Resin soap is largely employed as a size. The soap is applied to the bibulous paper, and then decomposed by an acid water, leaving a fine film of resin spread upon the surface, susceptible of the highest polish.

The following tables will exhibit the results of an extensive series of experiments, which were undertaken after the publication of Sir J. Herschel's memoir, On the Chemical Action of the Rays of the Solar Spectrum, in which he has given a table of results, obtained with different preparations on various kinds of paper ; but as he has not established the influence of the paper, except in a few instances, independent of the preparation, it became desirable to do so ; and the result of several years' experience has proved the correctness of the conclusions then arrived at.

In pursuing this inquiry, it was found that the same description of paper, from different manufacturers, gave rise to widely different effects ; so that the most carefully conducted experiments, several times repeated, have only given approximations to the truth. The form of experiment was to select a number of specimens of paper,—prepare them with great care in precisely the same manner, and expose them to the same solar influences. They were partly covered with a piece of colourless glass,—the object of this being to determine whether under it the action was quicker or slower than when exposed uncovered. Sir John Herschel has shown that there are some peculiar differences in this respect; and this method offered a very correct mode of determining the relative effects.

I. Papers Prepared With Chloride Of Sodium And Nitrate Of Silver

a. Superfine satin post...... Considerable exalting effect.

b. Thick wove post.......... Depressing influence.

c. Superfine demy........... Slight exalting effect.

d. Bath drawing card....... Changes slowly.

e. Thick post................. Slight exalting effect.

f Common bank post...... Ditto.

g. Thin post.................. Very tardy.

h. Tissue paper............. Considerable exalting effect.

II. Papers Prepared With Chloride Of Barium Anal Nitrate Of Silver

a. Superfine satin post...... Slight exalting influence.

6. Thick wove post.......... Ditto, but stronger.

c. Superfine demy............ similar to a.

d. Bath drawing card....... similar to a.

e. Thick post.................. Considerable exalting influence.

f. Common bank post....... similar to a.

g. Thin post................... similar to e.

h. Tissue paper............... Results uncertain.

III. Papers Prepared With Muriate Of Ammonia And Nitrate Of Silver

a. Superfine satin post...... Strong exalting influence.

b. Thick wove post.......... Results uncertain—dependent on the size employed.

c. Superfine demy............ Slight exalting effect.

d. Bath drawing card........ Results uncertain.

e. Thick post.................. Results uncertain.

f. Common bank post....... Very slow.

g. Thin post................... Ditto.

h. Tissue paper............... Strong exalting influence.

IV. Papers Prepared With Iodide Or Bromide Of Potassium And Nitrate Of Silver

a. Superfine satin post...... Darkens slowly.

b. Thick wove post.......... Results uncertain.

c. Superfine Demy .......... Strong exalting influence.

d. Bath Drawing card.......Very slowly changes.

e. Thick post.................. Depressing influence.

f. Common bank post....... Slight exalting effect.

g. Thin post,................. Ditto.

h. Tissue paper.............. Results uncertain.

Unsized paper has been recommended by some, but in no instance have I found it to answer so well as paper which has been sized. The principal thing to be attended to in preparing sensitive sheets—according to my experience, extended over a period of ten years, although it is opposed to the views of some young but able photographers—is to prevent, as far as it is possible, the absorption of the solutions into the body of the paper—the materials should be retained as much as possible upon the very surface. Therefore the superficial roughness of unsized sheets, and the depth of the imbibitions, are serious objections to their use. It must not, however, be forgotten, that these objections apply in their force only to the silver preparations; in some modifications of the processes, with the bichromate of potash, the common bibulous paper, used for filtering liquids, has been found to answer remarkably well, on account of the facility with which it absorbs any size or varnish.

Great annoyance often arises from the rapid discolouration of the more sensitive kinds of photographic drawing paper, independent of the action of light, which appears to arise from the action of the nitrate of silver on the organic matters of the size. Unsized paper is less liable to this change. If we spread a pure chloride of silver over the paper, it may be kept for any length of time without any change of its whiteness taking place in the dark. Wash it over with a solution of nitrate of silver, and, particularly if the paper is much sized, a very rapid change of colour will take place, however carefully we may screen it from the light. From this it is evident that the organic matter of the size is the principal cause of the spontaneous darkening of photographic papers prepared with the salts of silver.

The most curious part of the whole matter, is, that in many cases this change is carried on to such an extent that a revival of metallic silver takes place, to all appearance in opposition to the force of affinity. This is very difficult to deal with. I can only view it in this light—the nitric acid liberates a quantity of carbonaceous matter from the paper, which, acting by a function peculiarly its own, will at certain temperatures effect the revival of gold and silver, as proved by Count Rumford's experiments.