'I ^: : './EKAar}CA v L' : AGRICULTURAL CHEMISTRY [2-3 



duce the supply of air, or the gas will take fire below, which 

 then gives off unpleasant-smelling products, and may melt 

 the rubber tubing attached to the burner. If the burner 

 should thus take fire below, at once turn off the gas, reduce 

 the air supply by turning the ring, and relight. 



For heating a large surface a * rose-top ' may be used. 

 It consists of a metal cap perforated with holes to distribute 

 the gas. The cap is slipped on to the top of the burner 

 when it is required for use. 



2. The Spirit-lamp. — Where gas is not available the 

 spirit-lamp (fig. 2) forms a convenient heating-lamp. Like 



the Bunsen burner the flame is non- 

 luminous, and does not smoke; but the 

 temperature is not so high, and the 

 flame cannot be so conveniently regu- 

 lated. It consists of a reservoir con- 

 taining methylated spirit, into the neck 

 of which passes a wick-holder which 

 supports the cotton-wick. When not 

 in use the spirit-lamp should be covered 



with the ground-glass cap (a) to prevent evaporation of the 



spirit. 



Special forms of spirit- lamps, known as ' spirit-bunsens,' can 



now be purchased. They burn a mixture of spirit-vapour and air, and 



give a much higher temperature than does the ordinary spirit-lamp. 



3. The Blowpipe. — The mouth blowpipe shown in 

 fig. 3 is useful for obtaining a high temperature. On blowing 

 a stream of air by means of this apparatus through an ordinary 

 luminous flame a small hot flame is obtained. In order to 

 produce this effect turn off the air supply in the Bunsen 

 burner (fig. 1), so as to obtain the luminous flame. Next 

 turn down the gas supply until the flame is nearly two 

 inches in height. Now place the jet of the blowpipe just 

 over the orifice of the burner and blow a gentle current of 

 air into the flame. The flame will then lose its luminosity 

 and appear as a finely pointed tongue of flame (fig. 4). In 

 the use of the blowpipe the student should practise so as 



