I have not only tried every purchasable collodion, but my experiments have been innumerable, especially in respect to the ammoniated salts, and I may, I think, safely affirm that all preparations containing ammonia ought to be rejected. Often, certainly, great rapidity of action is obtained; but that collodion which acted so well on one day may, on the following, become comparatively useless, from the change which appears so frequently to take place in the ammoniacal compounds. That blackening and fogging, of which so much has been said, I much think is one of the results of ammonia; but not having, in my own manipulations, met with the difficulty, I have little personal experience upon the subject. The more simple a collodion is the better; and the following, from its little varying and active qualities, I believe to be equal to any now in use. A great deal has also been said upon the preparation of the simple collodion, and that some samples, however good _apparently_, never sensitize in a satisfactory manner. I have not experienced this difficulty myself, or any appreciable variation. The collodion made from the Swedish filtering paper, or the papier Joseph, is preferable, from the much greater care with which it is used. If slips of either of these papers be carefully and completely immersed for four hours in a mixture of an equal part (by weight) of strong nitric acid or nitrous acid (the aqua fortis of commerce) and strong sulphuric acid, then _perfectly_ washed, so as to get entirely rid of the acids, the result will be an entirely soluble material. About 100 grains of dry paper to a pint (twenty ounces) of ether will form a collodion of the desired consistence for photographic purposes. If too thick, it may be reduced by pure ether or alcohol. However carefully this soluble paper or the gun cotton is prepared, it is liable to decompose even when kept with care. I would therefore advise it to be mixed with the ether soon after preparation, as the simple collodion keeps exceedingly well. Excellent simple collodion is to be procured now at the reasonable price of eight shillings the pint, which will to many be more satisfactory than trusting to their own operations. _To make the sensitizing Fluid._--Put into a clean stoppered bottle, holding more than the quantity required so as to allow of free shaking, six drachms of iodide of potassium and one drachm of bromide of potassium; wet them with one drachm of distilled water first, then pour into the bottle ten ounces of spirits of wine (not alcohol); shake frequently until dissolved. After some hours, if the solution has not taken place, add a few more drops of water, the salts being highly soluble in water, though sparingly so in rectified spirits; but care must be taken not to add too much, as it prevents the subsequent adhesion of the collodion film to the glass.