1823.] Royal Society. 151 



this earth in the oolites has not been ascertained with certainty, 

 and it has not yet been discovered in the chalk of this country ; 

 though Brononiart has detected it in that near Paris; and he 

 ascribes the sterility of Champagnes, to its presence in the chalk, 

 of that province. Magnesia, also occurs in the tertiary forma- 

 tions which, in many countries, succeed the chalk; its presence 

 in the London clay is shown by the efflorescence of its sulphate 

 from the bricks which are made of it, and the springs of Epsom 

 mav also be adduced in proof of this : it has likewise been found 

 in the calcaire grossier of France, and in a specimen of the same 

 formation containing nummulites, from near Verona. 



After some remarks on the importance of the subject, in its 

 connexion with building and with agriculture, from the modifica- 

 tion by the magnesia of the characters of the rocks into which it 

 enters, and from the effect of that earth upon vegetation; 

 Dr. D. proceeded to describe the chemical methods he employed 

 in his examination of the magnesian rocks. He stated that our 

 object be merely to detect the presence of magnesia in a lime- 

 stone, we may adopt Dr. Wclkiston's process, by carbonate of 

 ammonia and phosphate of soda; or that in which oxalate of 

 ammonia is used in conjunction with those salts ; but these can- 

 not be employed in ascertaining the quantity of it which may 

 exist ; for in both of them a portion of magnesia is thrown down; 

 and in the first, apart of the lime remains unprecipitated. For 

 the last mentioned purpose, Dr. D. suggested the following pro- 

 cess, for the details of which he referred to his paper on the 

 subject lately published in the Edinburgh Philosophical.! ournal. 

 Dissolre the substance under examination in nitric acid, evapo- 

 rate the solution to dryness, so as to expel the nitric acid ; then 

 dissolve the lime and magnesia in acetic acid, and separate them 

 by converting them into sulphates : the only objection to this 

 method of separating the two earths is, that by too great a heat, 

 part of the acid may be driven off from the sulphate of magnesia, 

 and so the proportion of that earth underrated ; or, in consequence 

 of not employing sufficient heat, some water may remain in the 

 salt, and thus the quantity of magnesia maybe overrated. 



The paper concluded with a catalogue of the secondary rocks 

 in which the author has found magnesia, with some account of 

 their external characters ; the primitive rocks were omitted 

 because the presence in them of the earth in question is so well 

 known. Some of the characters of magnesian limestones afford a 

 presumption of their nature ; their colour varies from honey- 

 yellow to saffron-red, and their fracture presents small crystalline 

 points, having a glimmering lustre : slight effervescence with 

 acids, of course, is a test only when the proportion of magnesia 

 is large ; but it is a sufficiently accurate one for agricultural 

 purposes. 



At this meeting likewise, the following paper was read : 

 — Corrections applied to the great Meridional Arc extending 

 from Latitude 8° W 38-39", to Latitude 18° 3' 23-64" to reduce it 



