correct to two decimals, or to the nearest million, etc. Absolute errors 

 are more important in addition and subtraction: e.g., in a column of 

 figures the absolute errors in tlie third place of a sum or a difference 

 may be great enough to make the second place unreliable. Relative errors 

 (x, -x)/x are connected with the number of significant figures and are 

 usually expressed as percentages. These errors are important in multi- 

 plication and division. In a product or quotient the number of signif- 

 icant figures is equal to the number in the weakest factor. Many solu- 

 tions need not be prepared more carefully than 5% while others must be 

 made with care to insure sufficient accuracy. If in doubt as to the 

 precision required consult with the investigator or with the person in 

 charge. This information and that given above is to be used as a guide 

 by the staff in the use of the equipment in the Chemical Room. 



ALCOHOL DILUTION: 



For ordinary histological work special strengths of alcohol may 

 be prepared by taking the number of milliliters of 95% alcohol equal to 

 the strength desired in a graduate and adding enough distilled water to 

 make 95 ml. (Example: to prepare 60% take 60 ml. of 95% alcohol and 

 add 35 ml. of distilled water making 95 ml. of the strength of the 

 alcohol used . ) 



CLEANING METHODS: 



Scrubbing with a 2% solution of alconox and hot water followed 

 by a liberal rinsing with tap water will remove most chemicals. A 

 small amount of trisodium phosphate applied with the fingertips or with 

 a small brush will remove pencil markings, most greases, xylene, and 

 films of paraffin. No abrasives should be used in cleaning volumetric 

 glass ware. When cleaning solution (sulfuric acid-dichromate ) is used 

 continued rinsing is necessary. Ten rinsings with water, seven of 

 tap and three of distilled, are necessary for adequate removal of the 

 dichromate from the surface of the glass. Since chromic acid is toxic 

 to living organisms, this is important. For many purposes 1 part con- 

 centrated nitric acid to 9 parts water is superior to chromic acid as 

 the nitric acid oxidizes organic material without leaving an adsorbed 

 residue on the glass. Aqua regia will remove what can be removed by 

 cleaning solution and will wash off completely with tap water. To clean 

 staining jars use a little dilute hydrochloric acid. For the few dyes 

 not removed by this, use a strong solution of sodium hydroxide. Another 

 useful cleaning fluid is 1-5% trisodium phosphate. Rubber stoppers 

 may be cleaned by boiling in dilute sodium hydroxide, then rinsing with 



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