The advance of photography : its history and modern applications (1911)

22 THE ADVANCE OF PHOTOGRAPHY beating up white of egg to the consistency of snow, and allowing it to settle. The glass plates, after being coated with this solution, were dried, and then dipped into a solution of nitrate of silver. Iodide of silver was formed in this manner — the coating turned yellow, and became very sensitive to light. Niepce put these glass plates in the place of the image in the camera obscura, and suffered the light to act upon them. No change was at first visible, but one became clearly perceptible when the picture was immersed in a solution of gallic acid. Thus Niepce obtained a negative on glass without the blemishes which appeared on the paper negatives. He prepared prints from this negative by exactly the same process that had been employed by Fox Talbot. Niepce invented his method in 1847. It excited much attention, but had its draAvbacks : the preparation of the albumen and the treatment with salts of silver and gallic acid was a dirty process. Therefore the method appeared to those who had been accustomed to the daguerreotype, dirty and unpleasant, and many were deterred from trying it. On the other hand, the advantages of the new process in multiplying prints were so evident that it could not be overlooked ; therefore, even those who had an objection to soiling their fingers nevertheless zealously devoted themselves to the work. The readiness with which albumen decomposes was, however, a great disadvantage in the new process. Hence the experimenters sought to avoid this by adopting a more durable substance. Discovery of Collodion. — This was afforded by the discovery of gun-cotton made by Schonbein and Bottcher in 1847. Schonbein found that ordinary cottonwool dipped in a mixture of nitric and sulphuric acids assumes