366 PHILOSOPHICAL TRAKSACTIONS. [ANNOl73t|. 



after a progress of some paces, disappeared. Animals, which happened to 

 graze where these passed, were all killed by it ; and likewise a Teresian friar, 

 who inadvertently breathed the vapour of one of these damps. 



Exper. 0. — Having placed the barometer in the vapour, it underwent no 

 change, but the thermometer fell somewhat more or less. A lighted torch, 

 thrust into them at two palms from the ground, was soon extinguished by the 

 action of the damp. 



Exper. 7. — ^These damps grew gradually weaker in their pernicious effects, 

 for above 3 months, even to the subsequent autumn; as has been generally 

 found in other former eruptions, or when they happened to issue out of 

 their vents. 



Exper 8. — Concerning the salts which are generated in abundance in Vesu- 

 vius, I have, by order of the Academy, examined them by accurate experi- 

 ments, My intention was to know, if besides sal ammoniac, there were also 

 sea-salt, vitriol, nitre, or any other salt. I thought there was no better way of 

 proceeding in this inquiry, than by crystallization ; because it is universally al- 

 lowed, that salts in crystallizing constantly retain one certain and determinate 

 figure ; sea-salt concreting into cubes, vitriolic salt into rhomboidal parallepi- 

 peds, alum into octaedrons, and nitre into rectangular prisms on hexagonal 

 bases. I imagined, that if the salt of Vesuvius happened to contain any par- 

 ticles of the salts abovementioned, it would discover them after crystallization. 

 This way of reasoning was confirmed by experiment : for the Vesuvian salt, in 

 crystallizing, left on the sides of the vessels small parcels of crystallized salts, 

 which, observed through a microscope, resembled a tree with its branches, on 

 the ends of which there appeared several pyramids of an irregular figure, but 

 very sharp-pointed ; and between the branches there were interspersed iti some 

 places a group of prisms, in others some small cubes: whence I inferred, that 

 the salt was ammoniacal, and indeed a genuine and efficacious sal ammoniac, 

 with insensible portions of nitre and sea-salt. Which coincides with the sen- 

 timents of the Royal Academy of Paris in 1 705 ; with those of Thomas Cor- 

 nelius in his Progymnasma de Sensibus; of Dominicus Gulielmini in his 

 Treatise de Salibus; of Dr. Boerhaave in his chemistry, and many other 

 writers. 



Exper. 9. — In order to be convinced whether this salt was really ammonia- 

 cal, and of the nature of neutral salts, I mixed it with spirit of vitriol, and 

 spirit of salt, without producing the least fermentation. I afterwards put some 

 of it into oil of tartar per deliquium, but could not perceive any ebullition; so 

 that it is to be ranked among the neutral salts. 



