TECHNICAL INDEX AND GLOSSARY 



Piston Engine. A form of engine invented by Denis Papin 

 about 1688. The name is still used to distinguish certain types 

 of engines (such as the locomotive) from rotary engines, turbine 

 engines, etc. See "Captive Molecules: The Story of the Steam 

 Engine," Vol. VI, p. 79. 



Pitch-blende. A heavy, black, pitchy-looking mineral, found 

 principally in Saxony, Bohemia, Cornwall, and Colorado. It 

 was while experimenting with this substance that M. Henri 

 Becquerel discovered the so-called "Becquerel rays" in 1896. 

 See "Radio-activity," Vol. V, p. 97. 



Plagues. The name given to epidemics of various diseases 

 which caused great loss of life during the middle ages and until 

 quite recent times. 



Polonium. An element discovered by Mme. Skoldowska Curie 

 in 1898. Mme. Curie discovered it while examining the mineral 

 pitch-blende (q.v.) and named it in honor of her native country, 

 Poland. For chemical symbol and atomic weight, see "Element," 

 in the present index. 



Ponderable Matter. See "The Ether and Ponderable Mat- 

 ter," Vol. Ill, p. 283. 



Power. In mechanics and physics, the application of energy 

 through which work is performed; also, the rate at which work 

 is performed that is, the amount of work performed per unit 

 of time. See "How Work is Done," Vol. VI, p. 29. The subject 

 of power as generated by muscles, wind, water, steam, and 

 electricity will be found in the chapters dealing especially with 

 these subjects. See general index. 



Predynastic Period. The period in Egyptian history antedat- 

 ing the historic period. 



Printing. For the full account of the development of printing, 

 see "The Printing and Making of Modern Books," Vol. VIII, 

 p. 119. In the same volume are the chapters dealing with the 

 closely allied subjects, "The Manufacture of Paper," p. 159; and 

 "The Reproduction of Illustrations," p. 184. 



Protoplasm. An albuminous elementary organic compound 

 which enters into the composition of organized tissues of all 

 kinds. See "The Cell Theory Elaborated," Vol. IV, p. 122. 



Psychology. That branch of knowledge which deals with 

 the mind. See "The New Science of Experimental Psychology," 

 Vol. IV, p. 245- 



Pulley. A wheel turning on a pin, having a groove on its cir- 

 cumference in which runs a rope for turning it. It is a form 



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