SOUECES OF ILLUMINATION 79 



more than four and a half to five cubic feet of acetylene gas, 

 and that only when a well-constructed generator is used. It is 

 essential that the carbide be of good quality : in fact, its purity 

 is of the utmost importance. The chief impurities to be met with 

 in the gas are ammonia, and sulphuretted and phosphoretted 

 hydrogen. In any generating apparatus adopted it should 

 be seen that suitable purifying apparatus is included so that 

 these impurities are removed ; otherwise choking of the burners 

 will result and the light will be of lower illuminating power than 

 should be the case. The light given out on burning the gas 

 is very actinic, and spectroscopically the nearest in value to 

 sunlight ; so that there is a very equal distribution of light 

 throughout its spectrum no colour in particular predominating. 

 The consequence is that the light appears and is, in fact a 

 very white one. It has an additional advantage for photo- 

 micrographic work in that the amount of radiant heat is not 

 very great ; hence expansion of the apparatus, as the result of 

 the heat emitted by the illuminant, is not such a disturbing 

 factor as is the case with an oil lamp or a Welsbach gas lamp. 

 The exposures necessary with it will be found to be about 

 one-seventh to one-tenth of the time required with the oil 

 lamp. The light itself is absolutely steady, and its luminosity 

 is uniform throughout the area of the flame ; so that in cases 

 where critical illumination is required, satisfactory illumination 

 may be obtained even with low-power objectives. 



Acetylene Generators. The generator adopted may be either 

 automatic or non-automatic. By automatic generators are 

 meant those which have a storage capacity for gas less than the 

 amount that the charge of carbide is capable of producing, and 

 in which when the consumption of gas is stopped, no more 

 water is allowed to reach the carbide, and so a further evolution 

 of gas does not take place. 



Non-automatic generators have a gas-holder of sufficient 

 capacity to receive the whole of the gas from a given charge 

 of carbide. 



The automatic generators may be generally divided into 

 two classes ; or at least for our purpose it is not worth while 

 considering any others. In the first type the carbide, which is 

 in a granulated form, is allowed to drop in quantities as required 

 into a chamber of water underneath. These usually have the 



