184 STUDIES FOR STUDENTS 
freezing of the included water than if it were laid on the bed. 
In case the stone is laid on edge, the pressure required to split 
off lamina will ordinarily be much less than if the stone is laid 
on the bed. In the first case the force occasioned by the freez- 
ing of the water which collects between the layers is augmented 
by the superincumbent pressure of the wall. If the stone is laid 
on the bed, the water is less apt to penetrate along the parting 
planes, and even though it should circulate with equal freedom 
in this position, the superincumbent pressure of the wall would 
tend to force the expansion in directions parallel to the bedding. 
Furthermore, when stone is laid on edge the difference in tex- 
ture of the various laminae are much more strikingly emphasized 
than where the stone is laid on the bed. When laid on edge 
the different blocks, as a whole, will exhibit different rates of 
wear, instead of the minor inequalities ordinarily shown by the 
different laminae when the block is laid on the bed. 
In important structures one ought to avoid laying any stone 
on edge which shows stratification or schistosity for the reason 
that in this position it is inherently weaker and permits a more 
ready absorption of water, with the attendant dangers from alter- 
nate freezing and thawing. 
AGENTS OF CHEMICAL DECOMPOSITION 
In artificial stone constructions the decomposition of the min- 
eral constituents of a rock proceeds much more slowly than dis- 
integration. The forces which are at work breaking down the 
chemical compounds have a much greater task to perform than 
those which have simply to overcome adhesion and cohesion. 
Water.—The active agent producing chemical changes in the 
rock is water. Water generally contains in solution, besides 
mineral salts, one or more acids, either sulphuric, sulphurous, 
carbonic, or organic. Thus the water is often a very dilute acid 
solution. As it percolates through the rocks it dissolves small 
quantities of mineral matter in one place and deposits it in 
another. Through these agents the minerals composing the rocks 
of both the igneous and sedimentary series are decomposed, and 
transfers of large quantities of mineral substances take place. 
