Land round Melbourne. 



233 



simply pulverised it is an excellent manure for the improve- 

 ment of sour soil, and is much valued as a plastic material 

 for artistic purposes. 



Gypsum is soluble in water to a certain extent only ; that 

 is, till the water becomes saturated with it, it being found 

 by experiment that 1 of gypsum will be dissolved in 400 of 

 water. When exposed to a heat of 424"^ Fahr.^ the water 

 entering into the composition of this remarkable mineral is 

 expelled by evaporation, and the gypsum becomes possessed 

 of a peculiar antipathy to any combination with water. If, 

 however, heated to 318^ only, it readily re-unites with water, 

 heat is evolved from it, and a dense crystalline mass appears 

 as the result of this combination. This (the efficient 

 degree of heat) is the secret of producing plaster of Paris 

 from the gypsum in its raw or crystallised state. 



In a purely scientific point of view these crystals possess 

 a considerable degree of interest. They occur both in 

 clino-rhombical foi^ms and polygonal columns. Twin or 

 double crystals, of which there are specimens now before the 

 Society, also exist, together with the single ones. These 

 remarkable combinations, presenting a varied field of obser- 

 vation to the ardent admirer of nature, are so perfectly 

 developed that the crystallographcr does not readily distin- 

 guish or individualise the twin parts of which they are 

 formed. 



These specimens are of a dirty greyish or soapy colour : 

 specific gravity equal to 2*40; and their chemical composi- 

 tion, calcium 33, sulphuric acid 46, water 21. That these 

 are still in course of formation in the manner which I have 

 attempted to delineate there can be but little doubt. This 

 supposition applies particularly to those places where fissures 

 and cavities are formed in the parched mud, by the intense 

 heat of the summer sun — the crystalising power having, in 

 such spots, abundant room for operation, besides obtaining 

 large supplies of sulphuretted hydrogen, derived from the 

 decomposition of organic matters on the surface. 



In order to exhibit a more succinct view of the process 

 which I have here endeavoured to describe, I have sketched 

 out the opposite classification of details. ( Vide plate,) 



In the same basalt formation in other parts of Victoria I 

 have found magnesite, opal, carbonate of iron, carbonate of 

 lime^ carbonate of strontium, mesotype, and steatite (soap- 

 stone). The component parts of all these minerals are held 

 in solution in common sea water, whence I am of opinion 

 that they derive their origin from the enormous evaporation 



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