160 PROPERTIES OF GLASS. 



The reason given for these phenomena is the swelling of the 

 tubes towards the fire by the heat, which is known to expand all 

 bodies. For, say the adopters of this hypothesis, granting the ex- 

 istence of such a swelling, gravity must pull the tube down when 

 supported near its extremities ; and a fresh part being exposed to 

 the fire, it must also swell out and fall down, and so on. Unf, 

 without going farther in the explanation of this hypothesis, it 

 may be here remarked, that the fundamental principle on which 

 it proceeds is false: for though fire indeed makes bodies expand, 

 it does not increase them in weight ; and therefore the sides of 

 the tube, though one of them is expanded by the fire, must still re. 

 main in equilibrio ; and hence we must conclude, that the cause of 

 these phaenomena remains yet to be discovered. 



4. Glass is less dilatable by heat than metalline substances ; and 

 solid glass sticks are less dilatable than tubes. This was first dis. 

 covered by Colonel Roy*, in making experiments in order to re- 

 duce barometers to a greater degree of exactness than hath hitherto 

 been found practicable ; and since his experiments were made, one 

 of the tubes eighteen inches long, being compared with a solid glass 

 rod of the same length, the former was found by a pyrometer to ex- 

 pand four times as much as the other, in a heat approaching to that 

 of boiling oil. On account of the general quality which glass has of 

 expanding less than metal, M. de Luc recommends it to be used in 

 pendulums: and, he says, it has also this good quality, that its ex- 

 pansions are always equable and proportioned to the degrees of 

 heat ; a quality which is not to be found in any other substance yet 

 known. 



5. Glass appears to be more fit for the condensation of vapours 

 than metallic substances. An open glass filled with water, in the 

 ummer time, will gather drops of water on the outside, just as far 

 as the water in the inside reaches ; and a person's breath blown on 

 it manifestly moistens it. Glass also becomes moist with dew, 

 when metals do not. 



6. A drinking.glass partly filled with water, and rubbed on the 

 brim with a wet finger, yields musical notes, higher or lower as the 

 glass is more or less full, and will make the liquor frisk and leap 

 about. 



7. Glass is possessed of extraordinary electrical virtues. 



Phil. Trans, vol. Uxii. p. 608. 



