Kotes on Barometer Construction. S^ 



sudden mechanical shocks, or the slightest scratch upon its 

 strained inner surface. But glass may be in a condition of 

 high tension and may at the same time possess very marked 

 properties of permanence. If we optically examine vessels 

 of De la Bastie's touohened fflass we find them showinoj in a 

 beam of polarised light the black cross indicative of strain, 

 and we know that these specimens of glass will resist 

 mechanical shocks of great violence, and that they have 

 some other marked properties conducive to permanence; 

 but if sufficient external force for the fracture of one of these 

 vessels be employed, it does not simply break as annealed 

 glass would break, but goes off with a report and is shattered 

 throughout into a complete ruin of small particles. The 

 "Bologna vial" and the "Prince Rupert's drop"* are 

 each permanent in this sense, and each under proper 

 conditions liable to disruption ; and, in fact, we have to 

 distinguish between irregular and symmetrical strain in- 

 order to gain a clear insight into the question of fracture 

 of glass tubes, especially fracture due to imperfect annealing. 

 Just as the Bologna vial is safe as long as you hammer its 

 external surface, but flies into fragments as soon as you 

 scratch ever so slightly its strained interior surface, which 

 has cooled and contracted after the exterior layers have 

 become solid, so a large proportion of the glass tubes found 

 in commerce are permanent enough as long as we do not 

 suddenly heat them, and so long as we do not bring hard 

 substances in contact with their inner surfaces. Experience 

 has taught the glass manufacturer that, unlike pieces of com- 

 plex form, thick glass tubes with little annealing, and thin 

 glass tubes with none at all, or next to none, are sufficiently 

 permanent to serve most of the purposes of commerce. Take 

 a stout glass barometer tube and pass through it an iron wire 

 so as to rub the inner walls of the tube with the latter, the 

 chances are great that after this treatment the tube will very 

 soon crack ; indeed it is unsafe to touch the interior surfaces of 

 stout glass, tubes Avith iron at all, as no instrument made 

 with tube thus treated will be afterwards reliable. Regard 

 the inner surfaces of your glass tubes as possessing in degree 

 the physical properties of the inner surface of the Bologna 



* The latter are called by the French " Larmes Bataviqne ;" concerning 

 the properties of which bodies the reader is referred to an interesting 

 memoir by M. Victor De Luynes in the Annales de Cliemie et de PhysiqiWi 

 3rd series, Vol. XXX. p. 289. 



