On definite Proportions. 45 
thuriatic acid. For, ifit is impossible to exhibit the muriatic acid in_ 
a separate state, it would have been requisite that the carbon should 
separate from the radical not only the small portion of oxygen 
which distinguishes oxymuriatic from muriatic acid, but the whole 
quantity that exists in the combination. Now the attraction of 
_carbon for oxygen is much weaker than that of the muriatic ra- 
dical, and the experiment cannot be admitted as a valid proof 
that the oxymuriatic acid contains no oxygen. 
A second preliminary remark relates to the water of crystat- 
lization of salts. If we examine a solid that contains water, 
we must not forget that the water may be either in a state of 
chemical combination, or merely retained mechanically in the 
pores of an aggregate. The internal parts of crystals are not 
completely continuous, but interrupted by very small irregular 
interstices, occupied by a small portion of the mother liquor, in 
which the crystals have been formed. For this reason, regular 
saline crystals, although they derive their origin from a power 
which detaches them from all chemical connexion with the sub- 
‘stances remaining in solution, still contain a small quantity of 
the foreign substances which had been present, however carefully 
their surfaces may have been washed. Crude saltpetre, for in- 
stance, crystallized from the first solution, exhibits yellowish- 
brown crystals. But this saltpetre is not chemically combined 
either with the colouring extractive matter, or with the alkaline 
muriate, which is found init. By the help of a magnifying 
glass we see clearly that the white salt contains a yellow sub- 
stance in its pores; and the same must he true of the impurities 
which are not distinguishable by their colour. The less pure the 
mother liquor, the more must the crystals be loaded with foreign 
substances ; and the purification by repeated crystallizations de- 
pends on the mechanical admixture of a mother liquor which 
becomes less and less charged with impurities at each step of the 
process. A large crystal contains in its pores a quantity of the 
mother liquor proportionally larger than a small crystal. Thus, 
in the refineries of sugar, the syrup which is separated during 
the continual agitation that is employed for obtaining lump sugar, 
by crystallization in small grains, is commonly more coloured 
than the syrup from which sugar candy has been separated du- 
ring a slow evaporation, in large yellow or even brown crystals. 
In the same manner it has been usual to purify nitre for fire- 
works by causing it to precipitate in fine grains; and we know 
from the experiments of MM. Thenard and Roard, that alum, 
may easily be purified by a similar process. . 
If we powder a salt that has been crystallized in very small 
grains, and well dried, it affords immediately a dusty dry powder. 
If, 
