260 HOW TO WORK 



agitate so as not to soil the neck of the bottle, and set aside in a 

 dark cool cupboard for three or four days, or longer ; then carefully 

 pour off, without any shaking, the half into a clean dry close 

 stoppered bottle for use, or better, into one of the 4 o/.. capped 

 pouring bottles, called " cometless." The formula given has been 

 for only 4 oz. of collodion. The absolute alcohol can sometimes be 

 a little increased. 



338. Nitrate Bath. The nitrate bath may be prepared as 

 follows : 



Freshly distilled water, 4 ounces. 

 Re-crystallised nitrate of silver, 600 grains. 



Dissolve ; test for acidity by blue litmus paper ; if acid, neutralise 

 by a little fresh pure oxide of silver, or by a few drops of a very 

 weak solution of carbonate of soda; dissolve in a drachm or 

 two of water 



Iodide of potassium, i grain. 



Then drop into it a few drops of the strong solution of nitrate of 

 silver until it produces no further turbidity. Wash the precipitated 

 yellow iodide of silver, pour off the washings, and add the iodide to 

 the strong silver solution ; stir, make up the quantity of fluid to 

 20 oz. by distilled water, and filter, or allow it to settle, then carefully 

 pour off close, and filter the remaining portion into a small bottle. 

 This can be used in the after intensifying process, or if filtered 

 through a washed filter, added to the stock for the nitrate bath. 



The strong solution of silver is oftener rather alkaline than acid 

 to test paper ; if this be the case, adxt a few drops of a solution con- 

 taining i drop of glacial acetic acid to i drachm of distilled water, 

 until the test paper remains slightly reddened, or the same propor- 

 tions of nitric acid in water may be used ; the latter often works 

 remarkably well with the bromo-iodised collodion, not giving at first 

 intense but remarkably sharp clean negatives, permitting of a rather 

 longer exposure to the strong sunlight without staining, and con- 

 siderable intensifying qualities without blocking out the finest lines. 

 To keep up the strength of this nitrate bath, add occasionally a plain 

 solution of re- crystallised nitrate of silver in distilled water, in the 

 proportion of 50 grains to the ounce. 



It is desirable to keep this nitrate bath perfectly free from dirt 

 and substances likely to injure it. As there is considerable difficulty 

 in obtaining the gutta percha baths without impurities, and the 

 porcelain ones are sometimes too porous, a vertical glass bath with 

 cover is much to be preferred. It is often desirable, after a full 



