In between the old-fashioned flash powder and the electronic flashes we use today, flashbulbs were what was used in photography to produce artificial light. These bulbs were used between the 1930's and 1960's. From the 60's-70's, the flash bulbs evolved into Flashcubes, Magicubes, and Flash Bars. The bulbs came in different sizes, and were available in either clear or blue shades. The clear bulbs were primarily used for black and white photography, and the blue was used for color daylight balanced film.

a bulb burning (towards the end of flash)

The bulbs produce light from a small explosion within it when the shutter is pressed. Bulbs tend to burn longer, brighter, and hotter than today's flashes. Because of their high light output, flashbulbs are still coveted by cave photographers today. Because of the heat generated from the bulb, they will most likely be hot to the touch after use, and will sometimes start to melt. Other things to keep in mind when shooting with flashbulbs is that you can only use them once, and you may need to use slower shutter speeds to ensure synchronization since the bulbs take longer to reach their full brightness.

used flashbulbs

In order to use a flashbulb, it must be fitted into a flashgun which is attached to the camera. Many different styles of flashguns were produced, all with some kind of camera synch, and a circular reflector behind the socket area. Below are a few of these styles... 

Nikon Rangefinder S2, w/ 50 F1.4 lens, bulb flash (flash attached by hot shoe)
Zeiss Contina IIA, w/ 45 F2.8 lens, Ikoblitz 0 flash
(flash attached via sync cord and mounted by shoe)

Beacon Two-Twenty Five (flash attached on top)

Canon Rangefinder VT De Luxe, w/ 50 F1.2 lens, flash Model V
(flash attached on side by screw lock)

Argus C3 (flash attached on side by "pins")

Leica S Selsy VIII Flash (attached by camera baseplate).

For more information on flashbulbs, check out these links:
* Camerapedia- Flashbulbs
* Flash Photography: An "Explosion" Of Light (info. from a U.S. Navy training manual)
* History of Flash and Ilford Flashguns
* Flashbulbs and Caving
* Digital Photography with Flashbulbs
* Flashbulbs Flickr Group
* Flashbulbs Repurposed for Wintery Decor