114 FKAGMENTS OF SCIENCE. 



long to permit of its thorough examination. The light 

 discharged from the cloud, at right angles to its own 

 length, was at first perfectly polarised. It could be 

 totally quenched by the Nicol. By degrees the cloud 

 became of whitish blue, and for a time the selenite 

 colours, obtained by looking at it normally, were ex- 

 ceedingly brilliant. The direction of maximum polari- 

 sation was distinctly at right angles to the illuminating 

 beam. This continued to be the case as long as the 

 cloud maintained a decided blue colour, and even for 

 some time after the blue had changed to whitish blue. 

 But, as the light continued to act, the cloud became 

 coarser and whiter, particularly at its centre, where it 

 at length ceased to discharge polarised light in the di- 

 rection of the perpendicular, while it continued to do 

 so at both ends. 



But the cloud which had thus ceased to polarise the 

 light emitted normally, showed vivid selenite colours 

 when looked at obliquely, proving that the direction of 

 maximum polarisation changed with the texture of the 

 cloud. This point shall receive further illustration 

 subsequently. 



A blue, equally rich and more durable, was obtained 

 by employing the nitrite-of-butyl vapour in a still more 

 attenuated condition. The instance here cited is rep- 

 resentative. In all cases, and with all substances, the 

 cloud formed at the commencement, when the pre- 

 cipitated particles are sufficiently fine, is blue, and. it can 

 be made to display a colour rivalling that of the purest 

 Italian sky. In all cases, moreover, this fine blue cloud 

 polarises perfectly the beam which illuminates it, the 

 direction of polarisation enclosing an angle of 90 with 

 the axis of the illuminating beam. 



It is exceedingly interesting to observe both the 

 perfection and the decay of this polarisation. For ten 



