PKOCEEDINGS OF GEOLOGICAL SOCIETIES. 



181 



It is with lime especially, and its compounds, that we have to do. Lime 

 is composed of one equivalent of calcium and one of oxygen. You are all 

 familiar with the properties of lime — an amorphous body, white, or more 

 or less coloured with impurity, and which, on the application of water, 

 slacks ; that is, it absorbs water, the water enters into combination with 

 it, and becomes solid, and on its passing from the liquid to the solid state, 

 a large amount of heat is evolved. The lime falls to pieces,* having become 

 hydrated, or, in common language, slacked. It is slightly soluble in water, 

 as every one knows, producing the well-known liquid called lime-water. 

 When combined with water — for it must be hydrated — it unites readily 

 with carbonic acid, forming carbonate of lime. 



Carbonate of lime is a mineral which occurs very extensively in nature, 

 forming beds of chalk, limestone, and two very important minerals, namely, 

 calcite and arragonite. Carbonate of lime is a compound of oneequivalent 

 of carbonic acid and one of lime. It is known in three distinct states. 

 There is, first, the amorphous or chalk-like state : we will call it chalk. It 

 is perfectly non-ciwstalline or amorphous. There is, then, the form of 

 arragonite which occurs in prismatic crystals, and belongs to the prismatic 

 system. The third form of carbonate of lime is that of calcite or calcspar, 

 which is rhombohedral, crystallizing in the beautiful rhombohedral crystals 

 with which every mineralogist is familiar. Arragonite varies in specific 

 gravity from 2'93 to 3'01. This is a point to note. The calcite has a lower 

 specific gravity, ranging from 2'69 to 2*75, so that not only in their crys- 

 talline system, but also in specific gravity, are these two minerals distin- 

 guished clearly from each other. 



The next point is the solubility of carbonate of lime. In treating this 

 subject of chemical geology, the lecturer was selecting all those points 

 which lie conceived had a direct geological bearing ; and it is requisite to 

 pay rather close attention, which, perhaps, may be considered tedious, to 

 this part of our subject. One part of carbonate of lime dissolves in 110,000 

 parts of pure water, in round numbers : it is 110,132 parts really. This car- 

 bonate of lime dissolves to a much greater extent when carbonic acid is 

 passed through the water, and it then forms what is termed bicarbonate of 

 lime. One part of carbonate of lime dissolves in 998 parts of water con- 

 taining carbonic acid, according to Bischoff, the carbonic acid being passed 

 through the water for an hour. He has made several experiments upon 

 this subject, which are remarkable. He finds that the solubility varies to 

 a great extent with the nature of the carbonate of lime operated upon. 

 Thus, 11*15 parts of chalk were dissolved in 10,000 parts of water by pass- 

 ing carbonic acid through for an hour. He performed this experiment 

 three times, and each time he obtained pretty nearly the same result ; 

 but, when he tried the experiment with carbonate of lime precipitated from 

 a salt of lime, passing the carbonic acid through for about the same time as 

 he did in the experiments with the chalk, he found that 28 parts dissolved 

 in 10,000 parts of water. There is another very striking statement, but 

 which will require further corroboration. It is, that burnt muscbelkalk 

 dissolved to the extent of 135 3 parts in 10,000 parts of water, by passing 

 carbonic acid through the water for an hour and a half. 



In passing, he would mention the fact, that when arragonite is exposed 

 to a red-heat, it falls to powder ; and it was supposed for a long time that 

 this powder consisted of minute rhombs of calcite. This, however, is 

 denied by Gustave Eose, who contends that the powder is strictly amor- 

 phous. 



Let us now consider the mode of formation of arragonite, or the condi- 



