SOLDERING PIPES. 37 



Expt. 31. To join Leaden Pipes by Soldering. When a 

 plumber wants to join two leaden water-pipes together he 

 frequently makes use of a somewhat analogous property possessed 

 by the mixture of metals known as " plumber's solder " (mostly 

 lead and tin) ; this mixture when pretty hot is quite fluid, but on 

 cooling a little becomes pasty before it finally solidifies ; in this 

 pasty condition the solder is spread round the two ends of the 

 pipes applied together, one end being slightly widened out to a 

 trumpet shape by means of an appropriate tool, and the other 

 shaved down so as to be capable of insertion into the mouth thus 

 formed. The portions of the lead intended to have the solder 

 applied to them are brightened by scraping so as to enable the 

 solder to stick firmly ; and the pasty solder is manipulated with a 

 greased piece of cloth so as to form ultimately a " wiped joint." 

 Although the knack of being able to manipulate hot pasty solder 

 in this way is by no means indispensable in the performance of 

 chemical experiments, yet the ability to make joints of the kind 

 is sometimes very useful. In the construction of apparatus 

 intended for the preparation of gases of various kinds, a similar 

 mode of making leaky corks and joints, &c., air-tight is frequently 

 used, some plastic material not too hot being usually employed, 

 such as melted sealing-wax that has cooled sufficiently to bear 

 touching, or a stiff" paste of linseed meal and a little water, or 

 ordinary dough of flour and water, clay, and similar pasty cement- 

 ing materials. Workers in glass take advantage of the pasty 

 condition assumed by molten glass as it partially cools to shape it 

 into bottles and numberless other articles ; and methods of 

 making small glass vessels, &c., for experimental use are constantly 

 adopted in laboratories, the heat being usually applied by lamps, 

 where the flame is urged by bellows forming a " blow-pipe " flame. 

 Glass tubing, when not too large and thick, may be easily softened 

 and bent in an ordinary gas flame (Expt. 48). 



CHAPTER III. 



BOILING OF LIQUIDS AND CONDENSATION OF VAPOURS. 



When water is placed in a kettle on the fire it is obvious that 

 tho heat of the fire mainly reaches the water through the bottom 

 of the kettle. Water and many other liquids when thus heated 



