RtfraB'ion of Heai.—Arragonite, 3^7 



before tlie affiftant thermometer was placed | of an inch diftant on one fide, and, cbn- 

 fequently, was expofcd to the direfl: aftion of the heated cylinder. The fcreen was never 

 advanced before this laft. During eight minutes, the thermometer in the focus being un- 

 covered for the firft minute, covered for the fecond, uncovered for the third, and fo forth, 

 came fo its maximum by alternate greater elevations and fraaller deprefllons, and afterwards 

 loft its heat by greater depreffions and fmaller elevations. But the other thermometer being 

 out of the reach of refraftion, acquired its maximum of heat gradually, in confequence of 

 an uniform expofure to the heated cylinder, and afterwards began to decline. 



After the firft eight minutes, the affiftant thermometer was removed to one fide of the 

 focus, fo as to participate of the alternate fcreening, and alfo to receive a fmall fhare of one 

 fide of the invifible heat image, which muft be formed in the focus of the lens. Here, in 

 confirmation of the reafoning flowing from the general fails and doftrines, the afliftant 

 thermometer fhould be afFefted by alternate rifings and fallings, but lefs confiderablc than 

 thofe in the focus. The changes of the focal thermometer were for fix alternations in ten 

 minutes — | + 4- — i^ + I — ^ + i> ^nd thofe of the afliftant thermometer were 

 — I + \—^ + -^ — i + |, all which, as our author obferves, fo clearly confirm the effe£l: 

 of the refraition of the lens, that it muft be evident that there are rays ifli'uing from hot iron, 

 which, though in a ftate of total invifibility, have a power of occafioning heat, and obey 

 certain laws of refraftion, very nearly the fame with thofe that aff*e£l: light. From the 

 whole retrofpeft of his paper, he adds, that it will be eafy to perceive that he has made a 

 good proof, in this divifion of his memoir, of the three firft of his propofitions. 



VI. 



On the Arragonite of JVerner. By Cit. Hauy*. 



jL he mineral which is the obje<S of this Memoir prefents itfelf in the form of hexahe- 

 dral cryftals of a dirty, uneven, violet colour ; fome of them have their bafes fmooth, but 

 commonly tarnifhed, whilft others inftead of bafes have a multitude of angular proje£lions. 

 Their fpecific gravity is 2,9455 ; their refraftion double in a very ftriking degree. They 

 eafily fcratch carbonated lime, and appear to poflTefs the fame hardnefs as fluated lime ; their 

 powder thrown upon burning coals gives a violet coloured phofphorefcence. They are en- 

 tirely foluble with a lively efFervefcence in nitric acid. They are found, in Spain between 

 the kingdoms of Arragon and Valencia, which is the reafon of their being called Arrago- 

 nites, by Werner, There are fome alfo to be met with near the Pyrenean Mountains. 



Their phofphorefcence, together with their colour, induced Born to fufpeft the prefence 

 of phofphate of lime ; but Klaproth found only carbonate of lime. 



• Bulletin de la Soc, Philomath, No, 33. toirie i. 



- Haujt 



