A Manual Of Photography

SELECTION OE PAPEB E0B PHOTOGBAPHIC PUEPOSES. 193 But what is very singular, this change is not equally rapid upon all hinds of paper,—a difference depending, no doubt, on the size employed; which, it may he observed here once for all, is of the utmost influence in all photographic processes. In one sort of paper (known by the name of blue wove post) it is instantaneous, taking place the moment the nitrate (if abundant) is applied. And yet I find this paper to resist discolouration, by keeping, better than any other, when the mordant base is silver instead of lead. On the other hand, a paper of that kind called smooth demy, rendered sensitive by a combination of lead and silver, was found to acquire, by long keeping, a lead or slate colour, which increases to such a degree as might be supposed to render it useless. Yet, in this state, when it is impressed with a photo- graphic image, the process of fixing with hyposulphite of soda destroys this colour completely, leaving the ground as white as when first prepared. This fortunate restoration, however, does not take place when the paper has been browned as above de- scribed. Some of the muriatic salts also are more apt to induce this discolouration than others, especially those with the earthy bases." It will be evident from these remarks that it is of the utmost importance to secure a paper which shall be as chemically pure as possible. Experience has proved that recently-manufactured paper does not answer equally well with that which has been made for a year or two. It has been thought by many that this 'was an unfounded statement, but it is not so ; and the causes operating to the improvement of paper by age are evident. The organic matter of the size is liable to a spontaneous change: this goes on for a considerable time, but at length the process becomes so exceedingly slow that it may, for all practical pur- poses, be said virtually to rest. Paper changes its colour by keeping from this cause, and I have found that such as I have selected from the shop-worn stocks of stationers has been gene- rally superior to that which has been more recently manufac- tured. Select, therefore, paper of a uniform texture, free irom spots, and of equal transparency, choosing the oldest rather than the newest varieties. Where the process is highly sensitive for which the paper is desired, it is important to treat it in the following manner: Having a shallow dish sufficiently large to receive the sheets of paper without in any way crumpling them, it is to be filled with very clear filtered water, to which a sufficient quantity of nitric acid has been added to make it slightly sour to the taste. Taking a sheet of paper, it should be laid on a porcelain slab, 0