278 Heaiifig by the Circulation of Fluids. 



tubes any one acquainted with locomotive engines is well 

 aware of. To this is to be added the deposition of sediment 

 in the whole line of the tubes, and the difficulties of origin- 

 ally stanching an extensive range of such apparatus, and 

 afterwards maintaining it so, against the effects of expansion 

 and contraction. 



The second mode proposed is by using a solution of 

 lime, or some other saline solution, instead of water. By 

 this no great increase of temperature is obtained, and the 

 corrosion of the apparatus is likely to be very great. It 

 would seem, however, that there are very few cases in which 

 it can be of any real advantage, to heat by a medium cir- 

 culating at a temperature greater than that of boiling water, 

 notwithstanding the speciousness of the arguments by which 

 the contrary may be urged. But, where any such advantage 

 is apparent, I now propose an arrangement at least free from 

 the objections of the preceding ones, whatever it may have 

 peculiar to itself. 



I propose to use, as my medium of circulation, any me- 

 tallic alloy which will melt at a tempei'ature not greatly 

 exceeding 212° Fahrenheit, for this purpose: that composed 

 of bismuth, tin, and lead will probably be found the most 

 convenient. 



Fig. 80. is a section of one of the simplest possible forms of 

 this apparatus. The large tube, shown by strong lines, con- 



tains within its whole length another range of small gas 

 tubes, the extremities of which are at a and b. The tubes 

 are arranged in any of the usual forms to produce circidation, 

 and have at one extreme a hollow cylindrical wrought-iron 

 boiler (c) which, together with the interspace between the 

 outside or larger tubes and the inner or gas tubes, are 

 filled with the fusible metallic alloy, poured in hot, through 

 tubes at d d, which have a loose plug or piston fitted to each, 

 and capable of being withdrawn. 



The extremities of the gas tube {a, b) communicate with a 

 very small steam-boiler. Now, supposing the apparatus all 

 full, but the metal cold and solid in it, and requiring to be 

 heated. By a shovelful of coals steam is got up in the 



