The acetylene gas system used to fuel the lighting system in the store was first to come under suspicion as the cause of the explosion. Water dripping onto calcium carbide chips results in a chemical reaction that produces acetylene gas that when ignited lights up gas lamps in a home or store like Benge & Pratt's (see diagram below).
Expert evidence was given that some 20 cubic feet of acetylene would be needed to produce an explosion in the building. Since the gas was turned off at around 10 pm (originally it had been suggested it was turned offf at 9:30 pm), it was estimated by experts that only 7 to 8 cubic feet could have leaked between that time and that of the explosion. Moreover, it would have required a leak or rupture of the gas piping for gas to have spread throughout the building. No one gave evidence of smelling gas at the time the store closed, at the time of the first alarm, or immediately prior to the explosion some thirty minutes later.
An examination of the generator after the explosion showed that it remained sealed and showed no signs of being ruptured either by the fire or the explosion.
A water-to-carbide acetylene gas generator of the type use to provide gas for household and business lighting in the late 1890s to early 1900s. Water was slowly released onto carbide chips in the generator (A) and the acetylene gas produced was stored in the gas tank (B) until the gas lighting was run.
Source: Evening Post, Report of Coroner's Inquest, 28 April 1914, p. 7.