PHOTIUS 



IC.Is 



PHOTO-ENGRAVING 



hut tl><> dicad di>ea>e known as pfio.^i/ jair, 

 which is a rotting of tin 1 jaw hones, has led to 

 the prohibition of its use in most conn 

 where matches are produced. All workmen en- 

 (1 in the manufacture of phosphorus suffer 

 from bone 



When burned in a good supply of uir, phos- 

 phorus forms tht' dense, white vapor called 

 phosphoric oxide, which immediately absorb- 

 water from the air and dissolves, forming phos- 

 phoric acid. When compounds of phosphorus 

 d to a high temperature in closed ves- 

 ihe phosphorus distills, and the vapor, when 

 collected over water, is changed to a yellow 

 liquid that settles to the bottom. The most 

 isive phosphorus works are at Oldbury, 

 md; Lyons, France; and Niagara Falls. 

 Bone ashes and phosphate rock are the sub- 

 res most generally used in its manufacture. 



Phosphorus is necessary to the growth of 

 plants and animals. The plants take phos- 

 phorus from the soil and animals obtain it from 

 the plants. It is found in the bones, and in 

 the brain and nerves in larger proportions than 

 in any other animal tissues. 



Red phosphorus, an allotropic form (see AL- 

 LOTROPY), is made by heating ordinary phos- 

 phorus to a high temperature in a closed vessel. 

 It is a brownish-red powder and does not burn, 

 nor does it melt except at high temperatures. 

 Phosphorus is a nonmetallic element (see 

 CHEMISTKY. subhead Elements). It is not 

 found free in nature, but exists in plants and 

 animals and in phosphate rocks. 



PHOTIUS, Jn'xhius (about 820 -about 891), 

 an eminent Byzantine prelate and statesman, a 

 man of great intellect and literary power, cele- 

 brated because of his interest in the separation 

 of the Ka-tern and Western churches. He was 

 born in Constantinople, and was elevated from 

 the standing of a layman to the patriarchal 

 dignity of secretary of state to Michael III. 

 Pope Nicholas I deposed him in 862, but Pho- 

 tius in turn deposed and condemned Nicholas 

 in 867, thereby laying the foundation of the 

 schism between the Eastern and Western 

 churches. In the same year Photius was exiled. 

 but later was restored to patriarchal dignity. 

 He was again exiled in 886 by Leo the Philoso- 

 pher, and he died in an Armenian monastery. 

 Biographers disagree as to the character of Pho- 

 tius; those of the Roman Catholic Church ac- 

 cuse him of craft, perfidy and violence, while 

 the Greek Church has canonized him. 



PHOTO-ENGRAVING, fo' toh engrayv'ing, 

 is the process of preparing engraved plates or 



blork> for printing by means of photog- 

 raphy and etching. The plates are prepared on 

 two general plans, one in which the background 

 died away, leaving the design or picture 

 to be printed in relief, and the other in which 



the desmn is etched into the plate, leaving the 

 background in relief. The first is known as /-, - 

 Inf photO-engravmg, and the second a8 intniil'm 



photo-engraving, or photogravure. Both de- 

 pend upon the principle that certain substance-. 

 Mich as gelatin, some forms of albumen and 

 bitumen, harden under the action of light. BO 

 that they are not soluble. 



Relief Photo-Engraving. In making a plate 

 in relief for the reproduction of linework. a 

 glass plate coated with a iilm of gelatin and po- 

 tassium bichromate is exposed under a black 

 and white negative. The film is then soaked in 

 water, and the lines swell that were protected 

 by the dark parts of the negative, while the 

 other parts of the film do not swell. The film 

 is then laid in a mold, and a wax or pla>ter 

 impression of it is taken. From this impression 

 an electrotype is made. With the mounting of 

 the electrotype on the block or plate for the 

 printing press the process is completed. 



The halftone (which see) is the most com- 

 mon form of relief photo-engraving, and may 

 be made one of the most effective. 



Photogravure. In the photogravure process 

 a copper plate, covered with a sensitized film. 

 is exposed to a black and white positive, or 

 photograph, on glass. The parts under tin- 

 clear spaces are hardened and those under the 

 shaded parts corresponding to the shades in the 

 picture remain soluble in varying degrees. The 

 plate is then washed and the soluble portions 

 of the film are removed, leaving the metal bare. 

 The plate is then etched and hardened. The 

 grain often seen in photogravure pictures is 

 produced by sprinkling the plate with powdered 

 resin, which is heated in. 



The photogravure process is employed in the 

 reproduction of etchings, fine engravings and 

 other works of art where an exact reproduction 

 is desired. It is also in general use in preparing 

 illustrations for books. For this purpose a less 

 expensive process of preparing the plate j> u>ed. 

 and plates are often prepared for rotary pn 

 for rapid work. 



Consult Jenkins' A nixtiit-.'K lltnnlhoul.- <>f 



Related Sub'jectf*. The reading of the follow- 

 ing articles in connection with this topic is recom- 

 mended : 



Kimra\ iiiK Halftone 



Etching Intaglio 



