Architectural and engineering drawings can be copied with clearness and accuracy upon paper or calico which has been dyed with primuline diazotized and dried. The diazo-paper for this purpose can be kept in the dark for some time without undergoing decomposition. The copy is developed by brushing over the surface a solution of fi-naphthol or a-napthylamine.
Transparent photograph pictures can be produced upon glass by coating with a film of gelatine containing primuline, then diazotizing with nitrous acid, printing, say, from a carbon or a silver positive, and developing as before. In all the above application the primuline may be replaced by the higher homologues, already mentioned, and for producing designs upon silk dehydrothiotoluidine sulphonic acid, and its homo logues may be employed, which are equally sensitive to light.
The above described process is probably the first which has been devised in which derivatives of the aromatic hydro carbons are employed for the production of a positive image capable of development in color.
A process was recently invented and patented by A. Feer,* based upon the fact that when the diazo compounds are treated with an alkaline bisulphite they are converted into the diazo-sulphonates.f These compounds are sensitive to light, the action of which is to set free the diazo group from its combination, but they do not react with phenols and amines as do the diazo compounds. The mixture of a diazo
sulphonate with the latter is unattended by any color reaction, but on exposure to light, the diazo group being set free in presence of a phenol, the development of an azo color takes place pari passu. The photographic surface is a mixture of a diazo-sulphonate with the alkali com pound of a phenol applied to any suitable material. On exposure to light under a transparency, development of color takes place in proportion to the quantity of light transmitted, giving there fore, a reversed reproduction or negative picture. When printed the unattached mixture is dis solved away by copious washing, leaving the picture already developed in the azo-color permanently fixed upon the fabric or material.
It will be seen that the latter process is in point of simplicity inferior to the primuline process, nor are the results superior. The primuline process can be adapted to a very large number of uses. No technical knowledge is re quired. The description of the process already given will be found sufficient. Most delicate pictures can be printed upon all kinds of materials, either from photographic positives or from natural subjects, such as fern leaves, etc. It also bids fair to supersede the old blue process for the reproduction of maps, drawings, etc. The images obtained will resist the action of hot soap and water. They are also exceedingly fast to light.