320 MEASURING SLIDES AND COVERS [Ch. X 



With these measures or gauges one should be certain that the index 

 stands at zero when at rest. If the index does not stand at zero it 

 should be adjusted at that point, otherwise the readings will not be 

 correct. 



As the covers are measured, the different thicknesses should be put 

 into different glass boxes and properly labeled. Unless one is striving 

 for the most accurate possible results, cover-glasses not varying 

 more than 0.06 mm. may be put in the same box. For example, if 

 one takes 0.15 mm. as a standard, covers varying 0.03 mm. on each 

 side may be put into the same box. In this case the box would con- 

 tain covers of 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, and 0.18 mm. 



§ 497. Dichromate cleaning mixture for glass. — The cleaning 

 mixture used for cleaning slides and cover-glasses is that commonly 

 used in chemical laboratories: (Dr. G. C. Caldwell's Laboratory 

 Guide in Chemistry). 



Dichromate of potash (K^Cr^O?) 200 grams 



Water, distilled or ordinary 800 cc. 



Sulphuric acid (ILSO4) 1 200 cc. 



As great heat is developed in the reaction on mixing the sulphuric 

 acid with the watery solution of dichromate, it is necessary to use heat- 

 resisting vessels. The best so far employed are those made of pyrex 

 glass. Use ordinary tap water and the commercial dichromate and 

 strong sulphuric acid. Chemically pure ingredients are not demanded. 



Dissolve the dichromate in the water by the aid of heat. Use for 

 this an agate dish. Now place the pyrex dish in the sink on some 

 asbestos or a piece of board. Pour the warm solution of dichromate 

 into the pyrex dish, and then add the sulphuric acid, stirring the 

 liquid with a glass rod. The reaction is so great that the liquid will 

 boil violently. An abundance of chromic acid crystals will form as 

 the sulphuric acid is added. Let the pyrex dish remain in the sink 

 until the cleaning mixture is cool and then pour it into a glass- 

 stoppered bottle for storage. 



If the dichromate is well pulverized it can be put directly into the 

 pyrex dish with the requisite amount of water, and the sulphuric 

 acid added as directed. 



