187 



as they show what circumstances require particular attention, in 

 comparing even artificial crystals. In this part he proves, that the 

 sooner crystals are measured after their deposition the better; that 

 the solutions should be placed under precisely similar circumstan- 

 ces ; that even the position of the crystals in the vessel in which 

 they are formed, and the length of time that they have remained in 

 contact with the mother-liquid, are circumstances of great import- 

 ance, since even the system of crystallization may be changed 

 by the ktter. The author concludes by the consideration of two 

 objections, drawn from observations made on artificial salts. The 

 first of these is the well-known objection by Dr Clark of Aber- 

 deen. This he refutes on the ground that the case in question is 

 not a strict case of isomorphism, as the term is employed by Mit- 

 scherlich ; and he brings forward abundant proof in support of this 

 fact. The last objection is that which refers to the isomorphism of 

 potass and ammonia; here, however, he shews that the diflBculty 

 falls to the ground, if the new and evidently more correct notion 

 regarding the constitution of ammoniacal salts be substituted for 

 the older opinion. 



Monday, 5th February. 



Sir GEORGE MACKENZIE, Bart., in the Chair. 

 The following Donations were presented. 

 Comptes Rendus Hebdomadaires des Seances de I'Academie des 



Sciences, (1837, 2™= Semestre), No. 26, et (1838, I" Se- 



mestre), Nos. 1, 2— By the Academy. 

 Bulletin de la Societe de Geographie, 2"^ Serie, Tome xiu—Bythe 



Society. 

 Bulletin de la Societe Geologique de France. Tome viii. Fenilles 



21-25 By the Society. 



Journal of the Asiatic Society of Bengal for June 1837 — By the 



Society. 



The following Communications were read. 



1. Observations on the Parr and Young of the Salmon. 



By Dr Parnell. 



2. Notice of the remarkable Mathematical Properties of a 



certain Parallelogram. By John Scott Russell, M. A., 

 F.R.S.E. 



