to become dry; or if some portions of the paper forming the original basis of the gelatine, be suffered to become detached long in advance of the remainder, so that the warm water acts directly on the soluble matter in patches, the result will be uneven development, the portions last uncovered remaining darker than the rest of the print; and it will be difficult to equalize the tint, even by long-continued development.
Blisters During Development.—If, in mounting the tissue with the India-rubber solution, perfect contact in all parts be not secured, blisters will arise in the course of development, which will show as marks or defects in the finished print. They are also caused by small holes in the paper, or air which remained between the two varnished surfaces, or by want of contact in the rolling. If from the first cause, they will dry down, disappear, and do no harm. If from the second or third causes they may be pricked with a fine, sharp needle, from the back of the paper, and so rendered harmless. There is, however, once in a great while, a very refractory blister which will spoil a print, or which can only be removed by scraping, and retouching with Indian-ink, after the print is mounted. With great care in placing the two varnished surfaces together, before rolling, and care in rolling with a pretty heavy, steady pressure, the blisters may be entirely avoided.
Over-Exposure.—An over-exposed print will develop tardily, and continue, under ordinary treatment, too dark. After all the soluble chromic salts are removed, the temperature of the water may be raised, and by long soaking in hot water the depth may be reduced considerably. Mr. Swan has found that immersion for a short time in a very weak solution of chloride of lime, or of hypochlorite of soda, or in chlorine water, or per-
