110 THE BOOK OF THE LANTERN. 



Many of these old processes differ only in the kind of 

 preservative fluid applied to them, and from the nature 

 of this preservative they usually are named. Thus we 

 have the Tannin process, the Honey process, the Coffee 

 process, &o. Taking the first named, let me cite it as an 

 example of the others. 



The glass-plate which is to bear the picture is first of all 

 carefully cleaned. It is then edged with india-rubber 

 solution, albumen, or some other body which will prevent 

 the film from slipping off the glass during subsequent 

 operations. Next it is coated with ordinary negative 

 collodion, to which two grains per ounce of bromide of 

 cadmium may be advantageously added. It should now 

 be dipped in a silver bath which has been made distinctly 

 acid, by the addition of a few drops of nitric acid. After 

 thus sensitising the plate, it must be well washed to remove 

 all free silver, after which the preservative is applied, 



Tannin ... ... 35 grains. 



Distilled water ... 4 ounces. 



After the plates are dry they are ready for use, but will 

 only remain good for a week or two. 



In another process which gives good results coffee is used 

 as the preservative. In this case the bath can be made 

 very acid, by the addition of one-fifth of its volume of 

 glacial acetic acid. The plate is edged, and sensitised in 

 the bath, and is then flowed over with an infusion of 

 ground coffee. After drying, these plates will keep for 

 some months, and will give fine results. The development 

 is brought about by a plain solution of pyrogallic acid, 



