THE PARAFFIN METHOD 119 



A good paraffin is an absolute necessity, if preparations are to be of 

 the highest grade. Some brands of parowax are fairly good if sections 

 do not need to be thinner than 10 m- For critical work, with sections 

 from 5 m down to |/x, Griibler's 52° C. paraffin stands at the head of the 

 list. It melts at the temperature indicated on the package. Since the 

 price rises with the melting-point, many paraffins are marked higher 

 than they really are. 



Some paraffins can be improved by heating almost to the boiling 

 point for several hours. If any scum appears, skim it off; if anything 

 settles to the bottom, pour the paraffin off gently and throw away the 

 sediment. If, with prolonged heating, the paraffin takes on a slightly 

 amber color, keep that paraffin for your best work, for it is likely to be 

 good. 



The addition of bayberry wax — a piece about a centimeter square 

 and 5 mm. thick — to a pound of paraffin is likely to improve any paraf- 

 fin except the best. 



Dr. Land added asphalt and secured a paraffin which yielded In 

 ribbons with a rotary microtome. 



Do not throw away the paraffin which you pour off, but put it in a 

 waste jar or beaker, or, still better, in a small tin lard pail, in which 

 you have made a lip to facilitate pouring. This can be placed in the 

 bath, or, in winter, on the radiator, and the xylol will gradually 

 evaporate. After long heating, the paraffin not only becomes as good 

 as new but even better, since it becomes more homogeneous and tena- 

 cious. If it contains dust or debris of any kind, it may be filtered with 

 a hot filter. 



The time required varies with the character of the material and the 

 thoroughness of the dehydrating and clearing. If this schedule has 

 been followed up to this point, the time will be much shorter than 

 most investigators now deem necessary. In dehydrating and clearing, 

 material could be left overnight at any stage; but in the paraffin bath, 

 the time must be reduced to the minimum. If 30 minutes is enough, 

 an hour may be ruinous; if an hour is right, 2 hours may mean dis- 

 aster. A few hints may be helpful. Fern prothallia of average size in- 

 filtrate perfectly in 20-25 minutes; onion root-tips, in 30-45 minutes; 

 ovaries of Liliimi philadelphicum or L. canadense at the fertilization 

 stage, from 45 minutes to 1 hour; 5 or 6 mm. cubes of the endosperm of 

 cycads, containing archegonia, 2-3 hours; median longitudinal sections 

 of the ovulate cones of Pinus banksiana, 4 or 5 mm. thick, may re- 



