Research on some Standards of Light 



489 



rubber tubes. The gauge E indicates the difference of pressure between 

 the two sides of this diaphragm. By the aid of this gauge B, the rate 

 of flow of the gas can be kept constant at any desired value. The 

 gauge G serves to regulate the ratio of the two gases. Two bells 

 float in mercury, the interior of one being placed in communication 

 with the oxygen inlet of the blow-pipe, the interior of the other with 

 the hydrogen inlet. The apparatus is merely a balance, the two 

 quantities weighed being the pressures of the two gases. The point of 

 support is empirically adjusted so that the system is in equilibrium 

 when the gases are flowing in the desired ratio, viz., four volumes of 

 hydrogen to three of oxygen. 



The instrument is very easy to construct, and its accuracy is amply 

 sufficient for the purpose in view. 



Fig. 7. — Blow-pipe. (Scale f full size.) 



Oxygen. 



Oxygen. 



O 



c 



Hydrogen. 



The blow-pipe is of a somewhat peculiar shape and is shown in fig. 7.. 

 About half the oxygen passes through the central nozzle N, which is 

 surrounded by a platinum nozzle P. The supply of hydrogen is led 

 in at C, and flows down between these two tubes. The entire arrange- 

 ment is surrounded by a gun-metal case, which is cooled by the water 

 circulation W. Half the oxygen flows down between the gun-metal 

 and platinum tubes, entirely surrounding the hydrogen where it issues 

 from the platinum nozzle, thus insuring its perfect combustion. 



The Furnace (F, fig. 6). — The outer shell consists of an ordinary 

 Fletcher furnace, 23 cm. in external diameter. The interior is filled 

 up to within 7 cm. of the top with sand, on which rests one of the 

 ordinary magnesia bricks used for smelting platinum. Their chemical 

 composition is — 



Magnesia 92 per cent. 



Lime 4-3 „ 



Silica 2-4 



Iron and alumina 1*3 „ 



