ALCOHOL. On this important substance the remarks interesting to a photographer will be, its special use in the collodion process, its effect as organic matter on the photogenic image, and its assist ance in developing. Pure alcohol as organic matter is nearly inert in photolytic processes, but when decomposed or oxidized with ether, by long use in the exciting bath, organic principles are formed which act like strong acids, and their effect is too injurious to be attributed to the formation of acetic acid. Such a bath, if evapo rated to dryness, will show that the organic matter is in combina tion with the nitrate of silver; for on fusion the salt froths violently as long as any organic matter is undecomposed. Added to the collodion developers, it has been found very beneficial in making them flow evenly and quickly over the plate ; but it has not been tried as it deserves in the development of paper negatives. When it is added to the solution of gallic acid, it is easy from its greater solvent power to get the proper strength, and there is less tendency to the formation of crystals and stains, than when concentrated aqueous solutions are employed. The spirit has greater power also of penetrating into the wax and size; but this property, though useful in negatives, forbids its use in paper positives, where surface vigour is requisite.
The most important use of alcohol is in collodion, and attention must be paid to its strength and purity.
Absolute alcohol of S, G. 796 or 75.25 above proof, is very difficult to keep in use, because, from its strong attraction for water, it rapidly absorbs it from the air. But this is not necessary to the photographer. Absolute alcohol is not the best for collodion, either as respects the physical or chemical qualities of the article. When iodide of potassium (the most generally useful iodide) is used in collodion, it is so little soluble in alcohol + ether, that if the strongest spirits are employed, the iodide is precipitated in the film before going into the N. S. bath. The consequence is seen both in the appearance of the film before going into the camera, and in the developed picture. The film before exposure appears by trans mitted light semi-opaque, bluish in colour, and by reflection dim yellow, like bad iodized paper, because the salt, as in paper, is not in solution when the nitrate attacki it, but when the iodide is properly formed in a film containing sufficient water, it remains more or less in a state of semi-solution, as in jelly, and the colour is bright yellow, perfectly transparent by transmitted light, and of a bl cast by reflection. The same want of transparency is observed
with the same colours, in whatever way the iodide of potassium be comes excessive. In the picture the result is a sensitiveness scarcely superior to that of albumen on glass, and very feeble reduction. Alco hol of S. G. 845 is very good for mixing with ether in making the plain collodion, and '810 in the iodizing solution. The water being in the ethereal mixture counteracts its tendency to precipitate the salt from the iodizer. The strength of pure alcohol is obtained by its specific gravity ; as it occurs in commerce it is generally from S. G. 820 to 850. What is called " proof sphit " is alcohol of S. G. 920. By spirit 60 degrees over proof, a strength which is very commonly sold, is understood a spirit 100 measures of which, added to 60 measures of water, will form proof spirit. Its S. G. is 830. Spirit of 50 degrees over proof needs only 50 measures of water to 100 of spirit to bring it down to proof, and so on. In the French formulfe, spirit of so many degrees is often spoken of. This refers to Baume's hydrometer, for comparisons of which with the S. G. and with proof spirit, see " Tables of Weights and Mea sures." With respect to the purity of alcohol, it is necessary to be very careful in collodion for the positive process. All alcohol is apt to contain organic matter, fusil oil, grain oil, &c., which, though in small quantities, will exercise a more decided photographic effect, prejudicial or otherwise, than a large quantity of the alcohol itself. In negative processes these impurities may be beneficial, but not where pure metallic silver is to be produced. In all eases alcohol from the gmpe seems preferable to that from grain or roots. The last,.when not purified, often leaves on distillation, a red residue in the retort, very offensive t.o the smell. The purity of alcohol is a much more important matter than its strength.