494 POWDEKS WITH A NITRATE BASE. 



is caused even by lead on lead, or lead on wood ; seldom by 

 copper on wood, never by wood on wood, without of course the 

 interposition of gravel. 



4. Powder absorbs a certain amount of moisture, principally 

 owing to the hygroscopic properties of the charcoal and the 

 impurities of the saltpetre; this amount varying from 0'5 in 

 dry magazines to 1*20 in damp magazines. The proportion of 

 water thus absorbed may rise to seven per cent, in a saturated 

 atmosphere, the temperature of which undergoes alternate 

 changes. When it exceeds a certain limit it causes the 

 separation of the saltpetre by eventual efflorescence, thus 

 destroying the powder. 



5. The density of powder has been considered from three 

 points of view : 



1st. The absolute density, denned in the sense in which it is 

 employed by physicists. 



2nd. The apparent density of the isolated grains, called real 

 density. 



3rd. The apparent density of unrammed powder, called 

 gravimetric density (weight of powder at the unit of volume). 

 The gravimetric density varies from 0'83 to 0'94, according to 

 the coarseness of the grain. 



The so-called real density is found by plunging a given 

 weight of powder into a given medium of which the variation 

 of volume is observed. The following substances have been 

 used: lycopodium, a solid body in a very fine powder, 

 essence of turpentine, water saturated with saltpetre, absolute 

 alcohol, and mercury ; the latter being the only liquid which 

 can be considered as exercising no dissolving action. 



In the tests, it is subjected to a fixed pressure (2 atm.) 

 during the operation. The results obtained in this way have 

 only a relative significance. 



The following have been found in this manner : 



Ordnance powder 1-56 to 1-72 



Rifle powders 1-63 to 1-82 



Sporting powder 1-87 



The absolute density, measured by the volumometer, is 2*50. 



(2) Products of Combustion of Powder. 



1. These products are those of the combustion of charcoal 

 and sulphur by oxygen, modified by the presence of nitrogen 

 and the reaction between these products and potassium, proceed- 

 ing from the saltpetre, at the high temperature of combustion. 



2. The proportions of the composition of powder are not 

 those of total combustion, oxygen being wanting ; they therefore 

 do not correspond to the greatest heat which might be liberated 

 by the oxidation of the sulphur and carbon by a given weight 



