GEHENNA 



2411 



GELATIN 



chiefly on insects, and is more or less active 

 at night. It is found in warm climates, and is 

 widely distributed throughout the world. It 

 is generally regarded with disfavor as being 

 poisonous and the carrier of disease, but there 

 is no evidence to support such an opinion. 



GEHEN'NA, the valley of Hinnom, just 

 south of Jerusalem, where children long before 

 the Christian Era were burned to death as 

 offerings to Moloch. From the terrible suffer- 

 ing and sorrow connected with the valley, its 

 name, which is an abbreviated form of two 

 Hebrew words, ge-hinnom, meaning valley of 

 Hinnom, grew to mean a place of eternal pun- 

 ishment, or hell. In ancient descriptions of 

 hell the awful scenes in this valley were used 

 as a foundation. King Josiah caused the val- 

 ley to be used for sewage and other refuse, 

 "that no man might make his son or his daugh- 

 ter to pass through the fire to Moloch" (77 

 Kings XXIII, 10). 



GEIKIE, ge'ki, SIR ARCHIBALD (1835- ), 

 Scottish geologist and author, considered one 

 of the greatest geological authorities of all 

 times. His textbooks are used both in Europe 

 and the United States. He was born in Edin- 

 burgh, and after graduation at the University 

 of Edinburgh became a member of the Geo- 

 logical Survey of Scotland, of which he was 

 later made a director. He began his long list 

 of works in 1858 with The Story of a Boulder; 

 or, Gleanings from the Note-Book of a Geolo- 

 gist. After teaching geology and mineralogy 

 in Edinburgh for eleven years, Geikie became 

 director of the Museum of Practical Geology in 

 London. He was elected president of the 

 Geological Society of London and also became 

 president of the British Association for the 

 Advancement of Science. He visited America 

 in 1897 and delivered a series of lectures at 

 Johns Hopkins University and at McGill Uni- 

 versity in Montreal. Among his most im- 

 portant works are A Textbook of Geology, 

 Ancient Volcanoes of Britain and The Founda- 

 tions of Geology. 



GEISSLER'S, gys'ler's, TUBES, a name 

 given to glass tubes used to show the bril- 

 liant effect produced by electricity when it is 

 passed through a rarefied gas. They are named 

 after Heinrich Geissler (1814-1879), a cele- 

 brated German scientific instrument maker, by 

 whom they were first constructed. These 

 tubes, made of very hard glass and of different 

 shapes (as shown in the illustration), have a 

 platinum wire sealed into each end, which 

 serves as an electrode. . The tubes are filled 



with various rarefied gases, such as hydrogen, 

 oxygen, nitrogen, air, and so on. By connect- 

 ing the electrodes to the opposite poles of an 

 electric machine or to an induction coil, an 

 electric current is passed through the tubes. 

 The color of the light produced depends upon 

 the shape of the tubes, the gas with which they 



VARIOUS FORMS OF GEISSLER'S TUBES 



are filled and the degree of rarefaction of the 

 gas, and by varying these conditions very 

 pleasing effects can be obtained. Geissler's 

 tubes are also valuable in scientific research, 

 such as examining various incandescent gases 

 with the spectroscope. 



Related Subjects. The reader is referred to 

 the following 1 articles in these volumes : 

 Cathode Rays Electrode 



Crookes Tubes Roentgen Rays 



Electricity Spectroscope 



GELATIN, jel'atin, a transparent, brittle, 

 yellowish substance obtained from the air blad- 

 der of fish and the bones, hoofs, horns, skin and 

 muscle of animals. When bones are made 

 into gelatin they are treated with muriatic 

 acid; the mineral substances dissolve and leave 

 only the cartilage. After this has been thor- 

 oughly boiled in water it dissolves and forms 

 a crude gelatin suitable only for glue. To 

 purify it, the gelatin is again soaked in hot 

 water and filtered, while hot, in ninety per cent 

 alcohol, a process which causes the impurities 

 to settle at the bottom. 



The best and purest gelatin is found in the 

 air bladder of the sturgeon and, as it is easily 

 digested, it is used as a food for invalids. 

 In cold water it increases to five or ten times 

 its weight, becoming an elastic, jellylike mass, 



