July 19 1799:

During Napoleon Bonaparte's Egyptian campaign,
a French soldier discovers a black basalt slab inscribed with ancient writing near the town of Rosetta, about 35 miles north of Alexandria.
The irregularly shaped stone contained fragments of passages written in three different scripts:
Greek, Egyptian hieroglyphics and Egyptian demotic.
The ancient Greek on the Rosetta Stone told archaeologists that it was inscribed by priests honoring the king of Egypt,
Ptolemy V, in the second century B.C.
More startlingly, the Greek passage announced that the three scripts were all of identical meaning.
The artifact thus held the key to solving the riddle of hieroglyphics, a written language that had been "dead" for nearly 2,000 years.

On July 21:

# 1983 - The world's lowest temperature is recorded at Vostok Station, Antarctica at −89.2°C (−129°F).

July 22 1793:

More than a decade before Lewis and Clark, Alexander Mackenzie reaches the Pacific Ocean,
becoming the first Euro-American to complete a transcontinental crossing north of Mexico.

On July 23:

1995 – Hale-Bopp, one of the most widely observed comets of the twentieth century, was discovered by two independent observers, Alan Hale and Thomas Bopp, at a very large distance from the Sun.
http://en.wikipedia.org/wiki/Comet_Hale-Bopp

On July 24:

1915 - Passenger ship Eastland capsizes in central Chicago, with the loss of 845 lives.
http://en.wikipedia.org/wiki/Eastland