264 Professor Fuchs on Opal, and the 



very inferior specific gravity, much inferior hardness, single re- 

 fraction of light, and especially by its chemical characters. 

 When reduced to powder, it unites with lime in the moist way, 

 and becomes hardened under water, a property which the quartz 

 does not possess, even when it is reduced to the finest powder, 

 as I have already shewn on another occasion. Quartz powder 

 is dissolved by a boiling solution of potash with great difficulty 

 and slowness, whereas opal powder is dissolved in a few minutes ; 

 nay even entire fragments of this substance do not resist very 

 long the power of the solvent. At the usual temperature quartz 

 is not at all acted on by potash, but the opal is gradually en- 

 tirely dissolved by it ; and that not merely when in powder, 

 but also in fragments. All the varieties of this mineral do not 

 exhibit in this respect the same phenomena ; some are dissolved 

 in two or three months, while others require four or five months, 

 which might be concluded from the considerable difference in 

 coherence. The hyalite offers the strongest resistance.* 



Accordingly, there is a great difference between quartz and 

 opal, and the cause can hardly be any other than that the former 

 is crystallized and the latter is not. But as the opal always 

 contains more or less water, many are of opinion that it is a hy- 

 drate of silica, and must therefore differ from quartz. Brei- 

 thaupt, who first noticed the peculiar condition of opal and simi- 

 lar minerals, has made the singular assertion, that it cannot be 

 crystallized because it is a hydrate. But the quantity of water 

 contained in opal varies from 3 per cent, or less, to 12 per cent, 

 and upwards, and is therefore not constant as in real hydrates, 

 and, for the most part, in such as are capable of crystallization ; 

 the opal also, when it has lost all its water by heat, is still opal, 

 has the same appeai-ance, and is dissolved by potash in the 

 same manner as before Even by fusion it is probable that it 

 would not be converted into quartz. I must here remark that, 

 generally, it is a property of most uncrystallized bodies to ab- 

 sorb more or less water, in uncertain quantities, or, when they 

 are found in the same way as opal, to retain the water when 

 they become dry. 



It has often been asserted that opal passes into those varieties 

 of quartz which have been called calcedony and flint, and which 



• Melted silica would probably j'ield the opal having the greatest co- 

 lierence. 



