On the Cold Caves of the Monte Testaccio at Rome. S15 



have endeavoured, as naturally as possible, to combine what 

 hypothetical and even speculative reasoning was necessary, with 

 those facts which a knowledge of the localities, an attentive con- 

 sideration of the subject, and the results of several experiments 

 enabled me to lay down ; and though it may seem like a " re- 

 duetto ad ahsurdum^'' that it seems impossible to explain the 

 phenomenon in any other way, even the gentleman whose sen- 

 timents I have ventured to oppose, after conceiving he has 

 proved, that to evaporation " no part of the effect can be as- 

 cribed," is compelled to come to the conclusion, that " it is dif- 

 ficult to conceive in what other way the temperature could be 

 reduced so low as 44°.'" Allow me farther to add, that that 

 philosopher's theory of ice caves is quite insufficient to explain 

 the coolness observed by Saussure in Ischia and at Terni, * 

 which he supposes to arise from crevices which descend from 

 the higher regions of the mountains. The mean temperature 

 of the latitude of Ischia is about 62° ; and as the height of the 

 very summit of the island is only between 1800 and 2000 feet, 

 the reduction of mean temperature at the top will only be 

 about 6° or 5Q° '^ whereas Saussure found the grotto 45 2°, and 

 was assured that it was still lower in summer, which, from the 

 analogy of the Monte Testaccio, we may naturally suppose. 

 Nearly the same remarks apply to the example at Terni. 



The theory which ascribes the coolness of the Monte Tes- 

 taccio to certain salts distilled from the pottery, may, I think, 

 be set at rest by the following experiment : — I took 160 grains 

 in powder, taken from a portion of an Amphora brought from 

 the hill itself, and boiled it with three ounces of water for some 

 time; I then threw it on a filter, and took a portion of the pure 

 liquor, adding to it a decoction of litmus ; but no change what- 

 ever on the colour of the litmus was produced. I then added 

 a little sulphuric acid, but not the smallest symptoms of pre- 

 cipitation appeared. I next took another portion of the super- 

 natant liquor, which I found had no taste, except a very slight 

 one of clay, or earthy matter, which, of course, arose from a 

 minute portion of the terra cotta remaining in mechanical mix- 

 ture with the water. I evaporated it to dryness, but no saline 

 residuum appeared. Something, however, being visible in the 



* Mentioned in the Philosophical Journalt viii. 13. 



