THE BOOK OF THE LANTERN. 107 



drain, and dry spontaneously on a slip of blotting paper. 

 The best collodion to use is " Mawson's Negative Col- 

 lodion." The plates should be fixed in hypo of the usual 

 strength, and should the image appear to be " dirty " it 

 can be rendered clear by being washed over with a solution 

 of iodine and iodide of potassium. 



After fixation and after the plates have been thoroughly 

 washed, they may be toned in a solution of chloride of 

 platinum, one grain to 4 ounces of water, and they 

 should remain in this solution until the deposit is darkened 

 throughout. 



Beginners very often fail in getting a good tone from 

 platinum, and complain that instead of darkening the 

 image the salt has the opposite effect. They are recom- 

 mended in some formulae to add nitric acid to the toning 

 bath, but this is useless unless the platinum salt be 

 neutralised in the first instance. The proper mode of 

 procedure is to break the tube containing the platinum 

 crystals (this chemical, like chloride of gold, is on account 

 of its deliquescent property always sold in an hermetically 

 sealed glass tube) into a certain quantity of distilled water. 

 A convenient plan is to break a tube containing 1 5 grains 

 into 15 drachms of distilled water : one drachm of the 

 liquid will then represent one grain of platinum chloride. 

 Test this liquid before use with litmus paper: if it show by 

 the paper turning red that it is acid, we may be quite sure 

 that it contains free hydrochloric acid, which will have a 

 bleaching effect upon the photographic image. The liquid 

 must therefore be neutralised by the addition of a few 

 grains of carbonate of soda, after which it must be 



