TRANSACTIONS OF THE SECTIONS. 27 



On a Specimen of Tin-ore hitherto urulescribed. By Fredeeick Field. 

 The author, some years since, piu'chased a specimen of what was called on the 

 label "Slime Tin." In a matrix of quartz and fiuor are disseminated streaks of 

 cassiterite, and in immediate connexion with these are found similar streaks of an 

 earthy-loolring matter of a yeUowish-brown colour. This matter is rather soft, 

 and can be easily cut with a knife. Whilst an-anging the mineralogical collection 

 of the Bath Literaiy and Scientific Institution, he met with two specimens thus 

 labelled : — "Tin Ore, new variety, Cornwall." The first of these (the gift of Mr. 

 Fox) is a mass of a duU earthy appearance, yellowish brown in colour, and so hard 

 in many places as to be with difficulty scratched with a file. Its specific gravity 

 is 4-4. On examining with a lens, there are to be found, in some parts, specks of 

 cassiterite and portions of what appears to be felspar. The second specimen (the 

 gift of Colonel Page) is darker in colour than the first, but of equal hardness. Its 

 specific gravity, however, is only 3'6, and from it the author could obtain no indi- 

 cations of tin. With regard to these specimens, the author thinks that the slime 

 tin is a variety of what is called " toad's-eye wood-tin," and Mr. Fox's specimen 

 is this form of cassiterite mixed with much earthy matter. As for the specimen of 

 Colonel Page, it has no claim whatever to be called an ore of tin. 



On the Artificial Production of Anhydrite. By Alphonse Gages. 

 The circumstances imder which natural anhydrite is formed have not yet been 

 well determined. The experiments already made, and bearing upon the various 

 properties of sulphate of lime, and its behavioiu at various temperatures, although 

 presenting gi-eat interest, have not yet given crystfiUized anhydrite under the same 

 conditions as it occurs in the saliferous formations. Gay-Lussac had already observed 

 that overheated gypsum partially lost its well-known properties, and he attributed 

 the cause of it to the partial formation of anhydrite. The experiments of Graham 

 proved, also, that gjpsum passes into a kind of amorphous anhydrite at a tempera- 

 ture of 204° C. At the Meeting of the British Association in Dublin, Dr. Sullivan 

 gave an account of some experiments by which he obtained anhydrous sulphate of 

 lime from its solution in water, at a temperature of about 300° C. 



A mixture of gypsum and common salt fused in a crucible, and treated by water, 

 leaves undissolved prismatic lamellar crystals of anhydrite. Left for manj^ days in 

 water, the quantity of moisture absorbed did not exceed that which some varieties 

 of natural anhydiite would have absorbed if placed under the same circumstances. 

 I give here the results obtained : — 



Water 0472 



Lime 39-531 



Sulphm-ic acid 60-142 



100-145 

 Gypsum melted with anhydrous sulphate of soda gives also crystals of anhydrite ; 

 but the formation of anhydrite in sulphate of soda requires a higher temperature ; 

 the crj'stals of anhydrite, separated by water from the sulphate of soda, were, how- 

 ever, perfectly anhydi-ous. Analysis gave the following, namely : — 



CaO 40-952 



So3 59 085 



100 037 

 Mitscherlich fused gj'psum at the highest temperature of the porcelain furnace, 

 and obtained a white crystalline mass of anhydrite. The occuiTence of anhydrite 

 amongst rocks of an undoubted sedimentary origin, and the necessity of a tempera- 

 tm-e higher than melted lava, have been the gTeat argument employed to prove the 

 impossibility of anhydrite having been formed by fusion. If we could consider 

 melted chloride of sodium as the solvent in which anhj'drite has been produced, 

 the problem would be solved at once, as gypsum dissolves and ciystallizes in com- 

 mon salt at a temperature far below melted lava, and not rising above a dull red 

 heat — a temperature to which local circumstances may give rise ; at such tempera- 

 tui'e the liqiud mass would possess a great fluidity. 



