World's Greatest Inventions

3. Ancient Romans are credited as one of the first societies to use --- in architecture, with Roman bathhouses and iconic sites such as the Colosseumand Pantheon dome constructed using --- mixed with volcanic ash, lime, and seawater.
7. German inventor Johannes Gutenberg invented the --- --- sometime between 1440 and 1450. A machine by which text and images are transferred from movable type to paper or other media by means of ink.
8. The earliest rudimentary --- is thought to date back to the 10th century in China, when people inoculated small scratches in the skin with small doses of smallpox to provide protection against the disease.

1. Before the invention of the --- in 3500 B.C., humans were severely limited in how much stuff we could transport over land, and how far. The --- itself wasn't the most difficult part of "inventing the ---." When it came time to connect a non-moving platform to that rolling cylinder, things got tricky.
2. The first --- was invented in China during the Han dynasty between the 2nd Century B.C. and 1st Century A.D.; it was made of lodestone, a naturally-magnetized iron ore, the attractive properties of which they had been studying for centuries.
4. The first --- dates back to 1800, when Italian physicist Alessandro Volta wrapped stacked discs of copper and zinc in a cloth, submerged it in salty water and discovered that it conducted energy.
5. In 1928, the Scottish scientist Alexander Fleming noticed a bacteria-filled Petri dish in his laboratory with its lid accidentally ajar. The sample had become contaminated with a mold, which turned out to be the fungus ---.
6. Like many famous inventions, the --- was discovered by accident. In 1895, German engineer and physicist Wilhelm Conrad Röntgen was undertaking a two-month study into the potential of radiation. He soon discovered that --- could pass through human tissues.