Sulphuric Acid.........................6 oz.

Nitric Acid............................4 "

Water................................2 "

Mix and the temperature will rise to 1700 F.

Inmerse dry cotton wool (best long fibre), be sure wool is dry, draw it in long flakes and pull it under acids with a stout glass rod; do not crowd in too much, take care that each tuft is well wetted with the acids before putting in a fresh tuft. Carefully cover the vessel and put it where any slight fumes may escape; leave it for at least 12 hours - 20 hours will not spoil it. When ready, lift cotton out and plunge it into a large quantity of water, quickly separating the tufts with glass rods, wash in changes of water, until no acid is left, then wring the cotton in a coarse towel until dry as possible and pull it all apart and place in the air to dry.

Collodion With Above

Alcohol............................ 5 oz.

Ether.............................. 10 "

Cotton...........................100 grains to iodize :

Alcohol............................. 5 oz.

Iodide Ammonium............... .... 60 grs.

Iodide Cadmium......................30 "

Bromide " ......................20 "

Dissolve the iodides in the 5 oz. alcohol or in the whole 10 oz., if desired, then put in the 100 grains cotton, shake well, then add the 10 oz. ether and shake till the cotton is all dissolved; it will be ready for use in a few hours, and will improve with age.

The Negative Bath

The silver solution for a negative bath is prepared in substantially the same manner as for a positive.

Two thirds of the amount of silver nitrate intended to be used, should be dissolved in the quantity of water required to make the bath, and a grain or two of iodide of potassium added and placed in the sunlight until the solution has changed color, become turbid, and again clear and colorless. This change is caused by a minute portion of the silver combining with the organic matter and with other impurities in the water, if there are any; a molecular change then takes places, oxygen is evolved and the infinitesimal portion of silver changes to a metallic state and sinks to the bottom, carrying with it the impurities, which induced the chemical action or change.

The solution may now be filtered, the remaining portion of the silver nitrate added, and with a few drops of pure nitric acid, the solution will be ready for use. But if on trial it is ascertained that it will not give satisfactory results, it will be found that it has not been sufficiently excited, or an acid reaction has not been induced.

If the first supposition is true, more iodide of potassium must be used, as a silver solution must receive as much or nearly as much of the iodide as it will hold in solution, before it will work harmoniously with the collodion; the grain or two put in the solution with the first portion of nitrate of silver have not been sufficient.

A very little more may be added, which will combine with the silver, forming a white curdy precipitate, which, on shaking the bottle, will in a short time redissolve. If any remains, it should be filtered out, as no more can be held in solution.

An other plan is rather more convenient, but requires more time. It is to coat with collodion as large a plate as the bath will receive; immerse it in the bath, leaving it there some hours, usually all night.

After trying one of these remedies, if the bath refuses still to yield good results, more nitric acid should be added, until the solution will turn blue litmus paper slowly red.

I have never known a silver bath to fail to come to terms under this treatment.

The silver solution, when in constant use, rapidly deteriorates, and unless there is a very large quantity, it will soon become unfit for use, and however much or little there may be, it is only a question of time as to when it will cease to act satisfactorily.

This result is caused partly by the gradual contamination of the solution by the alcohol and ether washed from the collodionized plates that have been sensitized in it. When the bath is seriously affected in this manner, it is indicated by the difficulty experienced in getting the developer to flow evenly over the plate, and also by the strong alcoholic odor of the silver solution.

The necessity for a change of the bath may be delayed for a time by the addition of alcohol to the developer, which causes it to flow more evenly.

Another source of evil to the bath is the continual absorption from the surfaces of the plates immersed of minute portions of the salts with which the collodion is excited. After the silver solution has taken up all it can dissolve or assimilate, the surplus is held in suspension and is called free iodide, which deposits itself on the surfaces of the plates, and when in quantity causes the plate when taken from the bath to appear as if fine sand had been sprinkled over it. These small crystals prevent the action of the light on the parts they cover, and when the plate has been developed and fixed, every crystal has produced a small transparent spot or pinhole, as it is termed, and many a lovely negative has been ruined by pinholes.

This trouble may be cured by increasing the quantity of the solution and adding more silver nitrate, which, if in sufficient quantity, will dissolve the free iodide. But if it may not be convenient to do this and no other solution is ready for use, then, after immersing your plate, tip the bath dish back so that the face of the plate may be inclined downwards, when the crystals will be deposited on the back of the plate and do no harm.