318 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



been hardened by the addition of chrome alum, and it is immediately 

 placed in cold water. The gelatine absorbs water where the liglit has 

 not acted, but refuses it in the other parts. When a roller charged 

 with greasy ink is passed over the surface the ink adheres to the dry 

 portions and leaves the moist parts intact. A perfect /amwwVe is thus 

 obtained with a minimum amount of labour, A good transfer can be 

 obtained five minutes after the positive print in gelatine leaves the 

 printing- frame. The picture is transferred to stone or zinc in the 

 ordinary manner. 



Several processes for obtaining printing-blocks to be set up with 

 type are extant. One of the most siiccessful is that of Gillot. He 

 transfers a true picture of an engraving in greasy ink to zinc and eats 

 away the metal by acids, thus leaving the lines in relief. Another 

 process which has been worked out by Captain Abney is dependent 

 on the electrical action set up between two metals when one is 

 deposited on the other in a fine state of division. He obtains a proof 

 on a metal plate in resin which has become sensitive to light by a 

 preparation of bichromate of potash, and which becomes insoluble in 

 acids where the light has acted. He then covers the plate with a 

 weak solution of nickel, platinum, silver, &c., from which these metals 

 are deposited in a fine state of division. It is then placed in a solution 

 of chlorine, hydrochloric acid, or other solvent, and the lines are left 

 in relief. The use of the deposited metal consists in allowing a solvent 

 of such a weak character to be employed that ordinarily it would not 

 attack the plate ; hence there is no undermining of the lines. 

 Warnerke's method appears to be similar. 



IT. BELIEF IMPRESSION. WOODBURY PROCESS. 



A curious process of heliography, derived from M. Poitevm's, has 

 been invented by Mr. Woodbury, who calls it relief impression. 

 After having obtained on a thin film of collodion, covered with 

 bichromated gelatine, the reliefs and hollows arising from the unequal 

 swelling in water of the gelatine under the influence of the light, the 

 plate is dried with a gentle heat. The swollen parts in relief are the 

 shadows of the image. This done, Mr. Woodbury submits the plate 

 in relief covered with a plate of metal (a mixture of type metal and 



