Mechanical Science, 8^c. 167 



fleeting light, and, therefore, appeared quite black. At first, it 

 was supposed that a thin film of opaque and finely-divided matter 

 had insinuated itself into a fissure of the crystal; but this opinion 

 was soon overturned, an'l Dr. Brew^ster concluded that the 

 efifect was due to the surfaces being composed of short slender 

 filaments of quartz, whose diameter was so exceeding small, that 

 they were incapable of reflecting a single ray of (Jie strongest 

 liglit. The surfaces were perfectly transparent to transmitted light ; 

 no detergent substances had any effect on them, nor had hot acids ; 

 but when immersed in oil of anise seeds, a substance, which ap- 

 proaches to quartz in its refractive power, the blackness disap- 

 peared, and the piece of quartz behaved like any other piece of 

 quartz. Upon removing the oil, the original state was restored, 

 and the filamentous or velvety nature of the surface was rendered 

 evident to the eye, by the slight change of tint produced by pres- 

 sing the filaments to one side. Dr. Brewster concludes that the 

 thickness of the filaments cannot exceed one-third of the one- 

 millionth part of an inch, or one-fourth of the thinnest part of a 

 soap-bubble. — Edin. Jour. Science^ i. 108. 



1 1 . Instrument for Examinations under Water. — An optical in- 

 strument for seeing through water, and exploring the bottom of 

 rivers, has been constructed by Mr. Leslie, of Lausenburgh, U. S. 

 It consists of a conical tube of variable length, about one inch 

 broad at the top, and ten inches at the bottom. It is glazed at 

 both ends, and when the broad end is immersed to some depth 

 in water, and the eye applied to the narrow extremity, there is no 

 interruption to, or deflection of the rays of light coming from ob- 

 jects in the water to the eye, and,,if the water be clear, things in it 

 may be seen with great facility. For use in the night, or in other 

 circumstances, it is fitted with lamps, suspended near the bottom, 

 in a shorter outer cylinder sliding on over the tube, and secured 

 at its lower extremity ; the mouth of this cylinder is glazed, so 

 that the light of lamps, placed in it, is thrown into the water, and 

 illuminates objects in it. Two tubes go to this close cylinder, one 

 entering at the bottom, the other at the top ; the one carries fresh 

 air down, the other conveys the smoke and foul air upwards. The 

 instrument is very useful in the speedy recovery of drowned bodies, 

 or of lost property ; in examining the beds of rivers, or other situa- 

 tions under water, to facilitate excavation, and on many other 

 occasions. 



12. Preservation of Copper-plates. — Dr. Mac Culloch has pointed 

 out the great injury done to fine engravings on copper when they 

 are laid aside, from oxidation. In large and expensive works, 

 only the impressions immediately required are printed, and the 

 plates are laid aside ; alter some time, they are re-worked, wheti 

 it is necessary first to remove the film of oxide which has formed ; 



