Two mechanisms have been proposed for the build-up of detonation by solid explosives: (a) In the self-heating mechanism, when heat is evolved during thermal decomposition of the explosive faster than it can be conducted away, the temperature of the mass and the consequent rate of decomposition rise more and more. Ultimately the whole mass deflagrates more or less violently. The mathematical condition for self-heating has been formulated, but experiments show that a further condition is required for transition from deflagration to detonation, which has not yet been formulated mathematically. (b) In the mass-flow mechanism, when the gas evolved during chemical decomposition of the explosive becomes comparable with the molecular mass flow required for stable detonation in the explosive, thermal decomposition changes into detonation. To test these mechanisms measurements of delay to detonation were made with loose masses of lead azide, both Service and dextrinated, ranging from 10 to 200 mg. using previously described apparatus. The azides were wetted with measured volumes of liquids with various boiling points, including: water, benzene, quinoline, diethylene glycol, glycerol, dibutyl phthalate, benzyl benzoate, nujol, tricresyl phosphate, and the effect on the detonation was observed. Mixtures of benzene with nujol and with dibutyl phthalate were also investigated. Comparative measurements were made on the deflagration of cyclonite in the same apparatus, both dry and with added liquids. The effect of liquids on the detonation of lead azide when heated was found to belong broadly to one of two classes: (i) For liquids with the boiling points considerably below the temperature at which the test was being carried out, detonation followed after a longer delay than in the absence of liquid. There was evidence that the liquid first evaporated, and then normal detonation of the azide grains took place in the vapour phase thus formed. This behaviour was shown by the following liquids: [Note: Table omitted. See the image of page 238 for this table.] (ii) For liquids with boiling points considerably above the temperature of test, no detonation was observed. However, under certain circumstances a new phenomenon was observed, in that the lead azide 'deflagrated' in a manner closely resembling the behaviour of the (self-heating) deflagration of an explosive such as cyclonite. This is quite different from the sharp detonation obtained with loose azide in air, when the masses are small. When the boiling point was in the neighbourhood of the testing temperature, or with mixtures of liquids with boiling points above and below the testing temperature, both classes of behaviour were observed, according to the conditions of test. Further, the temperature coefficient of the induction period for azide wetted with these intermediate liquids suggested that detonation occurred after the liquid had been displaced by nitrogen produced by thermal decomposition of some of the lead azide. From the experimental results, it is concluded that (a) With the masses used, lead azide will detonate only when the grains are surrounded by gas or vapour. (b) Lead azide can deflagrate by a self-heating mechanism even under conditions where it will not detonate, e.g. when wetted by a liquid of very high boiling point such as tricresyl phosphate. These conclusions support the view that the 'normal' mechanism of detonation of lead azide is controlled not by self-heating but by some process such as mass flow. When this normal mechanism fails to operate explosion may still occur by self-heating.