set; allow it to remain for twenty-four hours, break it up, wash, re-melt, and coat the paper or other support. One of the ad-vantages of alpha papers is that it is possible to obtain prints of great beauty, rivalling albumen in tone, and superior to them in surface, without the trouble of having to print out; it being a de-velopment printing process. It must be remembered that alpha prints can be toned ; but for a photographic purple the developed print should be a pinkish violet after development, and this is the best colour to aim at for all other tones. Having found the correct exposure for the particular negative, expose as many pieces of paper as prints are required, taking care to give exactly the same exposure by a metronome or watch, and then pro-ceed to develop them. Either ferrous oxalate, hydroquinone, eikonogen, or glycin can be used, but for warm tones hydro-quinone is preferable; and the formulae for these developers suggested for bromide paper may be used if dilated with two or three parts of water and some bromide of potassium added. The final tone depends to a great extent on the exposure, long exposures tending to give warmer tones. If the iron developer is used the prints must be placed in an alum and citric acid clearing-bath for two minutes, well washed, and then fixed. Alpha prints may be toned in a combined bath, when of course fixing need not be done as a separate operation, or preferably in the following : -
Chloride of gold ............ I gr.
Calcium chloride (crystal) ... ... ... 10 ,,
Water ............... 10 ozs.
Place the prints in this after well washing out the fixing solution, and tone till the desired colour is reached. The prints only want well washing and drying to be ready for mounting. Alpha prints can be rolled and burnished, provided too great a heat be not used. Over-exposure gives flat prints without any depth in the shadows and without pure whites, whilst under-exposure gives harsh prints with greenish tones.
Alum. Under this name a series of important double salts are classed. These salts are characterised as being double sulphates of monatomic and triatomic metals crystallising in pctahedra, and containing 24 molecules of water of crystallisation,