110 OUR NATIONAL FORESTS 



termine the effect of preservative treatment upon 

 the strength of timber. As a result of the large 

 number and variety of strength tests carried on by 

 this Laboratory the United States Government 

 now has a more thorough and comprehensive col- 

 lection of data on the mechanical properties of 

 wood than any other nation. 



Many studies are also conducted to determine 

 the physical properties and the structure of the dif- 

 ferent kinds of wood grown in this country. The 

 minute structure of the wood of many of our native 

 species has been studied by means of microscopic 

 slides. A study has also been made of a large 

 number of species to determine the specific gravity 

 of the actual wood substance. Other tests are 

 made to determine the specific heat of woods. 



The drying or seasoning of woods, more espe- 

 cially of certain species which have been found 

 difficult to season, has received a good deal of at- 

 tention. A new type of kiln, invented by a Forest 

 Service man, has been devised to season such woods 

 as the eucalyptus, which has always been very dififi- 

 cult to handle in drying. Western larch has been 

 seasoned with a loss of only 5 per cent., whereas 

 the loss in ordinary commercial kilns usually ran be- 



