514 Mr Haidmger'’s Cornparatlve View of the Series 
But, however remarkable this identity of relations among the 
simple forms may be, the analogy between the crystals of the 
two substances will still allow of more points of coincidence. 
Both species exhibit two faces of cleavage, of different perfec- 
tion ; of which the one, parallel to fff, is very easily obtained, 
while the other, parallel to T, is less apparent. There are also 
in both traces of cleavage, parallel to P. The crystals of both 
substances are commonly elongated in the direction of the short 
diagonal of their fundamental form, and implanted in the same 
direction. 
In prismatic Glauber-salt we may obtain very long prisms of 
that description from a solution of sulphate of soda, if allowed to 
cool rapidly, while the crystals produced by a slow evaporation 
are in general shorter, and present a greater number of well- 
pronounced additional faces than the former. The facility of 
obtaining excellent crystals of this substance, render it very va- 
luable for the study of the hemiprismatic forms. These cry- 
stals are perfectly limpid, and often of a considerable size. If 
exposed to the dry atmosphere, they are soon decomposed, and 
fall into powder ; but I have preserved them for many months 
in their natural state, by keeping them in a cool place, wrapt 
up in paper, which was occasionally moistened. The decom- 
position, however, is not so rapid as to be prejudicial to the em- 
ployment of the reflective goniometer, particularly if care be ta- 
ken to work at low temperatures. It generally begins at certain 
points, and from these it spreads, creeping, as it were, over the 
surface, and through the whole mass of the substance, and thus 
rendering some parts of the faces of crystallisation perfectly dull, 
while other parts of the same faces retain their original splen- 
dour. The dry or anhydrous sulphate of soda, which is pro- 
duced by the decomposition of prismatic Glauber- salt in a state 
of powder, can also be obtained crystallised. It is a well known 
fact, that the capacity of water for dissolving sulphate of soda 
arrives at its maximum when the temperature is considerably be- 
low the point of ebullition (33° centigr. or 106 J° Fahr. according 
to Gay-Lussac) * ; above this temperature crystals are formed 
in the fluid, which consist of anhydrous sulphate of soda. Their 
form is that of a scalene four-sided pyramid. Fig. 11., whose 
Annahs de Chimie^ tom. xi. p. 313. 
