CH. VII. ,] 



SLIDES AND COVER-GLASSES. 



145 



y-j-jr mm. may be put in the same box. For example, if one takes T W 

 mm. as a standard, covers varying j~| T mm. on each side may be put 

 into the same box. In this case the box would contain covers of y'j'V, 

 ir. toV and T y o - mm. 



JL_4 



Fig. 122. Zeiss Cover-Glass Meas- 

 urer. With i/iis the knife edge jaws 

 are opened by means of a /ever, and 

 the cover inserted. J lie thickness 

 may /hen be read off on /he face as 

 the pointer indicates /he thickness in 

 hundredths millimeter in /he outer 

 circle and in hundred/Its inch on /he 

 ae& inner circle. 



§ 227. Cleaning Mixtures for Glass. — The cleaning mixtures used 

 for cleaning slides and cover-glasses are those commonly used in chem- 

 ical laboratories : 



(A) Dichromate of Potash and Sulphuric Acid. 



Dichromate of potash ( K, Cr 2 O t ) - - - 200 grams. 



Water, distilled or ordinary - - - 1000 cc. 



Sulphuric acid (H 2 S0 4 ) - 1000 cc. 



Dissolve the dichromate in the water by the aid of heat. Pour the 

 solution into a bottle that has been warmed and surrounded by a wet 

 towel. Add slowly and at intervals the sulphuric acid. It is safer to 

 mix the ingredients in an agate-ware basin, and put into the bottle only 

 after the mixture is cool. 



For making this mixture, ordinary water, commercial dichromate and 

 strong commercial sulphuric acid should be used. It is not necessary 

 to employ chemically pure materials. 



This is a very 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. 



( Bj Sulphuric and Nitric Acid I\Iixture. 



Nitric acid (H NO, ) - 200 cc. 



Sulphuric acid (H, SOJ ----- 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 corrosive mixture than (A), and has the undesirable 



feature of giving off very stifling fumes, therefore it must be carefully 



10 



