128 
DR. WALTER WAHL; PHYSICO-CHEMICAL DETERMINATIONS 
slightly, and thus transmits the pressure to the glass body in as even a manner as 
possilde. 
Originally the washers consisted of two flat rings about 0'5 mm. thick each, the 
one on wliicli the glass rests directly being of ivory, the other one of flbre. The 
conical washer for temperatures up to about 70° C. has been of “ galalith,” a material 
used instead of ebonite for electric fittings, and for higher temperatures of fibre. 
With this arrangement of the windows a great many of the measurements up to 
about 1,600 kg./cin.^ have been carried out, and Avith some practical experience as to 
the way in which the glasses are fitted it is not difficult to get a pressure-bomb of 
this kind for optical investigations in working order. The glass used has chiefly been 
a borosilicate froni Schott & Genossen, of Jena, which is used for the manufacture of 
“ Durax-glass ” liigh-pressure-and-temperature tid)ing. Suital Je pieces of the glass 
were cut from glass rods, the pieces annealed very carefully, and the cones then 
ground from these pieces. When overstrained, a 20 mm. thick cone of this glass 
shoAvs interference colours of the first order at about 200 atmospheres beloAA^ the point 
at Avhich it breaks. It is always necessary to Avatcli tlie glass windoAvs in polarized 
light Avhen pressure is applied to tliem the first time after they liaAm been fitted into 
the steel bomb, as tlie appearance of inteiference colours at once suggests that the 
fitting is not satisfactory, for glass cones Avhich stand pressures of seAmral thousand 
atmospheres break at a feAA" liimdred if precaution is not taken to Avatch their 
behaAuour in ^^olarized hglit, and to refit them if they shoAv interference colours 
already at comjDaratively Ioav pressures. Later it Avas found tliat tlie limit of 
1,600 kg./cm.^ mentioned above, Avas not really due to the resisting strength of the 
glass being reached at that pressure, but to the circumstance that the ivory Avashers 
began to Aoaa^ readily at this pressure, and in consequence of the structure of the 
bone did not give way eAmnly, Avith the result that the support suddenl}^ became less 
in some parts than in others, and the glass cracked. Later, Avashers made out of tAvo 
layers of thin lead foil have been used, and also AAaashers made out of rolled plates 
