L. MECHANICS AND ENGINEERING. 543 



water. By reason of its absorption and disengagement, it is 

 always the same ammoniacal gas which is employed either 

 to push or to draw the train of pneumatic boxes. It is main- 

 tained that the use of the gas is much cheaper than the use 

 of a steam-engine to force air into or draw air out of the pneu- 

 matic tube. 6 B, 1873, LXXVII., 280. 



b 



IMPEOYED GUN-COrrON. 



According to Metchel, the best gun-cotton is that which 

 is most explosible, and at the same time most soluble in al- 

 coholic ether. For this purpose he boils for several hours 

 (renewing the water that evaporates from time to time) two 

 parts of cotton with one of carbonate of potash, and one hun- 

 dred parts of water, after which the cotton is to be washed 

 and then dried. He then, treats seven parts of the cotton 

 thus purified with a mixture of four parts, by weight, of ni- 

 tric acid, and 1.42 density, charged Avith nitrous vapor; and 

 with fouf parts of sulphuric acid of 1.84 density, after the 

 mixture has been cooled to about 80. The wliole is then to 

 be exposed to the air for four days; after which the cotton is 

 to be removed, washed in warm water, then in cold distilled 

 water, and finally squeezed out and dried. Eleven parts of 

 gun-cotton will be thus obtained perfectly white, burning 

 without any residue, and dissolving vvith great facility in 

 sulphuric or nitric ether. The gun-cotton will be still better 

 if submitted to a second treatment of the acid. 9 B, Aiigust 

 2, 1873 650. 



EXPLOSION OF WET GUN-COTTON. 



Tlie discovery that finely divided gun-cotton, when sus- 

 pended in water, can be exploded by detonation, but is not 

 efi*ected by exposure to fire of any kind, is considered of very 

 great moment as regards the safe administration of this pow- 

 erful substance, especially as it can be kept in this condition 

 for an indefinite period without undergoing decomposition ; 

 when dried, resuming all its ordinary properties. To efifect 

 its explosion it is only necessary to introduce a little loose, 

 dry gun-cotton to start the discharge ; and, curiously enough, 

 a small amount of water, instead of diminishing, has been 

 found to actually add to the force of the discharge. 



Professor Abel now proposes, instead of gunpowder, to 



