1 66 MOUNTING AND LABELING [CH. VII 



(A) Dichromate of Potash and Sulphuric Acid. 



Dichromate of potash (K 2 Cr 2 7 ) - - 200 grams 



Water, distilled or ordinary - - - 800 cc. 



Sulphuric acid (H 2 S0 4 ) - 1200 cc. 



Dissolve the dichrornate 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 (Pr. 

 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 break- 

 age from the great heat developed upon the addition of the sulphuric 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 exceed- 

 ingly 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. 



( B) Sulphuric and Nitric Acid Mixture. 



Nitric acid (HN0 3 ) - - 200 cc. 



Sulphuric acid (H 2 S0 4 ) _____ 300 cc. 



The acids should be strong, but they need not be chemically pure. The two 

 acids are mixed slowly, and kept in a glass stoppered bottle. This is a more cor- 

 rosive mixture than (A), and has the undesirable feature of giving off stifling 

 fumes, therefore it must be carefully covered. It may be used several times. It 

 acts more rapidly than the dichromate mixture, but on account of the fumes is not 

 so well adapted for general laboratories. 



MOUNTING, AND PERMANENT PREPARATION OF MICROSCOPICAL OBJECTS 



\ 243. Mounting a Microscopical Object is so arranging it upon some suit- 

 able 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. 



\ 244. 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 and made 

 ready 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. Sometimes 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 



