ribed, and though many have rejected concrete under these
conditions, other engineers equally conservative are using it freely and
without fear.

The writer consulted with his partner and others at some length, and,
considering all the advantages to accrue by the use of these concrete
caissons, decided to do so after taking all known precautions.

[Illustration: FIG. 1.]

These precautions consisted in:

First, the use of cement in which the chemical constituents were limited
as follows:

It was specified that the cement should not contain more than 1.75% of
anhydrous sulphuric acid (SO_{3}) nor more than 3% of magnesia (MgO);
also that no addition greater than 3% should have been made to the
ingredients making up the cement subsequent to calcination.

Secondly, to secure by careful inspection the most completely
homogeneous mixture possible, with especial care in the density of the
outer skin of the caissons.

Thirdly, a prolonged seasoning process before the new concrete should be
immersed in the sea water.

In addition to these well-known precautions, it was decided to try the
addition to the cement of a chemical element that should make with the
free lime in the cement a more stable and indissoluble chemical
combination than is offered by the ordinary form of Portland cement.
This was furnished by the patent compound known as "Toxement," which is
claimed by the inventor to be a resinate of calcium and silicate of
alumina, which generates a resinate of lime and a silicate of alumina in
crystalline form. It is further claimed that each of these materials is
insoluble in sodium chloride and sodium sulphate, 3% solution. It was
used in all the caissons, excepting Nos. 1 and 2, in the proportions of
2 lb. of Toxement to each 100 lb. of cement. The first two caissons were
not thus treated, and will be held under close observation and
comparison with the others, which were treated with this compound.

The mixture used was one of cement (Pennsylvania brand), two of sand,
and four of gravel. The sand and gravel were from the nearby Cow Bay
supply, and screened and washed. None of the gravel was larger than 1/2
in., grading down from that to very coarse sand. The sand was also
run-of-bank, and very well graded.

The caissons, after being placed, were filled with sand and gravel from
the adjoining beach up to about mean high-water mark, and the edges
outside all around were protected from tidal and wave scour by ri