ease of controlling it a greater variety of more artistic effects may be obtained.  In ordinary photographic printing tungsten lamps are widely used, but in blue-printing the white flame-arc and the mercury-vapor lamp are generally employed.  Not many years ago the blue-printer waited for the sun to appear in order to make his prints, but to-day large machines operate continuously under the light of powerful artificial sources.  How many realize that the blue-print is almost universally at the foundation of everything at the present time?  Not only are products made from blue-prints but the machinery which makes the products is built from blue-prints.  Even the building which houses the machinery is first constructed from blue-prints.  They form an endless chain in the activities of present civilization.

Artificial light has been a great factor in the practical development of photography and it is looked upon for aid in many other directions.  Although there is a multitude of reactions in photographic processes which are brought about by exposure to light, these represent relatively few of the photochemical reactions.  In general, it may be stated that light is capable of causing nearly every type of reaction.  The chemical compounds which are photo-sensitive are very numerous.  Many of the compounds of silver, gold, platinum, mercury, iron, copper, manganese, lead, nickel, and tin are photo-sensitive and these have been widely investigated.  Light and oxygen cause many oxidation reactions and, on the other hand, light reduces many compounds such as silver salts, even to the extent of liberating the metal.  Oxygen is converted partially into ozone under the influence of certain rays and there are many examples of polymerization caused by light.

Various allotropic changes of the elements are due to the influence of light; for example, a sulphur soluble in carbon disulphide is converted into sulphur which is insoluble, and the rate of change of yellow phosphorus into the red variety is greatly accelerated by light.  Hydrogen and chlorine combine under the action of light with explosive rapidity to form hydrochloric acid and there are many other examples of the synthesizing action of light.  Carbon monoxide and chlorine combine to form phosgene and the combination of chlorine, bromine, and iodine, with organic compounds, is much hastened by exposing the mixture to light.  In a similar manner many decompositions are due to light; for example, hydrogen peroxide is decomposed into water and oxygen.  This suggests the reason for the use of brown bottles as containers for many chemical compounds.  Such glass does not transmit appreciably the so-called actinic or chemical rays.