Proceedings of the P' 'tecenic Association. 797 



below, where the reaction takes place, and nitro-gljcerine is formed, 

 which falls to the bottom of the jar. Mr, Mowbray agitates his acids 

 with cold air. For this purpose he leads the cold air resulting from 

 the partial expansion of compressed air into the laboratory through 

 iron pipes, and over each jar of acid is a cock to which a rubber tube 

 is attached. On the end of this is a glass tube. During the reaction 

 in the jars, and while dense volumes of nitrous-acid are evolved, and 

 the heat which it is necessary to constantly keep down is rising, his 

 men stir the mixture with these glass tubes, admitting a current of 

 cold air which agitates, cools, and in escaping carries off the gas it is 

 so essential to get rid of, as soon as possible after it is formed. 



The next part of the process is the removal of these jars, and to 

 empty their contents through a trap or square opening in the center 

 of the floor, into a reservoir holding about forty gallons of water, for 

 the purpose of washing off all traces of acid. This reservoir is of 

 wood, lined with lead. 



After washing the nitro-glycerine, the reservoir, Wiich is balanced 

 on two journals, is turned over on its side gradually and the nitro- 

 glycerine emptied into glass and earthen receptacles. These £^re 

 removed to the magazine, a few rods distant. 



At the time I entered this magazine, there were 1,000 pounds of 

 nitro-glycerine there in jars, holding from three to five gallojis each, 

 resting on benches. 



Mr. Mowbray prepares his own nitric acid, near by, and also 

 concentrates the sulphuric acid he employs. It is probably by close 

 attention to the qualities of the materials he employs, and the thorough 

 agitation and carrying off of the nitrous acid gas, by the cold air intro- 

 duced into the jars for this purpose, and also to prevent elevation of 

 temperature, that he succeeds in obtaining the quantity and (piality of 

 nitro-glycerine he does. Forty-two pounds of glycerine yields him 

 ninety-four pounds of nitro-glycerine, which, at a temperature of 

 forty-eight degrees and upward, is perfectly transparent and without 

 color, A little below this temperatnre it becomes frozen, and then 

 resembles pounded ice. 



The men who are obliged to breathe the smoke^anesulting from the 

 explosion of the nitro-glycerine in the tunnel, informed me that they 

 experienced very little inconvenience from it, while formerly, when 

 they used the imported article, and which was, more or less, yellow 

 and brown, they were affected with intense headache. 



One physical difference, which will be appreciated by chemists, 



