32 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
CI-'' 
fact that the water in it is wholly constitutional. In fact, the mineral 
is stable far above the ordinary temperatures of dehydration, so that 
the water can be regarded only as an essential part of the molecule. 
In addition to the formula proposed for kaolin, the following expres-|| 
sions are possible without assumption of any higher molecular weight: 
(1) Si 2 7 .H 4 (A10) 2 
(2) Si 2 7 .H 2 (AlOH) 2 . Brauns's. 
(3) Si 2 5 .(AlH 2 2 ) 2 . Groth's. 
<4) H-o-Ai< 8i0 °- A1H °°° 
Al = Si0 4 =H 2 
< 5 ) 0< Al=Si0 4= H 2 
If the formula be tripled, then kaolin may be written as a basic 
trisilicate, thus : 
X)H 
(6) Al— Si 3 8 =H.( A1H 2 2 ) 2 
X Si 3 8 =(AlH 2 2 ) 3 
Of all these symbols only the last and the one originally chosen indi- 
cate the relations between kaolin and its parent species. As for these 
two, the formula 
/OH 
Al— Si0 4 ~H 3 
\si0 4 =Al 
is the simpler, and would seem to represent the greater stability. Kao- 
lin, under ordinary circumstances, is scarcely attacked by the strongest 
hydrochloric acid, a fact which seems to be most in harmony with the 
ortkosilicate expression. That expression, therefore, is to be preferred ; 
at least until more positive evidence is attainable. After dehydration 
at low redness, kaolin is completely decomposable by hydrochloric acid, 
but the ignited mass contains no silica soluble in sodium carbonate 
solution. These facts, developed by experiments made under my direc- 
tion by Mr. George Steiger, seem to indicate the formation of a salt 
Al :i Si 2 7 as the result of ignition, but other interpretations are possi- 
ble. The data are given here simply as data, which may become avail- 
able for a fuller discussion of the problem by and by. It will be seen 
later, when the other clays are considered, that their formulas are in 
harmony with that chosen for kaolin. 
IV. THE NORMAL ZEOLITES. 
By this title is indicated a well-defined group of hydrous silicates, 
unmistakably related to nephelite and the feldspars, and for which the 
term feldspathie zeolites would be equally appropriate. A few zeolitic 
species, ptilolite, mordenite,laubanite, etc., are excluded from the present 
