SOUKCES OF ILLUMINATION 81 



obtained at will. It is absolutely safe, and when exhaustion 

 takes place, the cylinder has only to be returned to the Company 

 to be refilled. Further, as the evolution of the gas does not 

 take place in the cylinder at all, it is delivered perfectly cool 

 and dry and at a steady pressure. Its only disadvantage is its 

 expensiveness, as an outfit costs some ten pounds ; but in 

 some cases the possessors of motor-cars may perhaps already 

 have the apparatus in use. 



Lime-light. Of the illuminants that are suitable for higher 

 power work what is known as oxy-hydrogen light or lime-light 

 will probably be adopted by a large number of workers. It is 

 suited in its best forms for use with the highest powers, and is 

 only exceeded by the electric-arc light in its intensity, and in 

 the actinic value of the light produced. 



Oxy-hydrogen light or lime-light is produced by raising 

 the temperature of the surface of a cylinder of lime to a white 

 heat, by means of a flame produced by the combustion of 

 hydrogen and oxygen. The necessary gases are now universally 

 supplied compressed into steel cylinders. Pure hydrogen is 

 not used, but is nearly always replaced by compressed coal-gas. 

 A substitute for the hydrogen or coal-gas may be a volatile 

 substance such as ether or gasoline, and in cases where it is not 

 easy to procure the cylinders of compressed gas, the latter is 

 perhaps preferable. If coal-gas and oxygen are allowed to 

 mix in certain proportions they form, if ignited, a highly 

 explosive mixture. Neither will explode if unmixed, nor if 

 there is a great excess of either beyond the explosive proportion. 

 It is therefore advisable to take reasonable precautions in 

 working lime-light, although, unless a serious escape of either 

 gas occurs, with modern appliances danger is now practically 

 non-existent. 



The mechanical contrivance that is generally used for 

 combining and burning the gases and bringing them into contact 

 with the lime surface is known as a lime-light jet. There are 

 two types of jet most generally used, known respectively as 

 the ' mixed jet,' and the ' blow-through jet.' 



In the mixed jet, which is the more powerful, the gases, 

 hydrogen and oxygen, are intimately mixed in proper propor- 

 tions in a chamber, before they issue from the orifice of the jet 

 and impinge on the lime. 



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