124 British Association. 



Brewster expressed much surprise at hearing for the first time of this 

 theory of Poisson, and that he should feel much obliged to Mr. 

 Lubboclv if he would give some details of it in a separate communica- 

 tion to the Section ; and he had little doubt but that it would be as 

 new and as acceptable to many gentlemen as to himself. He thought 

 that the near apparent approach of distant objects in certain states of 

 the air, as mentioned by Professor Lloyd and Professor Stevelly, 

 might perhaps be accounted for by supposing that on these occasions 

 the intervening air became actually converted into a large magnifying 

 lens. 



Section of Chemistry and Mineralogy, Sept. 1 3. 



Dr. Apjohn next exhibited a new variety of Alum, upon the sub- 

 ject of which he had lately read a paper before the Royal Irish Aca- 

 demy.* This mineral, which he received from Mr. Atherton, an Afri- 

 can gentleman, was found on the eastern coast of the African conti- 

 nent, about midway between Graham's Town and Algoa Bay. It 

 occurs in fibrous masses very similar to asbestos, having a beautiful 

 satiny lustre, and splitting into threads which would appear to be 

 quadrilateral prisms. In taste, solubility in water, and relation to 

 several re-agents, it closely resembles ordinary alum, but is distin- 

 guished from it by containing protoxide of manganese instead of an 

 alkali, and by not assuming the octahedral form. In symbols it is 

 represented by 



(3 S + Al 0) + (S + Mn 0)-f 25 H O, 



3 2 3 3 



a formula identical with that which belongs to the entire genus of 

 alum salts. Dr. Apjohn briefly alluded to the other varieties of alum, 

 both those in which the alkalies replace each other, and those in which 

 the alumina is replaced by the deutoxide of iron, chrome, or manga- 

 nese ; and pointed out the theoretical possibility of an alum contain- 

 ing no metal but manganese. 



This communication gave rise to much discussion. Dr. Fara- 

 day stated, that a specimen of the mineral in question was given to 

 him in London, that he had found it to contain oxide of manganese, 

 and that on this account, and because of the absence of an alkali, he 

 hesitated to admit it as a true alum j and that supposing, as conjec- 

 tured by Dr. Apjohn, a double salt should be formed, in which the 

 alumina and alkali of ordinary alum were replaced by equivalent 

 quantities of the deutoxide and protoxide of manganese, he could not 

 admit it to be considered as an alum at all.^ — Dr. Clarke took up a 

 difl'erent ground, and objected to the term alum being applied to the 

 salt in question, inasmuch as it could not be made to assume the oc- 

 tahedral form. — To the first objection Dr. Apjohn replied, that he 

 considered the mineral he had examined to be an alum, because, ac- 

 cording to his analysis, its composition accorded with the general for- 

 mula for alum ; and to the second, that the other well-known alums 

 presented other difficulties of as great magnitude as respects the laws 

 of isomorphism. That e. g. soda alum crystallizes as an octahedron, 



♦ See p 203. 



