Hollow Spheres of Metal by Heat. 97 



being loosened by heat in the experiments : ordinary soft solder 

 would frequently melt ; and hard brass solder would not admit 

 of the hemispheres being readily separated for alterations. 



The two hemispheres are now to be reduced to equal weights 

 by filling one of them with nitric acid ; then a very small hole 

 formed in the pole of each in a lathe, and stopped by melted 

 sealing-wax : the object of these holes is to prevent heated air 

 bursting the ball during experiments, and to prevent the ball 

 collapsing by cold after an experiment. The two cups are now 

 to be soldered perfectly water-tight all round, and the ball turned 

 a number of times in a lathe, shifting its position in the chuck 

 frequently, until by means of a callipers and template it is found 

 to be sufficiently round, and finally turned and polished until, 

 upon being placed upon the surface of water, it remains steady 

 in every position. 



A number of scratches should now be made upon the outside 

 of the ball across the soldered junction at intervals of 1 ox \^ 

 inch apart all round the ball, the hemispheres separated by means 

 of a penknife and weighed ; they will probably be now unequal 

 in weight, and will certainly be far too heavy for successful ex- 

 periments ; a pair composing a 4-inch ball will weigh from 2000 

 to 2500 grains, and will require to be reduced to about 500 or 

 600 grains. To reduce them to equal weights and to the requi- 

 site degree of thinness, they are placed upon small glass beakers in 

 a light place (for advantage in watching their surfaces), and filled 

 to within a quarter of an inch (or less) of their edges with mode- 

 rately strong nitric acid, to dissolve their inner surfaces gradually. 

 They must be frequently emptied, weighed, and examined for thin 

 places during this process ; and when they are becoming very thin, 

 they must be continually watched to prevent holes being formed in 

 them. The production of a hole is generally preceded by an inden- 

 tation in the outer surface. If by accident a small hole should occur 

 or a very thin place be formed, it must be covered on the inte- 

 rior with a thin film of the solder already mentioned, then heated 

 and rubbed with sealing-wax, and the process of dissolving con- 

 tinued until the two cups are of equal weights, and until, by 

 passing one's finger with gentle pressure over the outer surface 

 of the cujjs, slight indentations visible in the interior are pro- 

 duced, or, by passing the end of a blunt tool over the inner 

 surface, its progress may be observed on the outside. They are 

 now to be soldered accurately and water-tight together by aid 

 of the marks made upon them, the soldered junction filed and 

 trimmed with emery-cloth, and the ball cleaned by jjlacing it 

 securely in a cage of stiff brass wire, di])ping it into nitric acid 

 and then into water. Indentations accidentally produced in the 

 ball may, if the ball be sufficiently thin, be removed by suction 



Phil. Mar/. S. 4. Vol. 18. No. 1 18. Aur/. 1859. II 



