(979 ) 
has become the number of examples *) of substances which together 
form mixed erystals that one is obliged to include the behaviour of 
common substances such as quartz, inorganic salts and metals in 
respeet to water — where this miscibility in the solid state appears 
to oceur but rarely — in the cases where this miscibility is so 
small that it readily escapes observation. 
It is characteristic of a mixed erystal that it can take up, or lose, 
a certain quantity of one of the components (such as water) the 
erystal remaining homogeneous but also (what is closely connected 
with this) that according to a continuous line the vapour pressure 
of that component depends on the composition”). 
As will be seen, the imbibing crystals behave exactly like mixed 
erystals and even fall under the definition “mixed crystal” in my 
opinion. Can this theory, however, also explain swelling, or enlarge- 
ment by water absorption? Rereers *) showed in 1889 that in the 
case of mixed crystals the volume of the mixture at the first cont-ct 
is equal to the sum of the volumes of the components. When 
applied to the imbibition phenomenon this rule teaches us that when 
a swelling crystal with a volume of 1 mm.® takes up 1 mm.” of 
water as is the case with the crystallised Bence Jones’ albumose 
— the volume of the swollen crystal must be two mm.*; and 
when the crystal remains homogeneous, all the length measure must 
necessarily increase in size. 
Finally, the question arises: but are there also known other sub- 
stances, for instance some with a less elevated molecular weight, 
which form mixed erystals with water? It is chiefly TAMMANN ‘) and 
his pupil LÖweNsterN who have found a series of examples thereof. 
') A collection of a large number of examples is found at G. Brunt, Ueber feste 
Lösungen, Aurens’ Sammlung chem. and chem. techn. Vortrage 1901 and F. M. 
Jarcer, Zeitschr. f. Krystallographie 42, p. 286—276. 
2) Usually this criterion is expressed in properties of the melting point lire; 
this, however, is closely connected with the vapour pressure line so that this 
makes no vital difference. 
3) Nernst, Theoretische Chemie, 224 Ed., p. 121. 
4) Wiedemann’s Ann. N. F. 68, p. 16—22 and Zeitschr. f. physikal. Chem. 27, 
p 323-336. TAMMANN looks upon these crystals as solid mixtures of an anhydrous 
zeolithe greup with a chemical combination of that group with water (a hydrate). 
This explanation is not materially different from the one given here, but is only 
less general. For every compound is in part dissociated and on the other hand, we 
have serious reasons for believing that most of the mixtures of water with another 
substance contain hydrate-molecules. The proof that all the water is present as 
nydrate-molecules is difficult to furnish and has not been given by TAMMANN. | 
think, myself, it is safer to call them simply mixed crystals with water, 
