490 



Mr. J. E. Petavel. An Experimental 



These bricks are eminently suitable to resist high temperatures. 

 Owing to the presence of silica they could not be used alone for the 

 purpose we have in view, but they are very valuable as an outer 

 crucible in which to place the pure lime on which the platinum rests. 

 In a brick 11x11x7 cm. a cylinder of 7 cm. in diameter and 3 cm. in 

 depth is drilled out ; the hollow is filled up with chemically pure lime 

 in the form of powder, which is pressed tightly in. The platinum, 

 roughly hammered into shape, is forced into the bed thus prepared 

 until the lime is flush with the upper surface of the metal. Before the 

 temperature has reached the melting point of platinum the lime has 

 formed itself into a sufficiently coherent mass to support the weight 

 of the metal. Another brick 11x11x4 cm. forms the cover ; it is 

 held in a cast-iron frame by six J-inch screws equally spaced along 

 two adjacent sides of the square. A hole 1*6 cm. in diameter is 

 drilled through the centre of this cover. It is through this hole 

 that the blow-pipe plays vertically on the surface of the metal. 

 The lower surface of the cover should be not more than 1 cm. above 

 the platinum. Both blow-pipe and diaphragm are attached to the 

 same vertical axis A ; the electromagnet M, which controls the motion 

 of this axis, is placed in series with the magnet N, which cuts 

 off the supply of gas to the blow-pipe. The instant the gas is 

 stopped the axis swings round, bringing the 1 square centimetre dia- 

 phragm D above the hole in the cover of the furnace. A number of 

 screens are provided so as to prevent any light from the furnace itself 

 reaching the photometer. These screens have been removed in fig. 6 

 so as to give a better view of the furnace. 



The light from the platinum is reflected on to the photometer by a 

 mirror. The secondary standard is not placed in the axis of the 

 photometer, but at right angles to it, its light being also reflected by a 

 mirror. Each mirror is supported by a bar, which is held in a 

 socket provided with a V-shaped check, so that when one of the 

 mirrors is taken out of its supporting socket it can always be replaced 

 in the same position ; the fittings are made interchangeable. Thus by 

 interchanging the two mirrors any error due to their coefficient of 

 absorption can be eliminated. 



A metronome ringing every ten seconds gives the intervals at 

 which the photometric observations are to be made. To save time 

 the readings are not taken, but the position of the index on the 

 photometer bar is marked off". The distances are read at leisure 

 later on. 



The method by which the observations are obtained is as follows : — 

 Let us suppose that the platinum is in position, the standard of refer-, 

 ence adjusted, the metronome started, the " rate " and " ratio " gauges 

 calibrated, and that we are ready to start the blow-pipe. We turn on 

 the gas and increase the supply until the gauge shows that the hydro- 



