78 PKACTICAL PHOTO-MICKOGKAPHY 



gas mantle becomes painfully evident. The spectroscopic 

 value of the light, too, is such that when using colour screens, 

 which in most cases are absolutely necessary, the light is so 

 deficient in certain parts of the spectrum, that it becomes in 

 e ffect a very weak source of light . Where no other more powerful 

 illuminant is available, an oil lamp will be found preferable. 



Acetylene. Of the less powerful sources of light available, 

 acetylene gas has many advantages ; it is brilliant and yet the 

 flame is comparatively cool. Acetylene, or to give it its scien- 

 tific name, ethine (C 2 H 2 ), is a gaseous compound of twenty-four 

 parts by weight of carbon with two parts by weight of hydrogen. 

 It is evolved by the action of water upon calcium carbide, 

 which causes a double decomposition to take place, the oxygen 

 of the water combining with the calcium and forming lime, which 

 remains as a residue, while the hydrogen unites with the carbon 

 of the carbide to form acetylene. For many years, although 

 its production was possible, calcium carbide was a comparatively 

 rare substance, or at least so expensive to produce as to make 

 it prohibitive as the source of acetylene. When, however, 

 the electrolytic method of producing this substance was dis- 

 covered, the whole aspect was /changed, and calcium carbide 

 became a cheap substance, obtainable commercially in large 

 quantities. 



Calcium carbide is produced as the result of the fusion of 

 lime and carbon in an electric furnace. It has no explosive 

 qualities, it is non-inflammable, and in ordinary circumstances 

 combustion does not take place. But it is essential that it 

 should be stored under such conditions that moist air or water- 

 vapour does not reach it, otherwise deterioration sets in and 

 some acetylene gas is inevitably evolved. The volume of 

 acetylene gas required to obtain a given light is only about 

 one-fifteenth that of coal gas ; there is, therefore, less risk of an 

 admixture of air and gas resulting in an explosion, as any 

 escape would rarely be sufficiently large. The unburnt gas 

 has a strong odour which would at once indicate any escape ; 

 but on burning, if the gas is properly purified, there is practically 

 a total absence of smell. 



There are at present rather considerable differences in the 

 quality of calcium carbide supplied, and it is advisable to assume 

 that one pound of carbide of the best quality will yield not 



