SOURCES OF ILLUMINATION 85 



lime quickly takes place. This is to some extent overcome by 

 turning the lime at intervals of five or ten minutes the time 

 depending largely on the pressure of the gas so that a fresh 

 surface of the lime is exposed to the action of the jet. 



In the case of the blow- through jet, a softer form of lime 

 can be used ; as the surface of incandescence is larger and 

 one gas only is under pressure, pitting does not so quickly 

 occur in this type. bi 



It is advisable to use limes that are of good quality and truly 

 cylindrical. In putting them on the pin which supports them 

 on the jet, it should be borne in mind that the hole should be 

 sufficiently large to allow the lime to drop on easily ; otherwise, 

 when the jet is started, expansion of the metal results in a frac- 

 ture of the lime occurring at a very early stage. Lime cylinders 

 can now be obtained in sealed glass tubes, and it is advisable 

 to adopt these although they are somewhat more expensive than 

 those sold in tins. They are usually of better quality, and, 

 further, being absolutely protected from atmospheric influ- 

 ences, there is no chance of their absorbing water-vapour from 

 the air, and consequently they remain in good condition for 

 any length of time. The glass casing^ need only be broken 

 when they are required for use, and there is consequently no 

 opportunity for moisture to reach them, or for any expansion 

 to take place, as usually happens with limes that are not so 

 encased. 



There are substitutes for lime now to be obtained particu- 

 larly the Zirconium discs or cylinders of a smaller size than 

 the standard lime. They are sold by Messrs. Zeiss, and have 

 the particular advantage that they offer more resistance to the 

 high temperature, last for a longer time, and, being of smaller 

 size, constitute a smaller and therefore better source of light. 

 They do not deteriorate so rapidly as lime cylinders, nor 

 are they subject to atmospheric influences. 



When purchasing either the steel cylinders for the com- 

 pressed gases, or the gases themselves, it is of the first impor- 

 tance to go to a firm of good repute. The products of the British 

 Oxygen Company, formerly known as Erin's Oxygen Company, 

 may be mentioned as reliable ; their cylinders are thoroughly 

 tested before being sent out, and in addition to this, to prevent 

 the possibility of any accident in filling, they now make them 



