57 
spar, hornblende, and other silicates, in which it is also com- 
paratively inactive; and thirdly, as amorphous or soluble 
silica, or opal, as it is called by mineralogists, the only form 
of much importance in agriculture, since in this form it is 
soluble in the liquids of the soil. Far too little attention has 
been paid in analyses of soils and fertilizing minerals to 
their content of opal or soluble silica, whereas a soil may be 
rich in every other necessary ingredient, and yet, if deficient 
in this respect, be perfectly sterile for many crops. 
“An analysis of the straw of wheat, made by Weber, in 
the laboratory, and according to the method of H. Rose, 
showed 3°82 per cent. of total ash and in this, ‘68 per cent. 
of soluble silica. A ton of such straw must therefore obtain 
from the soil not less than 50 lbs. of silica.” 
The importance of the function of soluble and hydrated 
forms of silica in mineral fertilizers, like green sand, has 
been underrated. Though some vegetable spongioles may 
be able to cause quartz to pass into solution in the sap, yet 
it is of course the soluble forms that are thus chiefly taken 
up, constituting the vegetable skeleton, as phosphate of lime 
the animal. Silicie acid, though so minute an ingredient in 
actual animal nutrition is indirectly as essential to animal life 
as even carbonic acid. The author has presented to the 
Lyceum and to the American Association, peculiar views of 
the relations of oxygen and carbon to life, arrived at by the 
a posteriori method of studying the chemical changes now 
going on, and tracing them backwards throughout their ante- 
rior stages. By the application of the same process to the past 
history of silicic acid, equally curious generalizations regard- 
ing the relations of this material (which next to oxygen is 
the most abundant of all) to zoic history in the past and 
the present are pointed to with greater or less probability. 
The study is, however, as yet, far more difficult and uncertain 
than in the case of carbonic acid, for the reason that the 
facts as yet established by chemical research regarding the 
habitudes and migrations of silicic acid, and the parts which 
it sustains in the grand drama of Life, are comparatively few. 
