SECTIONAL TRANSACTIONS.— K*. 419 



Kilns, therefore, are provided with means for temperature and humidity control. 

 The latter is most important ; poor control of humidity is responsible for most of the 

 defects found in kiln-dried timber. Good humidity control gives kiln-seasoning its 

 chief advantage over air-seasoning. 



In all kilns some means intended to cause a movement of air through the piled 

 timber is provided. Separation of the layers of a kiln pile is the invariable practice 

 and, except in certain special kilns, it is necessary that the width of the pile should 

 not be too great, otherwise the drying at the side by which the air leaves may be too 

 glow for efficient working. 



Thus, in kiln-seasoning, provision is made for the control of the important factors. 

 Such control, however, is essential, and if it is inefficient the danger of damage to the 

 timber is greater than occurs in air-seasoning. 



But under the best conditions, not only does kiln-seasoning yield the highest 

 quality of material, but on account of the control it is possible to adjust the final 

 moisture content of the seasoned timber uniformly to the value suitable for any 

 particular purpose. 



The imjiortance of precision in the final moisture content of the timber used in 

 certain manufactures is probably cliiefly responsible for the present development of 

 kiln-seasoning. 



20. Mr. J. F. Hartley. — Moisture Movement in Wood. 



A series of experiments was described on the passage of moisture through 

 cylinders of Scots pine heartwood. 



The effect on the rate of moisture flow duo to altering the vapour pressure 

 difference at the opposite faces and of varying the cylinder length was investigated. 

 In addition the moisture gradients were examined, and the variation of the difiFusivity 

 of moisture in wood with the moisture content determmed. 



It was found that the rate of flow through the cylinders was proportional to the 

 vapour pressure difference between the two faces and also when allowance was made 

 for the existence of a surface resistance that the rate of moisture flow was inversely 

 proportional to the length of the cylinder, other factors being constant. 



The form taken by the moisture gradients showed that the diffusivity of moisture 

 in wood was about six times as great near the fibre saturation point as when dry. 



The conclusion drawn from these experiments was that if the law of diffusion 

 was to be applied to the movement of moisture in wood, it would have to be modified 

 by the inclusion of a term embodying the variation of the diffusivity with the moisture 

 content. 



The general differential equation of the modified law of diffusion woulil take the 

 form : — 



8/ ^x\^^ 'Sx 



where /(m) gives the variation of the diffusivity with the moisture content. 



For the purpose of comparison with a non-colloidal substance, a similar series of 

 experiments was carried out with plaster of Paris. Similar results were obtained as 

 with Scots pine, with the exception that there did not appear to be a variation of the 

 diffusivity with the moisture content, but it was not possible to decide this point 

 definitely as the moisture gradient in the cylinders was not determined. 



In conclusion, the bearing of the results obtained on the practical problem of 

 kiln-seasoning was discussed. 



21. Mr. Alex. Howard. — A Country without Trees. 



22. Joint Meeting with Section K. Prof. J. W. Bews. — The Ecological 



Evolution of Angiospermous Woody Plants. 



The evidence from fossil botany, though incomplete and unsatisfactory in many 

 ways, lends strong support to the view that the moist tropical flora was at one time 

 more widespread. Without relying on the supposed determinations of fossil leaf 

 impressions, it is possible, by adopting statistical methods, to compare the fossil 

 Angiosperm flora with modern types of vegetation. Comparisons of such features 

 as leaf-size, leaf-shape, leaf-texture, leaf-margins, and the characters of the wood, 

 show that the fossil Angiosperms from all over the world are ecologically most nearly 



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