Physical changes, description

The properties of thermally modified wood are highly dependent upon the thermal treatment employed, and it is very important to take these into account when comparing the various treatment methods employed. This chapter examines the effect of the process variables upon the properties of thermally modified wood, and then considers the chemistry of thermal modification. Studies of physical changes are discussed, followed by an overview of the biological properties of thermally modified wood. A short description of some recent literature on the use of thermal treatment combined with compression and on hot oil treatments is also included. 

Sublimation. Accdu to definition ziven in Perry (Ref 13, p660), it covers the physical changes encountered by a substance in passing from a solid phase to a gas and back to a solid phase. It is characterized by the absence of the liquid phase and is used for purification of volatile substances like iodine, camphor, etc. Detailed description of the process is given in Refs 12, 13, 15 18). 

It is relatively easy to talk and gesture about how chemistry either does or does not reduce to physics. It is much harder to spell out exactly what is required to make good on the claim that chemistry does (or does not) reduce to physics. Philosophers have a concept of supervenience. In the case we are focused on here chemistry putatively reducing to physics—supervenience requires that every chemical change be accompanied by a physical change. This is nearly universally held, for example, if two molecules are identical in all physical respects, they will not differ chemically. However, supervenience is not sufficient for the reduction of chemistry to physics. There could be downward causation, where it is the chemical facts and laws that drive the physical facts and laws, not the other way around. Robin Hendry (Chapter 9) argues that those committed to the reducibility of chemistry to physics have not ruled out the possibility of downward causation, and moreover, he presents substantial evidence from the manner in which quantum mechanical descriptions for molecules are constructed and deployed by chemists in favor of downward causation. Quantum mechanical descriptions of molecules that have explanatory and descriptive power are constructed from chemical—not physical—considerations and evidence. Here in precise terms, we see chemistry supervenient on physics, but still autonomous, not reducible to physics. 

Although the history of mankind contains innumerable references to the periods of starvation from which no nation and no century has escaped, only a few descriptions of the physical changes which resulted from such starvation are to be found and even fewer reports of the damage suffered by the endocrine glands are recorded. However, an occasional pertinent reference does exist, such as Jeremiah s mention of hyperpigmentation of skin Our skin was black like an oven because of the terrible famine (Lamentations 5 10), quoted by Keys et al. (1950) or the more recent accounts of lowered birth rates during periods of starvation (Tisdall, 1950). 

Some special situations, which may influence the result of a bioassay, are listed in the test descriptions. To extend that list, more information about possible chemical and physical changes of the degradation matrix due to microbial activity has been collected. It appears to be of most importance to identify probable impacts on the result of a bioassay caused by reasons other than the presence of toxic residues or metabolites. 

The classical microscopic description of molecular processes leads to a mathematical model in terms of Hamiltonian differential equations. In principle, the discretization of such systems permits a simulation of the dynamics. However, as will be worked out below in Section 2, both forward and backward numerical analysis restrict such simulations to only short time spans and to comparatively small discretization steps. Fortunately, most questions of chemical relevance just require the computation of averages of physical observables, of stable conformations or of conformational changes. The computation of averages is usually performed on a statistical physics basis. In the subsequent Section 3 we advocate a new computational approach on the basis of the mathematical theory of dynamical systems we directly solve a... 

This level of simplicity is not the usual case in the systems that are of interest to chemical engineers. The complexity we will encounter will be much higher and will involve more detailed issues on the right-hand side of the equations we work with. Instead of a constant or some explicit function of time, the function will be an explicit function of one or more key characterizing variables of the system and implicit in time. The reason for this is that of cause. Time in and of itself is never a physical or chemical cause—it is simply the independent variable. When we need to deal with the analysis of more complex systems the mechanism that causes the change we are modeling becomes all important. Therefore we look for descriptions that will be dependent on the mechanism of change. In fact, we can learn about the mechanism of... 

It is indeed a distressing prospect to contemplate the complications introduced by chemical changes into an otherwise orderly physical description. The chemical complications are intimately intertwined with the mechanical and physical effects, which are already understood to be more complex than present theory indicates. As the questions addressed in solid state chemistry are quite different from those addressed in prior work, new approaches are required to develop a scientific understanding of the field. 

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