290 On tJie Manufacture of Optical Glass, [1829. 



a slag and coke rake (89) ; and a stoking iron, with its extre- 

 mity bent, for the purpose of breaking the clinkers off the bars 

 from beneath upwards. 



Preparation of spongy platinum. The platinum used for 

 this preparation should be pure, and may be the refuse pieces 

 resulting from such plate and foil as has been in use for trays 

 in former experiments. This, after being taken out of the 

 pickle (93), and condemned as useless for other purposes in 

 the glass house, should be trimmed from all alloyed parts, if 

 any such are adhering to it, and then digested in a Florence 

 flask, with a mixture of five measures of strong hydrochloric acid, 

 one measure of strong nitric acid, and three measures of water. 

 But little heat should be applied at first until the action di- 

 minishes. According to Dr. Wollaston, one ounce of platinum 

 will be dissolved by about four ounce measures of such acid, 

 and it is advantageous to have a considerable excess of pla- 

 tinum present. The solution obtained is to be precipitated by a 

 strong solution of muriate of ammonia ; a bright yellow pul- 

 verulent substance will fall, and a mother-liquid having more 

 or less colour remain. The precipitate being allowed to sub- 

 side, the liquor is to be poured off, and the former then washed 

 with two or three portions of water. The washing liquors and 

 the mother-water may afterwards be concentrated together; 

 but it is better not to prepare spongy platinum for this particular 

 use from these fluids, but only from the precipitate which falls 

 on adding the muriate of ammonia. 



The yellow precipitate, when washed, is to be dried on a 

 filter, or in a basin, and then decomposed by the application of 

 a dull red heat. This may be done in a clean white earthenware 

 crucible. The heat should be continued until vapours cease to 

 arise ; but this will be found a long operation, in consequence of 

 the low temperature which is to be applied, and the exceedingly 

 bad conducting power of platinum for heat when in this spongy 

 state. The reduction may also be performed by putting the pre- 

 cipitate upon a piece of platinum foil in a layer about th of an 

 inch in thickness, and covering it with another piece of foil ; a 

 spirit lamp will then suffice to reduce the metal, but the foil 

 and powder must be turned occasionally, that both sides may be 

 exposed to the flame. The platinum will appear as a dull grey 

 spongy metallic mass. It should be broken up, mingled, and 



