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evolution of heat which has been observed in solid sulphur imme- 

 diately after cooling. There are, however, no sufficient grounds for 

 this view, and some of the observations which I have given are de- 

 cidedly adverse to it. 1 . The change readily takes place, even at tem- 

 peratures at which sulphur becomes opake, in the opposite direction, 

 namely, from the octohedron to the oblique prism. 2. The melting- 

 point of the opake sulphur coincides too nearly with its point of soli- 

 dification for it to be supposed that this change in it has taken place. 

 On extracting melted sulphur which had become opake, with bisul- 

 phide of carbon, I have constantly found present traces of insoluble 

 matter, even where the greatest precaution had been taken to avoid 

 elevation of temperature ; and this opacity appears to me to be due 

 to the hardening of the viscid sulphur, and the consequent deposition 

 of opake matter in the pores of the crystals, which is quite sufficient 

 to account for it. It remains to ascertain the cause of the evolution 

 of the heat. On this point also I will offer a suggestion. It is well 

 known that the appearance of opacity is delayed by pouring the sul- 

 phur into cold water, and that the sulphur thus formed is at first 

 viscid and transparent, and only after a time becomes solid and opake. 

 The received view, I believe, is that the hard sulphur thus formed 

 is the solid form of the viscid sulphur, in the same sense as ice is the 

 solid form of water. It appears to me more probable that these two 

 sulphurs stand in a different relation, and that the change which 

 takes place on solidification is an allotropic transformation of the 

 viscid sulphur into the insoluble sulphur and one of the other modi- 

 fications. In the case of sulphur gradually cooled this change takes 

 place with rapidity, and, like other similar transformations, is attended 

 with a sensible evolution of heat. Where the sulphur is tempered 

 the change takes place very slowly, and the heat evolved is not per- 

 ceived. This view is confirmed by a fact which I have discovered, 

 namely, that the viscid sulphur possesses another solid form. I 

 have found that when sulphur, melted at a high temperature, is sud- 

 denly exposed to intense cold the cold of solid carbonic acid and 

 ether the sulphur formed is not viscid, but solid, hard, and perfectly 

 transparent. When the temperature is allowed to rise to that of the 

 air, the sulphur becomes soft and elastic. It is probable that this is 

 the true solid form of the viscid sulphur. 



