( 603 ) 



consists of' silica and inclal-oxides. We may sii]»|)()sg thai llie silira, 

 wliicli is most likely to bo the principal part, will also have a pre- 

 valent influence on the physical charactei-s of such natural i^lass. 



A determination of the specific gravity of the glass is made nioi-e 

 difficult by the [)resence of many gas-bubbles. If this obstacle did 

 not exist, the specific weight would be a better e.\|)edient for a 

 (piick tem|)orary oi-ientation than the determination of the i-efractive 

 index, for which more instruments are necessary. With respect to the 

 specific gravity it could be stated, that \vith these rocks where the 

 value of the index the use of bi'omofoi-ni as liquid of comparison neces- 

 sitated and whose exponent pr(>ved to be greater than thatofbromo- 

 form (1,593), the specific gravity of tlie glass was still higher than 

 that of bromoform (2,88) . The small air-free, not to be isolated 

 grains, still saidv in this liquid. Now 1 have fried to find out in how 

 far tlie I'efractive index is dependent on the SiO, percentage. For 

 that purjK)se 1() rocks have been examined, forming a series of the 

 most acid to the most basie magmas, -which occur in jiature. 



The result has been comprised in the following table: 



