CH. AY] MOUNTING PREPARATIONS 251 



Dichromate of potash (K,Cr,0 7 ) - 200 grams 



Water, distilled or ordinary - 800 cc. 



Sulphuric acid (H 2 SO,) - 1200 cc. 



Dissolve the dichromate in the water by the aid of heat, using an agate or 

 other metal dish, then pour it into a heavy iron kettle lined with sheet lead 

 (Trans. Amer. Micr. Soc , 1899, p. 107) . Add the sulphuric acid to the 

 dissolved dichromate in the kettle. The purpose of the lead lined kettle is to 

 avoid breakage from the great heat developed upon the addition of the sul- 

 phuric acid. The lead is very slightly affected by the acid, iron would be 

 corroded by it. 



For making this mixture, ordinary water, commercial dichromate and 

 strong commercial sulphuric acid may be used. It is not necessary to employ 

 chemically pure materials. 



This is an excellent cleaning mixture and is practically odorless. It is 

 exceedingly corrosive and must be kept in glass vessels. It may be used 

 more than once, but when the color changes markedly from that seen in the 

 fresh mixture it should be thrown away. An indefinite sojourn of the slides 

 and covers in the cleaner does not seem to injure them. 



. MOUNTING, AND PERMANENT PREPARATION OF MICROSCOPIC 



OBJECTS 



\ 340. Mounting a Microscopic Object is so arranging it upon some 

 suitable support (glass slide) and in some suitable mounting medium that it 

 may be satisfactorily studied with the microscope. 



The cover-glass on a permanent preparation should always be considerably 

 larger than the object ; and where several objects are put under one cover-glass 

 it is false economy to crowd them too closely together. 



\ 341. Temporary Mounting. — In a great many cases objects do not need 

 to be preserved ; they are then mounted in any way to enable one best to 

 study them, and after the study the cover glass is removed, the slide cleaned 

 for future use. In the study of living objects, of course only temporary 

 preparations are possible. With amoebae, white blood corpuscles, and many 

 other objects both animal and vegetable, the living phenomena can best be 

 studied by mounting them in the natural medium. That'is, for amoebae, in 

 the water in which they are found ; for the white blood corpuscles, a drop of 

 blood is used and, as the blood soon coagulates, they are in the serum. Some- 

 times it is not easy or convenient to get the natural medium, then some liquid 

 that has been found to serve in place of the natural medium is used. For 

 many things, water with a little common salt (water 100 cc, common salt T % 

 gram ) is employed. This is the so-called normal salt or saline solution. For 

 the ciliated cells from frogs and other amphibia, nothing has been found so 

 good as human spittle. Whatever is used, the object is put on the middle of 

 the slide and a drop of the mounting medium added, and then the cover-glass. 

 The cover is best put on with fine forceps, as shown in Fig. 197. After the 



