WITHDRAWN FROM THE ACTION OF GRAVITY. 249 



them projected upon two rectangular vertical planes ; and it is seen that the 

 aspects of the system observed upon tAvo adjacent sides are inverse as regards 

 each other. The formation of this system presents a curious peculiarity. At 

 the commencement of the operation all the faces of the octohedron become 

 simultaneously hollow; the layers in progress of formation are plane, and 

 arranged symmetrically, so that the system tends towards the form repi'csented 

 at Fig. 20. But when a certain limit is attained, a sudden change occurs, the 

 layers become curved, and the system tends to assume the singular form which 

 we have mentioned. I have several times repeated the experiment, varying 

 the circumstances as much as possible, and the same effects are always pro- 

 duced. 



In the course of this memoir I shall point out another process for obtaining 

 laminar systems ; it is an extremely simple one, and has moreover the advan- 

 tage of producing all the systems in a complete state. 



36. In concluding our observations upon polyhedric liquids, I shall remark 

 that the triangular prism may be employed to produce the phenomena of dis- 

 persion. In this way a beautiful solar spectrum may be obtained by means of 

 a prism with liquid faces. But as the effect only depends upon the excess of 

 the refracting action of the oil above that of the alcoholic liquid, to obtain a 

 considerably extended spectrum the angle of refraction of the prism must be 

 obtuse; an angle of 110° gives a very good result. Moreover, it is evidently 

 requisite that the faoes of the prism should be perfectly plane, which is obtained 

 by using a carefully made frame ; by establishing exact equilibrium between 

 the density of the liquids ; and, lastly, by arresting the action of the syringe 

 exactly at the proper point. 



