140 DEPARTMENTAL REPORTS. 



rock samijles, so that they can be properl}" classified according to their 

 mineral composition. 



The most important of the physical properties which are made sub- 

 jects of investigation are : Hardness, or the property possessed by a 

 material to resist .the wear caused by the abrasion of the wheels of 

 vehicles and horses' feet, and toughness, or the adhesion between the 

 fine particles of a material which gives it the power to resist fracture 

 when subjected to the blows of traffic. This property, while distinct 

 from hardness, is intimately associated with it and can, in a measure, 

 make uj) for a deficienc}' in hardness. Hardness, for instance, would 

 be the resistance offered by a material to the grinding of an emery 

 wheel; toughness, the resistance to fracture when struck with a ham- 

 mer. Cementing value, which is probably the most important prop- 

 ertj^ to be sought for in macadam rocks, gravels, and clays, is the 

 property possessed by the dust of these materials to act, after wetting, 

 as a cement tothe coarser fragments eomposingthe rock, bringingthem 

 together to form a smooth, impervious shell over the surface. The 

 degree to which a material absorbs water is also an important prop- 

 erty, as is also the density of macadam rock. 



All samples received at this laboratory during the past year have 

 been subjected to the necessarj- tests for determining the degree to 

 which they possess the above properties. They have also been ana- 

 Ij^zed chemically or petrograjjliically, as the particular case required, 

 and properly classified. Besides these standard tests, several new 

 ones have been proj)osed and investigated. 



In addition to the regular tests certain lines of investigation have 

 been taken up, which, it is hoped, Avill be of benefit to the road 

 builder. Among the most important of these is a study of ])urnt 

 clays, together with the cementing value of clay. The results obtained 

 from this work warrant the expectation that satisfactory roads can be 

 built of moderately l)urned clay at a comparatively^ small cost. If 

 this be true it will be of great benefit to those citizens of our country 

 who live in districts where no hard materials for road building are 

 available. It is the purpose of this laboratory during the present 

 year to build an experimental road of burnt clay, in collaboration with 

 the Office of Pulilic Road Inquiries, and thus test in a practical way 

 the results ol)tained in the laboratory. 



During the past fiscal year 150 abrasion tests have been made, 184 

 cementation tests, 88 chemical analyses, 180 iietrographical analyses, 

 150 absorption tests, and the same number of specific densities. Two 

 hundred and seventy-one miscellaneous samples were received. These 

 samjjles have come from all parts of the United States, including some 

 of our new possessions, and in no instance has a sample been solicited 

 in any wa3^ Tlie}' have come almost exclusivelj" from practical road 

 builders, endeavoring to lessen the cost and improve the quality of 

 their work. 



DENDRO-CHEMICAL LABORATORY. 



Mr. W. H. Krug was made chief of this laboratorj". It was organ- 

 ized in collaboration with the Bureau of Forestry for the j^urpose of 

 instituting a complete chemical studj' of forest prolucts in all their 

 economical relations. The work done during the year ended June 

 30, 1902, was divided into the following groups: 



(1) Researches, in cooperation with the Association of Official 

 Agricultural Chemists, on analytical methods applied to feeding 

 stuff's, and also on methods of analj'sis of tanning materials. 



