THE PARAFFIN METHOD 127 



smeared with fixative. Of course, since the ribbon had not got into 

 contact with the fixative, much would be lost. It is a good plan to put 

 the slide on a metal bath with a piece of corrugated pasteboard under 

 the shde. Very convenient warming plates, which can be kept at a con- 

 stant temperature, are sold by most dealers in laboratory supplies. A 

 very convenient warming plate is easily made. Simply take a box, or 

 make one, about 2 feet long, 1 foot wide, and 6 inches deep, using a 

 piece of glass or a piece of brass | inch thick for the top. Heat it by 

 putting in such an electric bulb as will give the temperature desired. 

 With the bulb in one end of the box, there will be various temperatures 

 in different parts of the box. This is fine for straightening ribbons and 

 for use during imbedding. It can even be used as a paraflfin bath 

 where the times are so short that the temperature can be watched 

 constantly. 



After the sections have become smooth and the surplus water has 

 been removed, leave the slides where they will be warm, but well under 

 the melting-point of the paraffin, overnight or for 24 hours. If free 

 from dust, they may be kept for several days, or even weeks, before 

 staining. If the sections are very thick, so that they do not need to be 

 smoothed out on water, they may be laid carefully on the fixative, 

 patted down with the finger and they are ready for staining. Sections 

 may stick to the beginner's finger, but he should soon learn to avoid 

 such troubles. 



Land's fixative. — Mayer's fixative is so easily prepared and it keeps 

 so well that it is in universal use; but, in many cases, it will not hold 

 the section to the slide. Moss archegonia and moss capsules are likely 

 to wash off, especially if cut rather thick. Large sections of cones of 

 conifers are almost sure to float off as soon as the slide comes into the 

 xylol or alcohol. Sections of ovules of cycads, as soon as they attain a 

 length of 1.5-2 cm., are likely to wash off. For handling these more 

 difficult cases, Dr. Land devised a fixative which has proved satisfac- 

 tory, even in such extreme cases as sections of ovulate cones of Pinus 

 hanksiana 2 cm. long. Formula: 



Gum arabic 1 g. 



Dichromate of potash . 2 g. 



Water 100.0 c.c. 



The mixture will not keep ; the formula is given merely to indicate 

 its composition. Make a 1 per cent solution of gum arabic in water, 

 which will keep as well as Mayer's fixative; but make the dichromate 



