APPENDIX 337 



expose the precipitate to the daylight in such a manner that a 

 portion is covered by something opaque. Examine from time to 

 time. 



EXP. 90. Add enough hydrochloric acid to silver nitrate solu- 

 tion to effect complete precipitation. Add an excess of ammonium 

 hydroxide, and shake vigorously. Acidify with nitric acid; add 

 more ammonium hydroxide. 



EXP. 91. Add hydrochloric acid to 5 cc. of silver nitrate solu- 

 tion; allow the precipitate to settle; pour off the clear liquid; add 

 water to wash the precipitate; pour off the wash- water. Shake the 

 precipitate with a solution of sodium thiosulphate. 



EXP. 92. Clean a test-tube very thoroughly by heating nitric 

 acid in it. Pour away the acid and introduce 5 cc. of silver nitrate 

 solution. Add ammonium hydroxide until the precipitate which 

 formed at first is redissolved. Add a few cc. of grape sugar solution 

 and warm gently until the silver is deposited. Pour the liquid 

 away, and examine the color of the silver by transmitted light. 



LABORATORY MAXIMS 



There is no such phrase as " clean enough." 

 Never scrub off to-morrow what you can dissolve to-day. 

 Towels are used for drying, not for rubbing off dirt. 



Neighbors' eyes were not meant for targets, nor their noses for 

 fume-receptacles . 



Glass and porcelain are not able to stand sudden changes of 

 temperature. 



Weights and hot crucibles should not be held in the fingers. 



Note-books have good memories; jottings on loose paper are 

 useful when you can find them. 



An unrecorded experiment was never begun. 



Chemical equations explain reactions, but do not describe them. 



Too much of a reagent is as bad as too little; and the latter 

 fault can be remedied. 



Repairing damages takes much longer than avoiding them. 



