PRECIPITATES. 



17 



which being easily soluble in water does not precipitate in the 

 solid form, but remains dissolved. 



In order to boil a liquid in a test-tube, a test-tube holder is con- 

 venient, such as that represented in fig. 10, consisting of a brass 

 spring-clip furnished with a 

 handle. In default of such 

 a holder a piece of paper 

 may be twisted round the 

 upper part of the test-tube, 

 as in fig. 11. The tube 

 should never be held point- 

 ing towards the face of the 

 operator, or towards any 

 other person, because some- 

 times steam is generated so Fig. 10. Spring Test-tube Holder, 

 rapidly that a quantity of the hot fluid is projected bodily out 

 of the tube, which thus becomes a sort of miniature steam-gun ; 

 dangerous scalding might readily be brought about if the 

 boiling fluid struck 

 anyone on the face or 

 elsewhere, especially 

 w r hen acids or other 

 corrosive fluids are 

 contained in the 

 test-tube. Fig. 12 

 represents a method 

 of holding a test- 

 tube by means of a 

 clamp-stand instead 

 of by the operator's 

 hand. 



Expt. 12. To 

 produce variously- 

 Coloured Precipi- 

 tates from the same Fig. 11. Paper Test-tube Holder. 

 Solution. The following experiments further illustrate the pro- 

 duction of solid substances (as precipitates) by double decom- 

 position on mixing two liquids together. Into six test-glasses 

 (fig. 13), or ordinary wine-glasses, respectively pour a tablespoonful 

 of the solutions of the following compounds : (1) common salt 

 (otherwise known as chloride of sodium) * ; (2) potassium chromate ; 



* Chemists often use indifferently the terms " chloride of sodium" and 

 "sodium chloride" to indicate common salt; and similarly in other cases, 



