80 CHAPTER VI. 



objects are being treated in a watch-glass or 011 a slide 

 becomes cloudy after a short time, and fails to clear the 

 tissues. This is owing to a combination between the essen- 

 tial oil and moisture., derived, I think, rather from the air 

 than from the objects themselves. The cloudiness can 

 usually be removed by warming (as pointed out by HATCHETT 

 JACKSON, Zool. Anzeig., 1889, p. 630), but this remedy is not 

 always successful, for in ceriain moist states of the atmo- 

 sphere the cloudiness will persist, notwithstanding continued 

 warming. It is for this reason that I advise that clearing 

 be done, whenever possible, in shallow, well-corked tubes, 

 under which conditions the phenomenon rarely occurs. In 

 any case, be careful not to breathe on the liquid. 



107. Classification of De-alcoholisation and Clearing Agents. 



NEELSEN and SCHIEFPEKDECKEU (Arch. f. Anat. u. Phys., 

 1882, p. 206) examined a large series of ethereal oils (pre- 

 pai^ed by Schimmel and Co., Leipzig), with the object of 

 finding a not too expensive substance that should combine 

 the properties of clearing quickly alcohol preparations, not 

 dissolving out aiiiliii colours, clearing celloidin without dis- 

 solving it, not evaporating too quickly, and not having a too 

 disagreeable smell. 



Of these, the following three fulfil the conditions and can 

 be recommended: Cedar-wood, Origanum, Sandal-wood. 



To these should be added the others recommended in the 

 following paragraphs. 



See also the paper of JORDAN, Zeit. f. wiss. Mil:., xv, i, 

 1898, p. 50, which has special reference to the behaviour of 

 some essential oils towards celloidin. 



It would be important to possess a complete list of the exact indices of 

 refraction of the substances used for clearing. I have, unfortunately, not 

 been able to obtain sufficient information of a trustworthy nature for the 

 compilation of such a list. Cedar oil has nearly the index of crown glass 

 (tliis is true of the oil in the thick state to which it is brought by exposure 

 to the aiv, not of the new, thin oil, which is less highly refractive), it 

 therefore clears to about the same extent as Canada balsam. Clove oil has 

 a much higher index, and therefore clears more than balsam; cinnamon 

 oil higher still. Turpentine and bergamot oil have much lower indices, 

 and therefore clear less. 



The following short list, extracted from BEHRENS' Tabellen zum Ge- 

 brauch bei mikroskopischen Arbeiten, Braunschweig, 1892, p. 42, and 



