposure, and such a surface will give a complete positive picture after 40-180 seconds exposure.

Since every one may not possess a photographic positive, let us assume that it is desired to have printed on the silk a fern-leaf in a purple colour. To attain this, the printing-frame is laid upon its face on a table, the back taken out, and the sheet of glass (well cleaned) is placed in it; upon the glass the fern-leaf is placed, and then the sensitised silk above it. The back of the frame is put in place and secured by the usual springs, and then the face of it is turned towards a good light.