VI. METALLURGY. 547 



weight beiiig so adjusted that sufficient silver is present to neutralize a given 

 volume of the solution employed as a precipitant. This solution is usually 

 one of common salt, but hydrochloric acid or hydrdbromic acid may some- 

 times be used with advantage. The assay pieces, having been carefully 

 weighed, are placed in numbered bottles, and a definite amount of moderately 

 dilute nitric acid is added to each, the bottles being then moderately heated to 

 assist solution. The standard solution is then carefully introduced by means 

 of a pipette, and the bottles are vigorously shaken until the precipitate coheres 

 and the solution becomes clear. A cubic centimetre of " decimal " solution, 

 which is -j^th as strong as the " standard " solution, is then added to each 

 bottle, and they are again shaken. This is repeated until the decimal solution 

 produces either no cloud or a very slight one. This indicates the conclusion 

 of the operation, as the amount of silver present can be calculated when the 

 weight of salt which is required to saturate it is known. 



From the above description it will be seen that the only special apparatus 

 required in assaying silver by the method of Gay Lussac is a pipette for 

 measuring out the " standard " solution, and one for adding " decimal " solu- 

 tion. 1 and 2 are the forms of these used in the Eoyal Mint. The pipette 

 for standard solution, 1, is fixed in a vertical position and filled by an india- 

 rubber tube from below. The opening at the upper end of the pipette is 

 closed by th finger, the india-rubber tube is removed, and the solution thus 

 accurately measured is added to the contents of a bottle. No. 2 is divided 

 into cubic centimetres, and the additions made by means of it as already 

 described. When great accuracy is required the decimal solution may be 

 added drop by drop by means of the apparatus No. 3, designed by Chevalier 

 Van Eiemsdijk, of the Utrecht Mint. 



2772. Old Cupellation Furnace, supposed to have been the 

 one used by Sir Isaac Newton, when Master of the Mint, in some 

 experiments on the cupellation of silver. The Master of the Mint. 



In general construction it is precisely similar to those now in use, the only 

 difference being that, in modern forms, more perfect means are adopted for 

 regulating the draught. 



2773. Touchstone for the Assay of Gold, formerly used 

 in the Royal Mint. The Master of the Mint. 



The method is based on the fact that the greater the amount of gold con- 

 tained in an alloy, the brighter is the gold yellow colour of a streak drawn 

 with it on a black ground, and the less is it attacked by pure nitric acid or by 

 a " test " acid. In ascertaining the richness of the alloy under examination 

 its streak is compared with marks drawn with alloys whose richness is 

 accurately known. 



2774. Diagram, illustrative of a Westphalian blast furnace of 

 most recent construction, for the use of lecturers on metallurgy. 



Professor Dr. Durre> Aix -la- Chap die. 



2775. Diagram, illustrative of a lead smelting furnace on 

 Pilz's principle, for the use of lecturers on metallurgy. 



Professor Dr. Durre, Aix-la- Chape lie. 



These diagrams are drawn to scale, and can also be employed for teaching 

 constructive drawing and as designs for smelting works. 



2775a. Saturn Steel. M. Breguet, Paris. 



Mm 2 



