242 optics. 



5. Fir-wood exposed to the focus under water 

 did not seem changed, but when broken, the in- 

 side was burnt to coal. 



6. If a cavity was made in a piece of charcoal, 

 and the substances to be acted on were put in it, 

 the effect of the lens was much increased. 



7- Any metal thus enclosed in the cavity of a 

 piece of charcoal melted instantly, the fire spark- 

 ling like that of a forge. 



8. The ashes of wood, paper, linen, and all ve- 

 getable substances were turned into a transparent 

 glass. 



9. The substances most difficult to be wrought 

 on were those of a white colour. 



10. All metals vitrified on a China plate, when 

 it was so thick as not to melt, and the heat was 

 gradually communicated. 



11. When copper was thus melted, and thrown 

 quickly in cold water, it produced so violent a 

 shock, as broke the strongest earthen vessels, and 

 the copper was entirely dissipated. 



Though the heat of the focus was so intense as 

 to melt gold in a few seconds, yet there was no 

 heat at a small distance therefrom ; and the finger 

 might be placed in the cone of rays within an inch 

 of the focus, without receiving any hurt. 



Mr. Parker had the curiosity to try what the 

 sensation of burning at the focus was ; and having 

 put his finger there for that purpose, he says, it 

 neither seemed like the burning of a fire, nor a 

 candle, but the sensation was that of a sharp cut 

 with a lancet. 



By means of an ordinary burning glass, a piece of 

 wood maybe charred or burnt to a coal, in a decanter 

 of water, and yet the sides of the decanter, through 

 which the rays pass so very near the focus, will not 



