EVALUATION OF WOOD PRESERVATIVES 455 



The list is incomplete, but it serves to illustrate one important point 

 about laboratory evaluation tests, and that is the inevitable variation 

 that creeps naturally into explorative research. In the writer's experience 

 and opinion much of the variation in block size has been the result of a 

 sort of forced adaptation, on the part of the investigator, to the size and 

 shape of his laboratory glassware, coupled with certain practical prob- 

 lems of block procurement and manufacture. Obviously it is easy to use 

 thin sticks in test tubes, thin sticks or plaques in Petri dishes, and 

 relatively flat blocks in Kolle flasks. The ^-inch cubes, which in essence, 

 as has been stated before, are simply sections of the ^-inch stake, handle 

 easily in the soil-block cultures. 



Criticism of the shape of the %-inch cube did not become pointed 

 until after the publication of the first papers^^' ^^ on the Laboratories' 

 cooperative work with the Madison laboratory. It is argued — as men- 

 tioned before — that the evaporation of creosote from such blocks is 

 unfairly rapid. However, the losses reported by Rhodes et al,^^ which will 

 be discussed later, indicate that separation of the transverse faces will 

 not prevent creosote evaporation. Assuming properly calculated gradient 

 retentions, and the use of weathered blocks, it does not appear likely that 

 the shape of the blocks — if kept constant within any given comparative 

 test series — will affect the location of the treatment threshold retention. 



Toluene as a Diluent for Creosote Treating Solutions 



This subject is most controversial in this country. The use of acetone 

 or chloroform has been widely accepted in Europe, but Schulze and 

 Becker^'^'^ warn that the use of any diluent may change the rate of 

 evaporation of given creosote fractions, and may affect the rate of evapo- 

 ration of whole creosote. However, there are points in the debate which 

 can be stressed, namely: 



(a) The treatment of test blocks to low and uniform retentions without 

 a diluent is extremely difficult, if not practically impossible; 



(b) The rate of loss of creosote by evaporation, and any change in the 

 character of that loss that may result from toluene dilution might be 

 evident in freshly treated blocks but not in weathered blocks; and 



(c) The volatile fractions of undiluted creosote are lost fairly rapidly 

 from small saplings^^^ and from test blocks; and it is assumed that the 

 use of toluene does not cause loss in the fraction above 355°C. 



Discussion of Item (c) will be resumed in the section on weathering. 

 In view of the perplexing character of this toluene dilution question steps 

 are being taken to find out what happens. In the meantime the toluene 

 diluent which has not been found to exert any measurable toxic effect in 



