Chemical Science. 45S 



4. Chromates of Potash. — Dr. Thomson gives the following 

 as the composition of these salts : 



Chromate of Potash. 



Chromic acid 52 or ...♦ 108.33 



Potash 48 100 



Bi-chromate of Potash. 



Chromic acid 63.421 or .... 216.93 



Potash 31.579 100. 



5. Metallographical Application of fusible Metal. — This 

 alloy is composed of eight parts of bismuth, five of lead 

 and three of tin, and its property of fusing at the boiling point 

 of water is well known. M. Gassicourt has proposed a me- 

 tallographical use of it, founded upon the extreme accuracy 

 with which, in casting, it preserves the marks and traces on 

 the mould. He illustrates his new application of it in the fol- 

 lowing manner : Paste a piece of white paper at the bottom of 

 a china saucer, and let it dry.: then write on it with common 

 writing-ink, and sprinkle some finely -powdered gum-arabic over 

 the writing, which will produce a slight relief. When well 

 dried, brush ofi' the powder that does not adhere, and pour 

 fusible metal into the saucer, taking care to cool it rapidly that 

 crystallization may not take place. In this way a counterpart 

 of the writing will be obtained, impressed on the metal. By 

 immersing the cast in slightly-warm water, any adhering gum 

 may be removed, and then, if examined by a glass, the writing 

 may easily be read and seen to be perfect. Afterwards, by 

 using common printers' ink, impressions may be taken from it, 

 all of which will be Xx\i& facsimiles of the first writing. 



The difficulties in this new application of the fusible alloy, 

 are, to avoid unequal thickness in the plate of metal, which 

 causes it to alter in form and break under pressure ; and to pre- 

 vent the surface from crystallizing, when the ink will adhere 

 where it is not required. 



6. Reduction of Chloride of Silver. — Chloride of silver is, from 

 the various processes of analysis, ^c, constantly accumulating 



