98 KANSAS. CITY REVIEW OF SCIENCE. 
patent street coverings composed of wood-tar, coal-tar, pitch, rosin, etc., mixed with 
either sand, gravel, ashes, scoria, sulphur, lime, etc., or with two or more or all 
of them. Some of them will produce a tolerably fair sidewalk, but they are to- 
tally unfit for the surface of a carriage way. Some of the best of them will an- 
swer for carriage way foundations. 
The rock should be of the fine grained variety, of tolerably close texture, 
and composed of pure carbonate of lime so uniformly and homogeneously im- 
pregnated with the bitumen, that a cut made with a sharp knife will show neither 
pure white nor jet black spots, but be of a brownish liver color, mottled with 
gray. 
When asphalt rock of this character is heated to a ternperature of 200° to 
212° Fah., the bitumen becomes soft, the grains of limestone separate from each 
other, and the mass crumbles into a partially coherent powder. If this powder 
while still hot, be powerfully compressed by ramming, tamping, or rolling, the 
molecules will again unite, and the mass when cold will assume all the essential 
qualities of the original rock, but in a superior degree, as regards toughness, 
hardness, and incompressibility. This is the whole theory of asphalt road cover- 
ings, as applied to the street pavements in Paris and elsewhere. 
Mention has been made of the superior toughness, hardness and incom- 
pressibility, conferred on bituminous limestone by compressing it while hot. This 
property characterizes any genuine asphalt mixture suitable for paving purposes, 
and advantage has been taken of it, in first forming the material into 
rectangular blocks under a heavy pressure, and then laying them in 
courses across the street, substantially after the manner followed in constructing 
the best stone block pavement. It is, perhaps, needless to say that a pavement 
of this kind, composed of good materials, properly prepared, and laid upon a 
firm and unyielding foundation, should be a good one. Specimens of it have 
been on trial for some years in San Francisco, Cal. The blocks are made with 
Trinidad asphaltum, softened with 7 to g per cent. of the heavy oils or still bot- 
toms, used in preparing the asphaltic cement. This preparation is mixed with 
hot powdered limestone, or powdered furnace slag, and then compressed with a 
- force of about fifty tons into blocks measuring 4 inches by 5 inches by 12 inches. 
The pressure, which is applied to the narrowest face of the block, exceeds one 
ton to the square inch. The limestone or slag is not required to be of the fine- 
ness of impalpable powder, but is composed of grains of all sizes from dust up to 
the size of a small pea. 
Tne blocks are laid close together on their longest edges, in courses across 
the street, breaking joints lengthwise of the street, the joints being filled with 
suitable asphaltic cement so as to render the paveinent water tight. The foun- 
dation should be firm and stable, such as the best of those described on pages 
143 to 149. This pavement while new would be nearly as smooth as that of the 
continuous sheet of asphalt heretofore described, but the wear of heavy traffic 
would, in a short time, crumble off the edges of the blocks and open the joints 
