Heating hy the Circulation of Thuds. 279 



little steam-boiler; by which means, as it passes through the 

 gas tubes, the metallic alloy is fused ; during which operation 

 a fire is kindled under the boiler of the apparatus itself, 

 which fuses the metal therein, and as soon as the whole is in 

 fusion, circulation commences throughout the system, the 

 same as in any other fluid ; and its temperature may be 

 raised, if necessary, to any thing below that at which the 

 metal becomes volatile, or to a white heat, supposing the 

 apparatus capable of sustaining it. 



In this arrangement, then, there is neither pressure, 

 sediment, nor corrosion, but there are three great, but per- 

 fectly superable, difficulties to be overcome. First, the very 

 great expansion and contraction of the metallic alloy. To 

 correct this, glands^ or hollow stuffing-boxes, must unite the 

 outside pipes at various appropriate places ; these may either 

 be altogether metallic, or, what I should prefer, packed with 

 amianthus. The gas tubes must likewise be inserted into the 

 outside tubing, through similar stuffing-boxes. 



If there be a sufficient number of these glands, no danger 

 from irregular expansion need be apprehended in apparatus 

 of this kind, however large or complicated. The second 

 difficulty is, that these fusible alloys, in common with all 

 crystalline metals, expand at the moment of congelation ; and 

 that with such force as to rupture any part of the apparatus 

 incapable of yielding to it. To remedy this, all the tubes 

 must be of wrought iron, and not very thick. They should 

 gj be welded tubes (for the outside), and 



rolled or drawn into the section shown 

 in^. 81., or some modification of it. 



Thus the exterior tube will accommo- 

 date itself to the metal it contains at the 

 moment of solidification, and at its fusion 

 again will return to its former shape, by 

 the elasticity of the iron. The third 

 difficulty consists in the great statical 

 pressure of the fluid metal, owing to its 

 great specific gravity, on the parts of the apparatus; so that 

 the lower part of it would have to sustain a pressure of fifteen 

 pounds the square inch for about every three feet in height. 

 This would require all the parts so exposed to be strongly 

 put together, and the joints well made. 



In point of danger of leakage, the metallic medium pos- 

 sesses an advantage over either oil or water ; the law of the 

 capillary attraction of fluid metals being such as disposes 

 them not to pass with facility through minute apertures. 

 With regard to the great statical pressure, it is to be 



T 4 



