using the minimum quantity of magnesium, as it is only then that complete and rapid combustion can be depended upon. Explosive mixtures of magnesium with oxidising salts* as chlorate of potassium, are sometimes used, especially when a powerful flash of the briefest possible duration is required; but such mixtures should be made and used with extreme caution, as several fatalities are on record. As regards these precautions, we cannot do better than quote from an article in The Amateur Photographer, by a contributor writing as "Index": - "When an explosive mixture is used, the following may be taken as one which has worked satisfactorily: -
Magnesium powder... ... ... ... 60 grs.
Chlorate of potash............90 ..
The mixture should not be made up in bulk, but only as required. The powders should both be dry; they may be, if necessary, separately dried in an oven only moderately warm, and when cold mixed with a piece of card, or a feather, on a sheet of paper. Any lumps in the chlorate should be thoroughly crushed before mixing. There should be no friction with a hard substance during mixture or after the powders have been mixed, or a disastrous explosion may ensue. A cautious manipulator will so arrange matters that if at any instant the mixture should fire, neither he nor any one else will be seriously hurt. The mixture may be fired by a taper affixed to the end of a stick. If the exposure is to take place out of doors, a small quantity of sul-phide of antimony may be added, as in the following formula : -
Chlorate of potash............60 grs.
Magnesium powder......... ... 30 ,,
Sulphide of antimony ... ... ... 10 ..
Combustion is more rapid with this mixture, estimated, indeed, by Messrs. Gaedicke and Miethe to take place in from the fiftieth to the thirtieth of a second; but the fumes given off are so offen-sive and poisonous that it should not be used in the house unless in a large lantern or combustion chamber, furnished with a chimney leading into the open air." As a comment on the latter remark, we may mention that outdoor scenes, groups, and even buildings, have been successfully photographed by flash light, (See Magnesium.)