20 MODERN SCIENCE READER 



materials must be in a very finely divided state. Moreover, 

 in order that uniformity of effect may be obtained, purity 

 of the original substances, the percentage of moisture 

 present, and the density of the finished powder are of 

 importance. 



The weighed quantities of the ingredients are first mixed 

 in gun-metal or copper drums, having blades in the interior 

 capable of working in the opposite direction to that in which 

 the drum itself is traveling. After passing through a 

 sieve, the mixture (green charge) is passed on to the in- 

 corporating mills, where it is thoroughly ground under 

 heavy metal rollers, a small quantity of water being added 

 to prevent dust and facilitating incorporation, and during 

 this process the risk of explosion is greater possibly than at 

 any other stage in the manufacture. There are usually six 

 mills working in the same building, with partitions between. 

 Over the bed of each mill is a horizontal board, the "flash 

 board," which is connected with a tank of water overhead, 

 the arrangement being such that the upsetting of one tank 

 discharges the contents of the other tanks onto the corres- 

 ponding mill beds below, so that in the event of an accident 

 the charge is drowned in each case. The "mill cake" is 

 now broken down between rollers, the "meal" produced 

 being placed in strong oak boxes and subjected to hydraulic 

 pressure, thus increasing its density and hardness, at the 

 same time bringing the ingredients into more intimate con- 

 tact. After once more breaking down the material (press 

 cake) the powder only requires special treatment to adapt 

 it for the various purposes for which it is intended. 



Within the last half-century an enormous alteration has 

 taken place in artillery, the old smooth bore cannon, firing 

 a round shot, having gradually given place to heavy rifled 

 cannon, firing cylindrical projectiles and requiring very 

 large powder charges. This has naturally had its influence 

 on the powder used, and modifications have been introduced 

 in two directions first, alteration in the form of powder, 

 and second, in the proportions of the ingredients. As the 



