75 
But such long continued steaming will soften the fibers, weaken their 
coherence, and reduce the strength of the timber. Experiments made 
in Hanover show that the ultimate strength of Burnet t ized wood was 
reduced by one fourth to one-tenth, and its elastic limit by one-seventh. 
Some part of this reduction was probably due to the action of the anti- 
septic used — chloride of zinc; but there can be little doubt that it was 
also partly due to the steaming. 
Steaming has some other unfavorable features. A portion of the 
albumen, although coagulated, remains behind and partially stops up 
the cells, hinders the free circulation of steam and sap, and ultimately 
the entrance of the antiseptic solution. The process, moreover, does 
not fully remove the viscid ingredients of the sap, which, next to 
albumen, are most favorable to decay, while it deprives the wood com- 
pletely of the essential oils and of tannic acid, which are to some extent 
preventives of decay; and, finally, unless the process be followed by 
some process of drying 1 , as by superheated steam, and by application of 
the vacuum process (which will be desciibed hereafter), it leaves a con- 
siderable quantity of water of condensation in the wood, making it in- 
capable of impregnation with at least one of the best antiseptic sub- 
stances (heavy oil of tar), and hindering complete impregnation with, 
other preservatives. 
That a considerable quantity of water remains in the wood after sim- 
ple steaming, was proved by experiment, when it was found that of the 
water introduced into the vessel in the form of steam only two-fifths to 
four fifths were removed from the boiler as water of condensation, so 
that from one-fifth to three-fifths remained in the wood. 
(4.) The Vacuum Process. — In 1838 Burnett patented a method of pre- 
paring timber for receiving the preserving fluid, by placing the timber 
in a closed vessel and then using an air-pump for removing the air and 
vapor from the cells of the wood. He next filled the vessel with the an- 
tiseptic fluid, and applied a pressure as high as 150 pounds per square 
inch, by compressing the air above the solution. From the use of com- 
pressed air his was named the pneumatic process, and it retained that 
name even after the use of compressed air had been abandoned, and the 
pressure was directly produced by pumps. 
This process, preceded by steaming and vacuum, is the one now al- 
most universally used. 
The timber to be treated is placed on cars, which are then pushed 
into a boiler, G to 8 feet in diameter, and from GO to 110 feet long, of 
sufficient strength to safely bear an internal pressure of 150 pounds, and 
a collapsing pressure of 12 or 15 pounds. The boiler is then closed, and 
steam admitted under a pressure of 10 or 20 pounds. Steaming is kept 
up, as heretofore stated, for from three to eight hours, and is followed 
by exhausting the air and vapor still contained in the wood for from 
one to three hours, until the vacuum becomes constant on the stoppage 
of the pumps. The antiseptic solution is then admitted to the boiler, 
