60 



If I am correct, we have either of those two salts, that is, in the sulphate of iron and the 

 super-sulphate of potash and alumina, applied separately, the desideratum essential to the 

 preservation of wood ; their application is not expensive, nor need there be any waste of 

 material, nor diminution beyond what is absorbed by the timber conformably to the object 

 proposed. 



Of these two substances the salt of iron or copperas is to be preferred, because it costs 

 less and will be found equally efficient. A triple acid — not known in commerce, but which 

 can readily be prepared, and is as highly antiseptic as the super sulphate of potash and 

 alumina — is an alum composed of water, sulphuric acid, iron and clay. It will be found 

 when made on a large >scale not much if any more expensive than copperas. 



When desirable to subject timber to the action of two saline substances with a view to 

 obtain the precipitation of their bases in the sap vessels, two reservoirs will be found necessary ; 

 that is, one to contain each of the salts in solution and into one of which the timber is 

 to be first placed, and afterwards, when the sap vessels are full, it is to be removed to the other. 



This process has been heretofore practiced, but was found not to answer the object 

 contemplated, because after the vessels had become filled with a solution of one salt the 

 precipitation which followed the introduction of another solution closed the orifices, and pre- 

 vented the access of a further supply to the deeper cells of the wood. In consequence of this 

 artificial means have been resorted to and the process is now found to be highly beneficial 

 though attended with considerable expense. 



But where time is no important consideration, as with governments, if proper attention be 

 given to the process ; such as sinking one end of the timber to a considerably greater depth 

 i I the reservoir than the other, and, when thoroughly charged, removing- it from the solution 

 and suffering it to become perfectly dry before it be placed in the other saline preparation. I 

 believe the former process may be made to answer the purpose equally well, and may be 

 effected much more economically. 



This difference in the elevation of the ends of a piece of timber may be readily increased 

 till it becomes perpendicular, and throughout every degree of the change the forces acting in the 

 sap vessels, viz.: the pressure of the fluids in the reservoir against the air in them and 

 capillary attraction are increased; certainly the former, and this force could be considerably 

 angmented by depressing the timber in a vertical position to a greater depth, insomuch that 

 no doubt remains in my mind as to the efficacy of the preservation of timber by the mode 

 above described. 



