86 Mr Haidingcr on the Parasitic Formation of Minerals, 



form, especially in settled serene weather, attended by a high 

 barometer, and the air in a positive state of electricity. On 

 the contrary, if these clouds increase in size, the upper surface 

 takes the shape of the cumulus, which swells very rapidly in 

 size, becoming very dense, and of a most brilliant white colour 

 on the side exposed to the sun. All the smaller neighbouring 

 clouds are attracted by the larger. But the increase of capa- 

 city for electricity, which keeps pace with the increase of va- 

 porous surfaces exposed to air, cannot receive a supply from 

 the earth, and but very slowly from the air. At length its rela- 

 tive state of electricity, as compared with^the earth's surface, is 

 of sufficient intensity to overcome the resistance of the plate of 

 air between the earth and cloud, and the discharges of electric 

 matter pass upwards. This opinion is offered as the result of 

 many years observation, and as an humble attempt to explain 

 one of the causes which produce negatively electrified clouds, 

 and those local thunder storms which sometimes prevail for se- 

 veral weeks together in the summer months. 



- 



Art. XIV. — On the Parasitic Formation of Mineral Species, 

 depending upon Gradual Changes which taJ^e place in the 

 Interior of Minerals, while their External Form remains the 

 same. By William Haidinger, Esa. F. R. S. Edin.— 

 ( Concluded fro7n last Number, p. 9Q%) 



IX. Changes in some of the Earthy Minerals and others. 



The explanation of many of the cases enumerated above, de- 

 pends upon the ordinary laws, active in our chemical laborato- 

 ries. Carbonates are changed into sulphates, metallic sub- 

 stances are oxidized, copper is replaced by iron : in general 

 weaker affinities give way to stronger ones. The conversion 

 of sulphates into carbonates, and other cases, may perhaps de- 

 pend upon some process of mutual decomposition, in which 

 one of the products has been subsequently removed ; but the 

 specimens preserved in collections do not usually present any 

 explanation of the facts which they furnish. We must en- 

 deavour to ascertain the causes which have contributed towards 



