34 MATERIALS AND PROCESSES OF SLIDE-MAKING 



There is no more common cause of failure in the preparation of microscope 

 sHdes than imperfect dehydration and clearing. It is a sheer waste of time to 

 endeavor to embed an object in paraffin unless all the water and all the alcohol 

 have been removed from it, and it is a waste of time to endeavor to impreg- 

 nate it with xylene unless all the water has been removed from it by alcohol. 

 It is quite impossible to give any particular time schedule for any particular 

 object; experience is the only guide. It is easy to see, however, when an object 

 has not been dehydrated or cleared perfectly. The least trace of milkiness— 

 as distinct from opalescence— is an indication that the water has been imper- 

 fectly removed in alcohol, so that the object cannot be cleared properly. If 

 this slight milkiness is observed, the object must be returned to absolute 

 alcohol until such time as all the water has been removed. There is no simple 

 method of determining when all the alcohol has been removed by xylene. It 

 is usually safer to use three changes, allowing ample time in each, than to 

 embed an object which has been cleared imperfectly and which will be impos- 

 sible to section subsequently. 



Perfect clearing is just as essential in objects intended for wholemounts and 

 is much easier to determine since the essential oils are of sufficiently high 

 refractive index to make a properly cleared object appear glass-clear. No further 

 clearing will take place in Canada balsam, so that, unless the object appears 

 perfect in oil, it is a waste of time to mount it. 



A final point to remember is that dehydrating agents must of necessity be 

 hygroscopic, and that they will dehydrate the air as readily as they will dehy- 

 drate the specimen. It is desirable, therefore, either to use fresh absolute 

 alcohol from an unopened bottle or, if one is not using the whole bottle at 

 a time, to keep a layer of some good dehydrating agent at the bottom of the 

 bottle. The best dehydrant for use in absolute alcohol is anhydrous copper 

 sulfate, for this not only absorbs water readily but also indicates, by chang- 

 ing from white to blue, when it is becoming exhausted. Anhydrous calcium 

 sulfate, in the form commercially known as Drierite, is a somewhat better dehy- 

 drating agent but cannot be used as an indicator in alcohol. 



One may also anticipate that both the hydrocarbons and the essential oils 

 used for clearing will be in a water-saturated condition when purchased. 

 These, therefore, should always be dehydrated as soon as they have been pur- 

 chased—preferably using Drierite— but, since they have little tendency to 

 absorb moisture from the air, it is not necessary to keep them in bottles con- 

 taining a dehydrating agent. 



