3 64 Mr. Belaval on the Caufe of the 



rays of light, 1 procured fmall 

 vials of flint glafs, whofe form 

 is a parallelepipid; the height 

 of thefe, exclufive of the neck, 

 is about two inches, and the 

 bafe about one inch fquare j 

 their necks are cylindrical, 

 and are about two inches in 

 length. 



I covered the bottom, and 

 three of the fides of each of 

 thefe vials, with a black var- 

 nilh ; the cylindrical neck. A, 

 and the anterior fide, B, ex- 

 cept at its edges, were left 

 uncovered. 



I carefully avoided leaving 

 anv crevices in the varnifh, 

 that no light might be admit- 

 ted, except through the neck, 

 or the anterior fide, of the 

 vials. 



In thefe experiments, both 

 the inner and exterior fur- 

 faces of the glafs mud be 

 perfeftly clean; and thofe liquors, which are 

 apt to depofit a fediment on the bottom or fides 

 of the veflfels which contain them, tnuft be put 

 into the vials, only when the examination of 

 t^em is intended : thofe, which arc not fubjedl 



to 



