Complex sentence: Used in 18th century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis, the burning-glass was a useful contrivance in the days before electrical ignition was easily achieved.
Split into simpler sentences: Used in 18th century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis. A useful contrivance in the days before electrical ignition was easily achieved.