Toronto. The early light sensitive media were far too insensitive for dusk, night or indoor shots (studios used long exposures, a means to hold the subject very still, and large windows, ideally facing north, to let in copious amounts of sunlight). This deficiency (media insensitivity) was remedied by flash. As can be seen by this early 1900s portrait indoors of a young gentleman at left (the dry plate negative was bought some years back from Ms M Cook). The sharpness and shadows give away the secret of using flash powder for the shot.
Long before we had flash bulbs and electronic flash, photographers used a mixture of magnesium and nitrate in a long trough held over the photographer. This concoction was announced in 1887 by Adolf Miethe.
Ignition provided a brief and intense light. Too little and the plate was under-exposed. Too much and the plate was over-exposed. Way too much and disaster! Unfortunately, the mixture was also somewhat unstable and sometimes it accidentally ignited with often bad results.