738 



excess of caustic potash, passing carbonic acid gas through the solu- 

 tion to saturation, and concentrating the solution : evernesiate of 

 potash crystallizes out. From this the acid may be separated by 

 means of muriatic acid. It gives a yellow colour with hypochlorite 

 of lime. 



Formula of hydrated acid Cig Hg 0--f HO 



Formula of evernesiate of baryta . . Cig Hg + BaO + Aq. 

 Formula of evernesiate of silver .. CigHgOj + AgO. 



Orcin. 



This substance is always obtained when any of the colouring prin- 

 ciples of the lichens which yield red dyes with ammonia are subjected 

 to particular processes. The best way of obtaining it pure is to boil 

 the alpha-, or beta-orsellesic acid, or the erythrelesic acid in water 

 for about an hour. Carbonic acid is given off, and crystals of co- 

 lourless orcin are deposited. It gives a dark purple red colour with 

 hypochlorite of lime, quickly changing into deep yellow. 



Empirical formula CigHu 0-. 



Brom-orceide, Ci6H24Br O13 (empirical), is obtained by pouring bro- 

 mine into a concentrated aqueous solution of orcin ; when pure it 

 forms long Vv^hite adhering needles ; it has no taste or smell. 



Chlor-orceide, a similar compound, is obtained by passing chlorine 

 gas through a solution of orcin. 



Usnic Acid. 



This principle is found in Usnea florida, U. hirta, U. plicata, U. 

 harhata, Ramalinea caUcaris, R. Fraxinia, Evernia Prunastri, and Cla- 

 donia Rangefer'uia. It is best obtained from Cladonia Rangeferina 

 and Usnea florida, by the use of lime and muriatic acid. 



Its empirical formula is Cgs H^ O14. 



Februar}^ 17, 1848. 

 GEORGE REN2n'IE, Esq., Treasurer, in the Chair. 



" On a Formula for the Elastic Force of Vapour at different 

 Temperatures." By Captain Shortrede. Communicated by Lieut-. 

 Col. Sykes, F.R.S. 



The author adopts as the basis of his formula the first series of 

 experiments at high temperatures made by the French Academy, 

 and those of Magnus at low temperatures. For the Academy's 

 experiments, he adopts the indications of the smaller thermometer in 

 the steam in preference to those of the larger thermometer in the 

 water. Of Dr. Young's sort of formulae, he notices that of the Aca- 

 demy and several others with exponents varying from 5 to 7. From 

 the elasticity at freezing, as given by Magnus, compared with four 

 of the Academy's experiments, he shows that for the range of obser- 



