Scientific Intelligence — Mineralogy. 183 



enormous masses of gypsum and dolomite met with among the 

 Andes, that he has formed a theory to account for their presence. 



•' It follows, then, frem the analyses and geological observations 

 I have made, tliat if the waters be, as is generally believed, the 

 proximate cause of goitre and cretinism, we may refer the deleterious 

 action of the waters to the salts of magnesia, or, perhaps, to the 

 presence of the magnesia, and the absence, at the same time, of a 

 quantity of lime sufficient for the wants of the economy. Analysis 

 is required to solve this problem. 



" I have pointed out a preservative means in my Memoir. This 

 consists in separating the magnesia, by making the water pass 

 through filters, or into large reservoirs filled with carbonate of lime, 

 and a thin layer of lime." — (VInstitut, No. 772, p. 319.) 



MINERALOGY. 



3. Cause of Irised Colours on Minerals. — From M. Hausmann's 

 valuable Memoir, sent to us, on this branch of optical mineralogy, 

 we gather that the irised colours on minerals, like that on steel, are 

 duo to a thin film covering the surface ; and that the colours are 

 varied by a variation in the thickness of this film. It is often pro- 

 duced by a chemical change in the surface of the mineral, and some- 

 times by deposition of a foreign substance. Hydrated oxide of iron 

 is one of the most common of the substances that communicate irised 

 hues. This compound results from the decomposition of pyrites 

 either forming first a carbonate which is common in many waters, 

 and then by the evaporation of the water yielding the hydrate, or 

 forming the hydrate direct. The colours on anthracite and specular 

 iron often proceed from this source ; and an exposure of the latter 

 species to water containing the carbonate, afforded Hausmann, after 

 a while, irised specimens. 



Arsenic becomes irised through the action of hydrogen from the 

 atmosphere ; bismuth^ by a superficial oxidation ; arsenical cobalt, 

 nickel, and iron, by oxidation ; galena, probably from the formation 

 of a thin coating of sulphate of lead ; magnetic iron, and some ferru- 

 ginous silicates (as olivine, yenite, &c.), from a change in the oxide 

 of iron to a hydrate ; pyrites, from the formation of a hydrate of iron ; 

 copper pyrites and variegated pyrites, probably from the same, the 

 latter being very remarkable for the rapidity with which the change 

 takes place in a moist atmosphere ; antimony glance, and other an- 

 timony ores, from the formation of antimony ochre ; fahlerz, and 

 other arsenical ores, probably from the oxidation of the arsenic. 



These irised colours sometimes proceed from the absorption of 

 oxygen and the elimination of water, or from a disengagement of 

 carbonic acid with a loss of water, as in spathic iron and carbonate 

 of manganese. 



Irisation is often favoured by heating, as in the case of steel. 



