OTHER SURFACES. This part of photographic manipulation is one of some importance. The production of uniform films by means of vapours is very simple in execution, but it is doubtful in such cases as the daguerreotype, what is the exact composition of the sub stances formed. Their continual change of colour is an evidence that they are not definite, not to speak of the amazing difference in their photographic properties. In the production of films of true chemical compounds, if pure salts are to be examined, it is necessary they should be precipitated on glass or other surfaces absolutely inert, for if collodion or other matters be present, no argument can be built upon the assumption of their purity. The pure salts themselves must settle upon the glass and be carefully drained, or experiments may be tried upon them while still in suspension. In other cases, sensitive surfaces are formed directly on paper or gelatinous beds by immersion and floating, by glass rod, by spatula, or by brush. When a cream of the substance is to be applied, the
spatula must be used, but the plan is only useful in experiments, as the coating is too uneven for the production of fme results. When a little variation in the quantity or streng,th of the salts is of no moment, floating and immersion may be resorted to, but these plans quickly lessen the quantity of the solution, and when double de composition occurs, they also soon reduce its strength, and the pre pared fihns are not constant in their actinic properties. The glass rod and brush are the most exact methods of manipulation, and of these the rod is more correct, the brush more convenient. All photographic formulte should, if possible, be so constructed that the solutions, paper, &c., should continue unvarying.