DEVELOPMENTS IN GLASS INDUSTRY 147 



thermometer has been used to measure a high temperature. 

 Experiments showed that the simultaneous presence of pot- 

 ash and soda is particularly responsible for a large error 

 of this character. Finally, the brand known as Jena nor- 

 mal thermometer glass 16 III was evolved, and this is now 

 regularly manufactured in uniform quality. It is com- 

 monly formed with a red streak to render it readily dis- 

 tinguishable. Its coefficient of expansion is so nearly alike 

 to that of platinum that the metal can be fused into it, 

 making a gas-tight joint. Another special thermometer 

 glass is the boron glass 59 III, which is particularly well 

 adapted for high temperature thermometers ranging up to 

 500 C. This glass is even superior to the first mentioned, 

 having a lower coefficient of expansion. This, however, 

 brings with it the disadvantage that a gas-tight joint with 

 platinum cannot be made through it by fusion. 



In conclusion it may be said that the principal char- 

 acteristic of the advances made at the Jena works during 

 the last twenty-five years consist in the production of 

 special glass adapted for definite purposes. This was 

 rendered possible only by greatly increasing the variety of 

 types of glass prepared and by the utilization of elements 

 which previously had not been employed in glass making. 

 Nevertheless, had the demand been exclusively for glasses 

 for scientific purposes, this would never have been sufficient 

 to enable the manufacturer to meet the requirements and 

 at the same time secure a margin of profit. The com- 

 mercial possibility of the developments which have taken 

 place necessarily rested on their exploitation for general 

 technical purposes, as for example in the manufacture of 

 the Jena incandescent gas-light chimneys. 



