372 PHILOSOPHICAL TRANSACTIONS. [anNO 1788. 



having left behind to the e. a part which lies in the direction from n. to s. and it 

 is from this that the pieces of metal may have been taken with a chissel. From 

 this account one may venture to form the following hypothesis. I will suppose, 

 that the volcanic explosion happened in the spot where I discovered the brackish 

 spring ; that by the explosion a great quantity of earth was raised, and formed 

 the elevation of this part of the plain above the rest ; that it was originally much 

 Wgher, but the continual rains of the Chaco, which is overflowed a 3d part of 

 the year, are always acting to bring it to a level with the rest of the country. 

 The direction of the greatest portion of the mass of iron ejected was from e. to 

 w. and being heavier reached to the distance where it is found. Another portion 

 of the matter smaller, and perhaps more fluid, separated and took another direc- 

 tion, running into several streams, as when water is thrown out of a pail. This 

 small portion of matter having cooled, and the earth which supported it being 

 washed away gradually by the water, must I apprehend have formed what is now 

 called the tree of iron. 



The saline and antimonial matters, which usually accompany all minerals, 

 must have been scattered round about in a similar manner, and rendered the 

 ground barren. In the kingdom of Santa Fe de Bogota, there is found dust of 

 platina mixed with gold. Almost every body knows the great affinity there is 

 between these two metals, so that it is not surprising, considering all these 

 reasons, that the fire of the volcano should have melted the platina which lay 

 above the gold, and thrown it up. This principle of volcanos is the most natural 

 to account for the formation of those famous masses of silver found separate at 

 Guantajaia, about which so many extravagant and ridiculous stories have been 

 told. The mass of iron, which is the subject of this letter, according to its 

 cubic measure, and allowing it a little more specific gravity than iron, must 

 weigh about 300 quintals. 



Some specimens of the iron accompanied this paper, and were laid before the 

 Society ; who afterwards presented them to the British Museum. C. B. 



If^. Frigorjfic Experiments on the Mechanical Expansion of jiiir, explaining the 

 Cause of the Great Degree of Cold on the Summits of High Mountains, the 

 Sudden Condensation of Serial Fapour, and of the Perpetual Mutability of 

 Atmospheric Heat. By Erasmus Darwin, M. D., F. R. S. p. 43. 

 Having often revolved in my mind the great degree of cold produceable by the 

 well-known experiments on evaporation ; in which, by the expansion of a few 

 drops of ether into vapour, a thermometer may be sunk much below the freez- 

 ing point ; and recollecting at the same time the great quantity of heat which is 

 necessary to evaporate or convert into steam a few ounces of boiling water ; I 

 was led to suspect, that elastic fluids, when they were mechanically expanded. 



