TESTS OF STABILITY. 381 



and also a serious danger, for it tends to substitute pure nitro- 

 glycerin for a substance which is but little sensitive to shocks 

 or friction, while the former is, on the contrary, extremely 

 sensitive. 



It has been stated how freezing followed by thawing, and 

 even the action of water, might also give rise to exudation. 



8. The possible separation of the various ingredients of a 

 mixture under the influence of jolting arising from conveyance 

 by sea or land is also to be considered. 



9. The slow action which the metals, constituting metallic 

 cartridges, exercise on the saltpetre and the sulphur contained 

 in cartridges, especially if these are even slightly hygrometric, 

 may determine the oxidation and sulphurising of these metals at 

 the expense of the saltpetre and sulphur. Hence there arises 

 at length a certain weakening of the effects obtained with recent 

 powders, according to the experiments made by Colonel Pothier. 



We then see how the preservation of explosives gives rise to 

 very varied special problems. It suffices at present to have 

 pointed out the preceding. 



5. TESTS OF STABILITY. 



1. The tests of stability to which a given explosive is 

 subjected in practice, comprise the most essential conditions 

 among those which have been just enumerated. These are 



2. Stability on exposure to air. The substance must maintain 

 itself, when in contact with air, without evaporation, lique- 

 faction, or apparent alteration, even after having been kept 

 several days. It must not attract atmospheric moisture. 



3. Neutrality. It should in general be neutral and preserve 

 this neutrality; above all, it must not liberate acid vapours 

 even when heated for some minutes in a bath kept about 60. 



4. Exudation. It must not allow the liquid substances it 

 contains, such as nitroglycerin, to exude, either spontaneously 

 or by a slight pressure such as is applied when pushing back 

 the substance gently with a wooden piston in a brass tube 

 pierced with lateral holes. In this trial the piston should not 

 be pressed by hand but by a weight, which is gradually 

 increased until exudation takes place. 



When heated to about 55 to 60 in a bath, the substance 

 should not give rise to the separation of small drops even under 

 a slight pressure. 



When subjected to a temperature below zero, arid then 

 brought back to the ordinary temperature, and that several 

 times, it ought also not to produce exudation. 



Nor should exudation take place under the influence of air 

 saturated with moisture ; for instance, should the substance be 

 left for a fortnight in a chamber containing damp tow. 



